Copyright © 2003 W3C^{®} (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark, document use and software licensing rules apply.
This is a specification of a precise semantics, and corresponding complete systems of inference rules, for the Resource Description Framework (RDF) and RDF Schema (RDFS).
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
This is a W3C Last Call Working Draft of the RDF Core Working Group and has been produced as part of the W3C Semantic Web Activity (Activity Statement).
Detailed changes from the previous 5 September 2003 working draft are described in Appendix D. The requirement on character strings being in Normal Form C has been weakened from MUST to SHOULD; all the other changes are editorial.
This document is in the Last Call review period, which ends on 07 November 2003. This Last Call publication consolidates changes and editorial improvements undertaken in response to feedback received during the previous Last Call publication of the RDFCore specifications which began on 23 January 2003. A list of the Last Call issues addressed by the Working Group is also available. This document has been endorsed by the RDF Core Working Group.
This document is being released for review by W3C Members and other interested parties to encourage feedback and comments, especially with regard to how the changes made affect existing implementations and content.
In conformance with W3C policy requirements, known patent and IPR constraints associated with this Working Draft are detailed on the RDF Core Working Group Patent Disclosure page.
Comments on this document are invited and should be sent to the public mailing list wwwrdfcomments@w3.org. An archive of comments is available at http://lists.w3.org/Archives/Public/wwwrdfcomments/.
This is a public W3C Working Draft for review by W3C Members and other interested parties. This section describes the status of this document at the time of its publication. Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress. A list of current W3C Recommendations and other technical documents can be found at http://www.w3.org/TR/.
0. Introduction
0.1 Specifying a formal semantics:
scope and limitations
0.2 Graph Syntax
0.3 Graph Definitions
1. Interpretations
1.1 Technical Note (Informative)
1.2 URI references, Resources
and Literals
1.3 Interpretations
1.4 Denotations of Ground Graphs
1.5
Blank nodes as Existential variables
2. Simple Entailment between RDF graphs
2.1 Vocabulary interpretations
and vocabulary entailment
3. Interpreting the RDF vocabulary
3.1. RDF Entailment
3.2 Reification, Containers,
Collections and rdf:value
3.2.1 Reification
3.2.2 RDF Containers
3.2.3 RDF Collections
3.2.4 rdf:value
4. Interpreting the RDFS Vocabulary
4.1 Extensional
Semantic Conditions (Informative)
4.2 A Note on rdfs:Literal
4.3 RDFS Entailment
5. Datatyped Interpretations
5.1 DEntailment
6. Monotonicity of Semantic Extensions
7. Entailment Rules (Informative)
7.1 Simple Entailment Rules
7.2 RDF Entailment Rules
7.3 RDFS Entailment Rules
7.3.1 Extensional
Entailment Rules
7.4 Datatype Entailment Rules
Appendix A. Proofs of lemmas (Informative)
Appendix B. Glossary (Informative)
Appendix C. Acknowledgements
References
Appendix D. Change Log (Informative)
RDF is an assertional language intended to be used to express propositions using precise formal vocabularies, particularly those specified using RDFS [RDFVOCABULARY], for access and use over the World Wide Web, and is intended to provide a basic foundation for more advanced assertional languages with a similar purpose. The overall design goals emphasise generality and precision in expressing propositions about any topic, rather than conformity to any particular processing model: see the RDF Concepts document [RDFCONCEPTS] for more discussion.
Exactly what is considered to be the 'meaning' of an assertion in RDF or RDFS in some broad sense may depend on many factors, including social conventions, comments in natural language or links to other contentbearing documents. Much of this meaning will be inaccessible to machine processing and is mentioned here only to emphasize that the formal semantics described in this document is not intended to provide a full analysis of 'meaning' in this broad sense; that would be a large research topic. The semantics given here restricts itself to a formal notion of meaning which could be characterized as the part that is common to all other accounts of meaning, and can be captured in mechanical inference rules.
This document uses a basic technique called model theory for specifying the semantics of a formal language. Readers unfamiliar with model theory may find the glossary in appendix B helpful; throughout the text, uses of terms in a technical sense are linked to their glossary definitions. Model theory assumes that the language refers to a 'world', and describes the minimal conditions that a world must satisfy in order to assign an appropriate meaning for every expression in the language. A particular world is called an interpretation, so that model theory might be better called 'interpretation theory'. The idea is to provide an abstract, mathematical account of the properties that any such interpretation must have, making as few assumptions as possible about its actual nature or intrinsic structure, thereby retaining as much generality as possible. The chief utility of a formal semantic theory is not to provide any deep analysis of the nature of the things being described by the language or to suggest any particular processing model, but rather to provide a technical way to determine when inference processes are valid, i.e. when they preserve truth. This provides the maximal freedom for implementations while preserving a globally coherent notion of meaning.
Model theory tries to be metaphysically and ontologically neutral. It is typically couched in the language of set theory simply because that is the normal language of mathematics  for example, this semantics assumes that names denote things in a set IR called the 'universe'  but the use of settheoretic language here is not supposed to imply that the things in the universe are settheoretic in nature. Model theory is usually most relevant to implementation via the notion of entailment, described later, which makes it possible to define valid inference rules.
An alternative way to specify a semantics is to give a translation from RDF into a formal logic with a model theory already attached, as it were. This 'axiomatic semantics' approach has been suggested and used previously with various alternative versions of the target logical language [Conen&Klapsing] [Marchiori&Saarela] [McGuinness&al]. Such a translation for RDF and RDFS is also given in the L_{base} specification [LBASE]. The axiomatic semantics style has some advantages for machine processing and may be more readable, but in the event that any axiomatic semantics fails to conform to the modeltheoretic semantics described in this document, the model theory should be taken as normative.
There are several aspects of meaning in RDF which are ignored by this semantics; in particular, it treats URI references as simple names, ignoring aspects of meaning encoded in particular URI forms [RFC 2396] and does not provide any analysis of timevarying data or of changes to URI references. It does not provide any analysis of indexical uses of URI references, for example to mean 'this document'. Some parts of the RDF and RDFS vocabularies are not assigned any formal meaning, and in some cases, notably the reification and container vocabularies, it assigns less meaning than one might expect. These cases are noted in the text and the limitations discussed in more detail. RDF is an assertional logic, in which each triple expresses a simple proposition. This imposes a fairly strict monotonic discipline on the language, so that it cannot express closedworld assumptions, local default preferences, and several other commonly used nonmonotonic constructs.
Particular uses of RDF, including as a basis for more expressive languages such as DAML+OIL [DAML] and OWL [OWL], may impose further semantic conditions in addition to those described here, and such extra semantic conditions can also be imposed on the meanings of terms in particular RDF vocabularies. Extensions or dialects of RDF which are obtained by imposing such extra semantic conditions may be referred to as semantic extensions of RDF. Semantic extensions of RDF are constrained in this recommendation using the keywords MUST , MUST NOT, SHOULD and MAY of [RFC 2119]. Semantic extensions of RDF MUST conform to the semantic conditions for simple and RDF interpretations described in sections 1 and 3.1 of this document. Any name for entailment in a semantic extension SHOULD be indicated by the use of a vocabulary entailment term. The semantic conditions imposed on an RDF semantic extension MUST define a notion of vocabulary entailment which is valid according to the modeltheoretic semantics described in the normative parts of this document; except that if the semantic extension is defined on some syntactically restricted subset of RDF graphs, then the semantic conditions need only apply to this subset. Specifications of such syntactically restricted semantic extensions MUST include a specification of their syntactic conditions which are sufficient to enable software to distinguish unambiguously those RDF graphs to which the extended semantic conditions apply. Applications based on such syntactically restricted semantic extensions MAY treat RDF graphs which do not conform to the required syntactic restrictions as syntax errors.
An example of a semantic extension of RDF is RDF Schema [RDFVOCABULARY], the semantics of which are defined in later parts of this document. RDF Schema imposes no extra syntactic restrictions.
Any semantic theory must be attached to a syntax. This semantics is defined as a mapping on the abstract syntax of RDF described in the RDF concepts and abstract syntax document [RDFCONCEPTS]. This document uses the following terminology defined there: URI reference, literal, plain literal, typed literal, XML literal, XML value, node, blank node, triple and RDF graph. Throughout this document we use the term 'character string' or 'string' to refer to a sequence of Unicode characters, and 'language tag' in the sense of RFC 3066, c.f. section 6.5 in [RDFCONCEPTS]. Note that strings in an RDF graph SHOULD be in Normal Form C.
This document uses the NTriples syntax described
in the RDF test cases document [RDFTESTS]
to describe RDF graphs. This notation
uses a node identifier (nodeID)
convention to indicate blank nodes in the triples of a graph. While node identifiers such as '_:xxx
'
serve to identify blank nodes in the surface syntax, these expressions are not
considered to be the label of the graph node they identify; they are not names,
and do not occur in the actual graph. In particular, the RDF
graphs described by two NTriples documents
which differ only by renaming their node identifiers will be understood to
be equivalent
. This renaming convention should be
understood as applying only to whole documents, since renaming the node identifiers
in part of a document may result in a document describing a different RDF
graph.
The NTriples syntax requires that URI references be given in full,
enclosed in angle brackets. In the interests of brevity, the
imaginary URI scheme 'ex:' is used to provide illustrative examples. To
obtain a more realistic view of the normal appearance of the
NTriples syntax, the reader should imagine this replaced with
something like 'http://www.example.org/rdf/mt/artificialexample/
'.
The QName prefixes
rdf:
, rdfs:
and xsd:
are defined
as follows:
Prefix rdf:
namespace URI:
http://www.w3.org/1999/02/22rdfsyntaxns#
Prefix rdfs:
namespace URI:
http://www.w3.org/2000/01/rdfschema#
Prefix xsd:
namespace URI:
http://www.w3.org/2001/XMLSchema#
Since QName syntax is not legal NTriples syntax, and in the interests of brevity and readability, examples use the convention whereby a QName is used without surrounding angle brackets to indicate the corresponding URI reference enclosed in angle brackets, e.g. the triple
<ex:a> rdf:type rdfs:Class .
should be read as an abbreviation for the NTriples syntax
<ex:a>
<http://www.w3.org/1999/02/22rdfsyntaxns#type>
<http://www.w3.org/2000/01/rdfschema#Class> .
In stating general semantic conditions, single characters or character sequences without a colon indicate an arbitrary name, blank node, character string and so on. The exact meaning will be specified in context.
An RDF graph, or simply a graph, is a set of RDF triples.
A subgraph of an RDF graph is a subset of the triples in the graph. A triple is identified with the singleton set containing it, so that each triple in a graph is considered to be a subgraph. A proper subgraph is a proper subset of the triples in the graph.
A ground RDF graph is one with no blank nodes.
A name is a URI reference or a typed literal. A name is from a vocabulary V if it is a URI reference in V or a typed literal containing an internal type URI reference in V. The names of a graph are all the names which occur in the graph. These are the expressions that need to be assigned a meaning by an interpretation. Plain literals are not classified as names because their interpretation is fixed. Note that a typed literal comprises two names: itself and its internal type URI reference.
A set of names is referred to as a vocabulary. The vocabulary of a graph is the set of names of the graph.
Suppose that M is a mapping from a set of blank nodes to some set of literals, blank nodes and URI references; then any graph obtained from a graph G by replacing some or all of the blank nodes N in G by M(N) is an instance of G. Note that any graph is an instance of itself, an instance of an instance of G is an instance of G, and if H is an instance of G then every triple in H is an instance of some triple in G.
An instance with respect to a vocabulary V is an instance in which all the names in the instance that were substituted for blank nodes in the original are names from V.
A proper instance of a graph is an instance in which a blank node has been replaced by a name or a plain literal, or two blank nodes in the graph have been mapped into the same node in the instance.
Any instance of a graph in which a blank node is mapped to a new blank node not in the original graph is an instance of the original and also has it as an instance, and this process can be iterated so that any 1:1 mapping between blank nodes defines an instance of a graph which has the original graph as an instance. Two such graphs, each an instance of the other but neither a proper instance, which differ only in the identity of their blank nodes, are considered to be equivalent. We will treat such equivalent graphs as identical; this allows us to ignore some issues which arise from 'renaming' nodeIDs, and is in conformance with the convention that blank nodes have no label. Equivalent graphs are mutual instances with an invertible instance mapping.
An RDF graph is lean if it has no instance which is a proper subgraph of the graph. Nonlean graphs have internal redundancy and express the same content as their lean subgraphs. For example, the graph
<ex:a> <ex:p> _:x .
_:y <ex:p> _:x .
is not lean, but
<ex:a> <ex:p> _:x .
_:x <ex:p> _:x .
is lean.
A merge of a set of RDF graphs is defined as follows. If the graphs in the set have no blank nodes in common, then the union of the graphs is a merge; if they do share blank nodes, then it is the union of a set of graphs that is obtained by replacing the graphs in the set by equivalent graphs that share no blank nodes. This is often described by saying that the blank nodes have been 'standardized apart'. It is easy to see that any two merges are equivalent, so we will refer to the merge, following the convention on equivalent graphs. Using the convention on equivalent graphs and identity, any graph in the original set is considered to be a subgraph of the merge.
One does not, in general, obtain the merge of a set of graphs by concatenating their corresponding NTriples documents and constructing the graph described by the merged document. If some of the documents use the same node identifiers, the merged document will describe a graph in which some of the blank nodes have been 'accidentally' identified. To merge NTriples documents it is necessary to check if the same nodeID is used in two or more documents, and to replace it with a distinct nodeID in each of them, before merging the documents. Similar cautions apply to merging graphs described by RDF/XML documents which contain nodeIDs, see RDF/XML Syntax Specification (Revised) [RDFSYNTAX].
RDF does not impose any logical restrictions on the domains and ranges of properties; in particular, a property may be applied to itself. When classes are introduced in RDFS, they may contain themselves. Such 'membership loops' might seem to violate the axiom of foundation, one of the axioms of standard (ZermeloFraenkel) set theory, which forbids infinitely descending chains of membership. However, the semantic model given here distinguishes properties and classes considered as objects from their extensions  the sets of objectvalue pairs which satisfy the property, or things that are 'in' the class  thereby allowing the extension of a property or class to contain the property or class itself without violating the axiom of foundation. In particular, this use of a class extension mapping allows classes to contain themselves. For example, it is quite OK for (the extension of) a 'universal' class to contain the class itself as a member, a convention that is often adopted at the top of a classification hierarchy. (If an extension contained itself then the axiom would be violated, but that case never arises.) The technique is described more fully in [Hayes&Menzel].
In this respect, RDFS differs from many conventional ontology frameworks such as UML which assume a more structured hierarchy of individuals, sets of individuals, etc., or which draw a sharp distinction between data and metadata. However, while RDFS does not assume the existence of such structure, it does not prohibit it. RDF allows membership loops, but it does not mandate their use for all parts of a user vocabulary. If this aspect of RDFS is found worrying, then it is possible to restrict oneself to a subset of RDF graphs which do not contain any such 'loops' of class membership or property application while retaining much of the expressive power of RDFS for many practical purposes, and semantic extensions may impose syntactic conditions which forbid such looped constructions.
The use of the explicit extension mapping also makes it possible for two properties to have exactly the same values, or two classes to contain the same instances, and still be distinct entities. This means that RDFS classes can be considered to be rather more than simple sets; they can be thought of as 'classifications' or 'concepts' which have a robust notion of identity which goes beyond a simple extensional correspondence. This property of the model theory has significant consequences in more expressive languages built on top of RDF, such as OWL [OWL], which are capable of expressing identity between properties and classes directly. This 'intensional' nature of classes and properties is sometimes claimed to be a useful property of a descriptive language, but a full discussion of this issue is beyond the scope of this document.
Notice that the question of whether or not a class contains itself as a member is quite different from the question of whether or not it is a subclass of itself. All classes are subclasses of themselves.
This document does not take any position on the way that URI references may be composed from other expressions, e.g. from relative URIs or QNames; the semantics simply assumes that such lexical issues have been resolved in some way that is globally coherent, so that a single URI reference can be taken to have the same meaning wherever it occurs. Similarly, the semantics has no special provision for tracking temporal changes. It assumes, implicitly, that URI references have the same meaning whenever they occur. To provide an adequate semantics which would be sensitive to temporal changes is a research problem which is beyond the scope of this document.
The semantics does not assume any particular relationship between the denotation of a URI reference and a document or Web resource which can be retrieved by using that URI reference in an HTTP transfer protocol, or any entity which is considered to be the source of such documents. Such a requirement could be added as a semantic extension, but the formal semantics described here makes no assumptions about any connection between the denotations of URI references and the uses of those URI references in other protocols.
The semantics treats all RDF names as expressions which denote. The things denoted are called 'resources', following [RFC 2396], but no assumptions are made here about the nature of resources; 'resource' is treated here as synonymous with 'entity', i.e. as a generic term for anything in the universe of discourse.
The different syntactic forms of names
are treated in particular ways. URI references are treated simply as logical
constants. Plain literals are considered to denote themselves, so have a fixed
meaning. The denotation of a typed literal is the value mapped from its enclosed
character string by the datatype associated with its enclosed type. RDF assigns
a particular meaning to literals typed with rdf:XMLLiteral
, described
in section 3.
The basic intuition of modeltheoretic semantics is that asserting a sentence makes a claim about the world: it is another way of saying that the world is, in fact, so arranged as to be an interpretation which makes the sentence true. In other words, an assertion amounts to stating a constraint on the possible ways the world might be. Notice that there is no presumption here that any assertion contains enough information to specify a single unique interpretation. It is usually impossible to assert enough in any language to completely constrain the interpretations to a single possible world, so there is no such thing as 'the' unique interpretation of an RDF graph. In general, the larger an RDF graph is  the more it says about the world  then the smaller the set of interpretations that an assertion of the graph allows to be true  the fewer the ways the world could be, while making the asserted graph true of it.
The following definition of an interpretation is couched in mathematical language, but what it amounts to intuitively is that an interpretation provides just enough information about a possible way the world might be  a 'possible world'  in order to fix the truthvalue (true or false) of any ground RDF triple. It does this by specifying for each URI reference, what it is supposed to be a name of; and also, if it is used to indicate a property, what values that property has for each thing in the universe; and if it is used to indicate a datatype, that the datatype defines a mapping between lexical forms and datatype values. This is just enough information to fix the truthvalue of any ground triple, and hence any ground RDF graph. (Nonground graphs are considered in the following section.) Note that if any of this information were omitted, it would be possible for some wellformed triple to be left without a determinate value; and also that any other information  such as the exact nature of the things in the universe  would, regardless of its intrinsic interest, be irrelevant to the actual truthvalues of any triple.
All interpretations will be relative to a set of names, called the vocabulary of the interpretation; so that one should speak, strictly, of an interpretation of an RDF vocabulary, rather than of RDF itself. Some interpretations may assign special meanings to the symbols in a particular vocabulary. Interpretations which share the special meaning of a particular vocabulary will be named for that vocabulary, e.g. 'rdfinterpretations', 'rdfsinterpretations', etc. An interpretation with no particular extra conditions on a vocabulary (including the RDF vocabulary itself) will be called a simple interpretation, or simply an interpretation.
RDF uses several forms of literal. The chief semantic characteristic of literals is that their meaning is largely determined by the form of the string they contain. Plain literals, without an embedded type URI reference, are always interpreted as referring to themselves: either a character string or a pair consisting of a character string and a language tag; in either case, the character string is referred to as the "literal character string". In the case of typed literals, however, the full specification of the meaning depends on being able to access datatype information which is external to RDF itself. A full discussion of the meaning of typed literals is described in section 5 , where a special notion of datatype interpretation is introduced. Each interpretation defines a mapping IL from typed literals to their interpretations. Stronger conditions on IL will be defined as the notion of 'interpretation' is extended in later sections.
Throughout this document, precise semantic conditions will be set out in tables which state semantic conditions, tables containing true assertions and valid inference rules, and tables listing syntax, which are distinguished by background color. These tables, taken together, amount to a formal summary of the entire semantics. Note that the semantics of RDF does not depend on that of RDFS. The full semantics of RDF is defined in sections 1 and 3 ; the full semantics of RDFS in sections 1, 3 and 4.
A simple interpretation I of a vocabulary V is defined by: 1. A nonempty set IR of resources, called the domain or universe of I. 2. A set IP, called the set of properties of I. 3. A mapping IEXT from IP into the powerset of IR x IR i.e. the set of sets of pairs <x,y> with x and y in IR . 4. A mapping IS from URI references in V into (IR union IP) 5. A mapping IL from typed literals in V into IR. 6. A distinguished subset LV of IR, called the set of literal values, which contains at least all character strings and all pairs consisting of a character string and a language tag. 
IEXT(x), called the extension of x, is a set of pairs which identify the arguments for which the property is true, that is, a binary relational extension. This trick of distinguishing a relation as an object from its relational extension allows a property to occur in its own extension, as noted earlier.
The assumption that LV is a subset of IR amounts to saying that literal values are thought of as real entities that 'exist'. This amounts to saying that literal values are resources. However, this does not imply that literals should be identified with URI references. Note that LV may contain other items in addition to those listed. There is a technical reason why the range of IL is IR rather than restricted to LV. When interpretations take account of datatype information, it is syntactically possible for a typed literal to be internally inconsistent, and such illtyped literals are required to denote a nonliteral value, as explained in section 5.
The next sections define how an interpretation of a vocabulary determines the truthvalues of any RDF graph, by a recursive definition of the denotation  the semantic "value"  of any RDF expression in terms of those of its immediate subexpressions. These apply to all subsequent semantic extensions. RDF has two kinds of denotation: names denote things in the universe, and sets of triples denote truthvalues.
The denotation of a ground RDF graph in I is given recursively by the following rules, which extend the interpretation mapping I from names to ground graphs. These rules (and extensions of them given later) work by defining the denotation of any piece of RDF syntax E in terms of the denotations of the immediate syntactic constituents of E, hence allowing the denotation of any piece of RDF to be determined by a kind of syntactic recursion.
In this table, and throughout this document, the equality sign "=" indicates identity.
if E is a plain literal "aaa" then I(E) = aaa 
if E is a plain literal "aaa"@ttt then I(E) = <aaa, ttt> 
if E is a typed literal from V then I(E) = IL(E) 
if E is a URI reference in V then I(E) = IS(E) 
if E is a ground triple s p o s, p and o are in V, I(p) is in IP and <I(s),I(o)> is in IEXT(I(p)) otherwise I(E)= false. 
if E is a ground RDF graph then I(E) = false if I(E') = false for some triple E' in E, otherwise I(E) =true. 
If the vocabulary of an RDF graph contains URI references that are not in the vocabulary of an interpretation I  that is, if I simply does not give a semantic value to some name that is used in the graph  then these truthconditions will always yield the value false for some triple in the graph, and hence for the graph itself. Turned around, this means that any assertion of a graph implicitly asserts that all the names in the graph actually refer to something in the world. The final condition implies that an empty graph (an empty set of triples) is trivially true.
Note that the denotation of plain literals is always in LV, and that those of the subject and object of any true triple must be in IR; so any URI reference which occurs in a graph both as a predicate and as a subject or object must denote something in the intersection of IR and IP in any interpretation which satisfies the graph.
As an illustrative example, the following is a small interpretation for the
artificial vocabulary {ex:a, ex:b, ex:c
} plus all typed literals
with one of these as the type URI. Integers are used to indicate the nonliteral
'things' in the universe. This is not meant to imply that interpretations should
be interpreted as being about arithmetic, but more to emphasize that the exact
nature of the things in the universe is irrelevant. LV
can be any set satisfying the semantic conditions. (In this and subsequent
examples the greaterthan and lessthan symbols are used in several ways: following
mathematical usage to indicate abstract pairs and ntuples; following NTriples
syntax to enclose URI references, and also as arrowheads when indicating mappings.)
IR = LV union{1, 2}
IP={1}
IEXT: 1=>
{<1,2>,<2,1>}
IS: ex:a=>
1, ex:b=>
1,
ex:c=>
2
IL: any typed literal =>
2
Figure 1: An example of an interpretation. Note, this is
not a picture of an RDF graph.
The figure does not show the infinite number of members of
LV.
This interpretation makes these triples true:
<ex:a> <ex:b> <ex:c>
.
<ex:c> <ex:a> <ex:a>
.
<ex:c> <ex:b> <ex:a>
.
<ex:a> <ex:b>
"whatever"^^<ex:b> .
For example, I(<ex:a> <ex:b> <ex:c>
.
) = true if
<I(ex:a
),I(ex:c
)> is in
IEXT(I(<ex:b>
)), i.e. if <1,2> is in
IEXT(1), which is {<1,2>,<2,1>} and so does contain
<1,2> and so I(<ex:a <ex:b> ex:c>
)
is true.
The truth of the fourth triple is a consequence of the rather idiosyncratic interpretation chosen here for typed literals.
In this interpretation IP is a subset of IR; this will be typical of RDF semantic interpretations, but is not required.
It makes these triples false:
<ex:a> <ex:c> <ex:b>
.
<ex:a> <ex:b> <ex:b>
.
<ex:c> <ex:a> <ex:c>
.
<ex:a> <ex:b> "whatever"
.
For example, I(<ex:a> <ex:c> <ex:b> .
) = true
if <I(ex:a
), I(<ex:b>
)>, i.e.<1,1>,
is in IEXT(I(ex:c
)); but I(ex:c
)=2 which is not in
IP, so IEXT is not defined on 2, so the condition fails and I(<ex:a>
<ex:c> <ex:b> .
) = false.
It also makes all triples containing a plain literal false, since the property extension does not have any pairs containing a plain literal.
To emphasize; this is only one possible interpretation of this vocabulary; there are (infinitely) many others. For example, if this interpretation were modified by attaching the property extension to 2 instead of 1, none of the above triples would be true.
This example illustrates that any interpretation which maps any URI reference which occurs in the predicate position of a triple in a graph to something not in IP will make the graph false.
Blank nodes are treated as simply indicating the existence of a thing, without using, or saying anything about, the name of that thing. (This is not the same as assuming that the blank node indicates an 'unknown' URI reference; for example, it does not assume that there is any URI reference which refers to the thing. The discussion of Skolemization in appendix A is relevant to this point.)
An interpretation can specify the truthvalue of a graph containing blank nodes. This will require some definitions, as the theory so far provides no meaning for blank nodes. Suppose I is an interpretation and A is a mapping from some set of blank nodes to the universe IR of I, and define I+A to be an extended interpretation which is like I except that it uses A to give the interpretation of blank nodes. Define blank(E) to be the set of blank nodes in E. Then the above rules can be extended to include the two new cases that are introduced when blank nodes occur in the graph:
If E is a blank node then [I+A](E) = A(E) 
If E is an RDF graph then I(E) = true if [I+A'](E) = true for some mapping A' from blank(E) to IR, otherwise I(E)= false. 
Notice that this does not change the definition of an interpretation; it still consists of the same values IR, IP, IEXT, IS, LV and IL. It simply extends the rules for defining denotations under an interpretation, so that the same interpretation that provides a truthvalue for ground graphs also assigns truthvalues to graphs with blank nodes, even though it provides no denotation for the blank nodes themselves. Notice also that the blank nodes themselves are perfectly welldefined entities; they differ from other nodes only in not being assigned a denotation by an interpretation, reflecting the intuition that they have no 'global' meaning (i.e. outside the graph in which they occur).
For example, the graph defined by the following triples is false in the interpretation shown in figure 1:
_:xxx <ex:a> <ex:b> .
<ex:c> <ex:b> _:xxx .
since if A' maps the blank node to 1 then the first triple is false in I+A', and if it maps it to 2 then the second triple is false.
Note that each of these triples, if thought of as a single graph, would be true in I, but the whole graph is not; and that if a different nodeID were used in the two triples, indicating that the RDF graph had two blank nodes instead of one, then A' could map one node to 2 and the other to 1, and the resulting graph would be true under the interpretation I.
This effectively treats all blank nodes as having the same meaning as existentially quantified variables in the RDF graph in which they occur, and which have the scope of the entire graph. In terms of the NTriples syntax, this amounts to the convention that would place the quantifiers just outside, or at the outer edge of, the NTriples document corresponding to the graph. This in turn means that there is a subtle but important distinction in meaning between the operation of forming the union of two graphs and that of forming the merge. The simple union of two graphs corresponds to the conjunction ( 'and' ) of all the triples in the graphs, maintaining the identity of any blank nodes which occur in both graphs. This is appropriate when the information in the graphs comes from a single source, or where one is derived from the other by means of some valid inference process, as for example when applying an inference rule to add a triple to a graph. Merging two graphs treats the blank nodes in each graph as being existentially quantified in that graph, so that no blank node from one graph is allowed to stray into the scope of the other graph's surrounding quantifier. This is appropriate when the graphs come from different sources and there is no justification for assuming that a blank node in one refers to the same entity as any blank node in the other.
Following conventional terminology, I satisfies E if I(E)=true, and a set S of RDF graphs (simply) entails a graph E if every interpretation of the vocabulary of (S union E) which satisfies every member of S also satisfies E. In later sections these notions will be adapted to other classes of interpretations, but throughout this section 'entailment' should be interpreted as meaning simple entailment.
Entailment is the key idea which connects modeltheoretic semantics to realworld applications. As noted earlier, making an assertion amounts to claiming that the world is an interpretation which assigns the value true to the assertion. If A entails B, then any interpretation that makes A true also makes B true, so that an assertion of A already contains the same "meaning" as an assertion of B; one could say that the meaning of B is somehow contained in, or subsumed by, that of A. If A and B entail each other, then they both "mean" the same thing, in the sense that asserting either of them makes the same claim about the world. The interest of this observation arises most vividly when A and B are different expressions, since then the relation of entailment is exactly the appropriate semantic license to justify an application inferring or generating one of them from the other. Through the notions of satisfaction, entailment and validity, formal semantics gives a rigorous definition to a notion of "meaning" that can be related directly to computable methods of determining whether or not meaning is preserved by some transformation on a representation of knowledge.
Any process which constructs a graph E from some other graph(s) S is said to be (simply) valid if S entails E in every case, otherwise invalid. Note that being an invalid process does not mean that the conclusion is false, and being valid does not guarantee truth. However, validity represents the best guarantee that any assertional language can offer: if given true inputs, it will never draw a false conclusion from them.
This section gives a few basic results about simple entailment and valid inference. Simple entailment can be recognized by relatively simple syntactic comparisons. The two basic forms of simply valid inference in RDF are, in logical terms, the inference from (P and Q) to P, and the inference from foo(baz) to (exists (?x) foo(?x)).
These results apply only to simple entailment, not to the extended notions of entailment introduced in later sections. Proofs, all of which are straightforward, are given in appendix A, which also describes some other properties of entailment which may be of interest.
Empty Graph Lemma. The empty set of triples is entailed by any graph, and does not entail any graph except itself. [Proof]
Subgraph Lemma. A graph entails all its subgraphs. [Proof]
Instance Lemma. A graph is entailed by any of its instances.[Proof]
The relationship between merging and entailment is simple, and obvious from the definitions:
Merging lemma. The merge of a set S of RDF graphs is entailed by S, and entails every member of S. [Proof]
This means that a set of graphs can be treated as equivalent to its merge, i.e. a single graph, as far as the model theory is concerned. This can be used to simplify the terminology somewhat: for example, the definition of S entails E, above, can be paraphrased by saying that S entails E when every interpretation which satisfies S also satisfies E.
The example given in section 1.5 shows that it is not the case, in general, that the simple union of a set of graphs is entailed by the set.
The main result for simple RDF inference is:
Interpolation Lemma. S entails a graph E if and only if a subgraph of S is an instance of E.[Proof]
The interpolation lemma completely characterizes simple RDF entailment in syntactic terms. To tell whether a set of RDF graphs entails another, check that there is some instance of the entailed graph which is a subset of the merge of the original set of graphs. Of course, there is no need to actually construct the merge. If working backwards from the consequent E, an efficient technique would be to treat blank nodes as variables in a process of subgraphmatching, allowing them to bind to 'matching' names in the antecedent graph(s) in S, i.e. those which may entail the consequent graph. The interpolation lemma shows that this process is valid, and is also complete if the subgraphmatching algorithm is. The existence of complete subgraphchecking algorithms also shows that RDF entailment is decidable, i.e. there is a terminating algorithm which will determine for any finite set S and any graph E, whether or not S entails E.
Such a variablebinding process would only be appropriate when applied to the conclusion of a proposed entailment. This corresponds to using the document as a goal or a query, in contrast to asserting it, i.e. claiming it to be true. If an RDF document is asserted, then it would be invalid to bind new values to any of its blank nodes, since the resulting graph might not be entailed by the assertion.
The interpolation lemma has an immediate consequence a criterion for nonentailment:
Anonymity lemma. Suppose E is a lean graph and E' is a proper instance of E. Then E does not entail E'.[Proof]
Note again that this applies only to simple entailment, not to the vocabulary entailment relationships defined in rest of the document.
Several basic properties of entailment follow directly from the above definitions and results but are stated here for completeness sake:
Monotonicity Lemma. Suppose S is a subgraph of S' and S entails E. Then S' entails E.[Proof]
The property of finite expressions always being derivable from a finite set of antecedents is called compactness. Semantic theories which support noncompact notions of entailment do not have corresponding computable inference systems.
Compactness Lemma. If S entails E and E is a finite graph, then some finite subset S' of S entails E.
Simple interpretations and simple entailment capture the semantics of RDF graphs when no attention is paid to the particular meaning of any of any of the names in the graph. To obtain the full meaning of an RDF graph written using a particular vocabulary, it is usually necessary to add further semantic conditions which attach stronger meanings to particular URI references and typed literals in the graph. Interpretations which are required to satisfy extra semantic conditions on a particular vocabulary will be generically referred to as vocabulary interpretations. Vocabulary entailment means entailment with respect to such vocabulary interpretations. These stronger notions of interpretation and entailment are indicated by the use of a namespace prefix, so that we will refer to rdfentailment, rdfsentailment and so on in what follows. In each case, the vocabulary whose meaning is being restricted, and the exact conditions associated with that vocabulary, are spelled out in detail.
The
RDF vocabulary, rdfV, is a set of URI references in the rdf:
namespace:
RDF vocabulary 
rdf:type rdf:Property
rdf:XMLLiteral rdf:nil rdf:List rdf:Statement rdf:subject rdf:predicate
rdf:object rdf:first rdf:rest rdf:Seq rdf:Bag rdf:Alt rdf:_1 rdf:_2
... rdf:value 
The subset of rdfV
consisting of the first 3 items in the above list, {rdf:type
rdf:Property rdf:XMLLiteral
} is called the
central RDF vocabulary, crdfV.
rdfinterpretations
impose extra semantic conditions on crdfV
and on typed literals with the type rdf:XMLLiteral
, which is
referred to as the RDF builtin datatype. This datatype is fully
described in the RDF
Concepts and Abstract Syntax document [RDFCONCEPTS].
Any character string sss which satisfies the conditions for being
in the
lexical space of rdf:XMLLiteral
will be called a welltyped
XML literal string. The corresponding value will be called the XML
value of the literal. Note that the XML values
of welltyped XML literals are in precise 1:1 correspondence with the XML
literal strings of such literals, but are not themselves character strings.An
XML literal whose literal string is welltyped will be called a welltyped
XML literal; other XML literals will be called illtyped.
An rdfinterpretation of a vocabulary V is a simple interpretation I of (V union crdfV) which satisfies the extra conditions described in the following table for all names in (V union crdfV), and all the triples in the subsequent table whose vocabulary is contained in (V union crdfV). These triples are called the rdf axiomatic triples.
The first condition could be regarded as defining IP to be the set of
resources in the universe of the interpretation which have the value I(rdf:Property
)
of the property I(rdf:type
). Such subsets of the universe will
be central in interpretations of RDFS. Note that this condition requires
IP to be a subset of IR. The third condition requires that illtyped XML literals
denote something other than a literal value: this will be the standard way of
handling illformed typed literals.
The rdfsinterpretations described in section 4 below assign further semantic conditions (range and domain conditions) to the properties used in the RDF vocabulary, and other semantic extensions MAY impose further conditions so as to further restrict their meanings, provided that such conditions MUST be compatible with the conditions described in this section.
For example, the following rdfinterpretation extends the simple interpretation in figure 1 to the case where V contains rdfV. For simplicity, we ignore XML literals in this example.
IR = LV union {1, 2, T , P}
IP = {1, T}
IEXT: 1=>
{<1,2>,<2,1>},
T=>
{<1,P>,<T,P>}
IS: ex:a=>
1, ex:b=>
1,
ex:c=>
2, rdf:type=>
T,
rdf:Property=>
P, rdf:nil=>
1,
rdf:List=>
P, rdf:Statement=>
P,
rdf:subject=>
1, rdf:predicate=>
1, rdf:object=>
1,
rdf:first=>
1, rdf:rest=>
1, rdf:Seq=>
P,
rdf:Bag=>
P, rdf:Alt=>
P, rdf:_1, rdf:_2, ...
=>
1
Figure 2: An rdfinterpretation.
This is not the smallest rdfinterpretation which extends the earlier example, since one could have made IEXT(T) be {<1,2>,<T,2>}, and managed without having P in the universe. In general, a given entity in an interpretation may play several 'roles' at the same time, as long as this can be done without violating any of the required semantic conditions. The above interpretation identifies properties with lists, for example; of course, other interpretations might not make such an identification.
Every rdfinterpretation is also a simple interpretation. The 'extra' structure does not prevent it acting in the simpler role.
S rdfentails E when every rdfinterpretation of the vocabulary of S union E which satisfies every member of S also satisfies E. This follows the wording of the definition of simple entailment in Section 2, but refers only to rdfinterpretations instead of all simple interpretations. Rdfentailment is an example of vocabulary entailment.
It is easy to see that the lemmas in Section 2 do not all apply to rdfentailment: for example, the triple
rdf:type rdf:type rdf:Property .
is true in every rdfinterpretation, so is rdfentailed by the empty graph, contradicting the interpolation lemma for rdfentailment. Section 7.2 describes exact conditions for detecting RDF entailment.
The RDF semantic conditions impose significant formal constraints on the meaning only of the central RDF vocabulary, so the notions of rdfentailment and rdfinterpretation apply to the remainder of the vocabulary without further change. This includes vocabulary which is intended for use in describing containers and bounded collections, and a reification vocabulary to enable an RDF graph to describe, as well as exhibit, triples. In this section we review the intended meanings of this vocabulary, and note some intuitive consequences which are not supported by the formal model theory. Semantic extensions MAY limit the formal interpretations of these vocabularies to conform to these intended meanings.
The omission of these conditions from the formal semantics is a design decision to accomodate variations in existing RDF usage and to make it easier to implement processes to check formal RDF entailment. For example, implementations may decide to use special procedural techniques to implement the RDF collection vocabulary.
RDF reification vocabulary 
rdf:Statement rdf:subject rdf:predicate
rdf:object 
Semantic extensions MAY limit the interpretation of these so that a triple of the form
aaa rdf:type rdf:Statement .
is true in I just when I(aaa) is a token of an RDF triple in some RDF document, and the three properties, when applied to such a denoted triple, have the same values as the respective components of that triple.
This may be illustrated by considering the following two RDF graphs, the first of which consists of a single triple.
<ex:a> <ex:b> <ex:c> .
and
_:xxx rdf:type rdf:Statement .
_:xxx rdf:subject <ex:a> .
_:xxx rdf:predicate <ex:b> .
_:xxx rdf:object <ex:c> .
The second graph is called a reification of the triple in the first
graph, and the node which is intended to refer to the first triple
 the blank node in the second graph  is called, rather
confusingly, a reified triple. (This can be a blank node
or a URI reference.) In the intended interpretation of the reification
vocabulary, the second graph would be made true in an
interpretation I by interpreting the reified triple to refer to a
token of the triple in the first graph in some concrete RDF
document, considering that token to be valid RDF syntax, and then
using I to interpret the syntactic triple which the token
instantiates, so that the subject, predicate and object of that
triple are interpreted in the same way in the reification as in the
triple described by the reification. This could be stated formally
as follows: <x,y> is in IEXT(I(rdf:subject
))
just when x is a token of an RDF triple of the form
aaa bbb ccc .
and y is I(aaa); similarly for predicate and object. Notice that
the value of the rdf:subject
property is not the
subject URI reference itself but its interpretation, and so this condition
involves a twostage interpretation process: one has to interpret
the reified node  the subject of the triples in the reification 
to refer to another triple, then treat that triple as RDF syntax
and apply the interpretation mapping again to get to the referent
of its subject. This requires triple tokens to exist as firstclass
entities in the universe IR of an interpretation. In sum: the
meaning of the reification is that a document exists containing a
triple token which means whatever the first graph means.Note
that this way of understanding the reification vocabulary does not interpret
reification as a form of quotation. Rather, the reification describes the
relationship between a token of a triple and the resources that triple refers
to. The reification can be read intuitively as saying "'this piece of
RDF talks about these things" rather than "this piece of RDF has
this form".
The semantic extension described here requires
the reified triple that the reification describes 
I(_:xxx
) in the above example  to be a particular token
or instance of a triple in a (real
or notional) RDF document, rather than an 'abstract' triple
considered as a grammatical form. There could be several such
entities which have the same subject, predicate and object
properties. Although a graph is defined as a set of triples,
several such tokens with the same triple structure might occur in
different documents. Thus, it would be meaningful to claim that the
blank node in the second graph above does not refer to the triple
in the first graph, but to some other triple with the same
structure. This particular interpretation of reification was chosen
on the basis of use cases where properties such as dates of
composition or provenance information have been applied to the
reified triple, which are meaningful only when thought of as
referring to a particular instance or token of a triple.
Although RDF applications may use reification to refer to triple tokens in RDF documents, the connection between the document and its reification must be maintained by some means external to the RDF graph syntax. (In the RDF/XML syntax described in RDF/XML Syntax Specification (Revised) [RDFSYNTAX], the rdf:ID attribute can be used in the description of a triple to create a reification of that triple in which the reified triple is a URI constructed from the baseURI of the XML document and the value of rdf:ID as a fragment.) Since an assertion of a reification of a triple does not implicitly assert the triple itself, this means that there are no entailment relationships which hold between a triple and a reification of it. Thus the reification vocabulary has no effective semantic constraints on it, other than those that apply to an rdfinterpretation.
A reification of a triple does not entail the triple, and is not entailed by it. (The reification only says that the triple token exists and what it is about, not that it is true. The second nonentailment is a consequence of the fact that asserting a triple does not automatically assert that any triple tokens exist in the universe being described by the triple. For example, the triple might be part of an ontology describing animals, which could be satisfied by an interpretation in which the universe contained only animals, and in which a reification of it was therefore false.)
Since the relation between triples and reifications of triples in any RDF graph or graphs need not be onetoone, asserting a property about some entity described by a reification need not entail that the same property holds of another such entity, even if it has the same components. For example,
_:xxx rdf:type rdf:Statement .
_:xxx rdf:subject <ex:subject> .
_:xxx rdf:predicate <ex:predicate> .
_:xxx rdf:object <ex:object> .
_:yyy rdf:type rdf:Statement .
_:yyy rdf:subject <ex:subject> .
_:yyy rdf:predicate <ex:predicate> .
_:yyy rdf:object <ex:object> .
_:xxx <ex:property> <ex:foo> .
does not entail
_:yyy <ex:property> <ex:foo> .
RDF Container Vocabulary 
rdf:Seq rdf:Bag rdf:Alt rdf:_1 rdf:_2
... 
RDF provides vocabularies for describing three classes of containers. Containers have a type, and their members can be enumerated by using a fixed set of container membership properties. These properties are indexed by integers to provide a way to distinguish the members from each other, but these indices should not necessarily be thought of as defining an ordering of the container itself; some containers are considered to be unordered.
The RDFS vocabulary, described below, adds a generic membership property which holds regardless of position, and classes containing all the containers and all the membership properties.
One should understand this RDF vocabulary as describing containers, rather than as a vocabulary for constructing them, as would typically be supplied by a programming language. On this view, the actual containers are entities in the semantic universe, and RDF graphs which use the vocabulary simply provide very basic information about these entities, enabling an RDF graph to characterize the container type and give partial information about the members of a container. Since the RDF container vocabulary is so limited, many 'natural' assumptions concerning RDF containers are not formally sanctioned by the RDF model theory. This should not be taken as meaning that these assumptions are false, but only that RDF does not formally entail that they must be true.
There are no special semantic conditions on the container
vocabulary: the only 'structure' which RDF presumes its containers
to have is what can be inferred from the use of this vocabulary and
the general RDF semantic conditions. In
general, this amounts to knowing the type of a container, and having a partial
enumeration
of the items in the container. The intended mode of use is that things
of type rdf:Bag
are considered to be unordered but to allow duplicates; things of
type rdf:Seq
are considered to be ordered, and things
of type rdf:Alt
are considered to represent a
collection of alternatives, possibly with a preference ordering.
The ordering of items in an ordered container is intended to be
indicated by the numerical ordering of the container membership
properties, which are assumed to be singlevalued.
However, these informal interpretations are not reflected in any formal RDF
entailments.
RDF does not support any entailments which could arise from enumerating
the elements of an rdf:Bag
in a different order. For example,
_:xxx rdf:type rdf:Bag .
_:xxx rdf:_1 <ex:a> .
_:xxx rdf:_2 <ex:b> .
does not entail
_:xxx rdf:_1 <ex:b> .
_:xxx rdf:_2 <ex:a> .
Notice that if this conclusion were valid, then the result of conjoining it to the original graph would also be a valid entailment, which would assert that both elements were in both positions. This is a consequence of the fact that RDF is a purely assertional language.
There is no assumption that a property of a container applies to any of the elements of the container, or vice versa.
There is no formal requirement that
the three container classes are disjoint, so that for example
something can be asserted to be both an rdf:Bag
and an rdf:Seq
.
There is no assumption that containers are gapfree, so that for example
_:xxx rdf:type rdf:Seq.
_:xxx rdf:_1 <ex:a> .
_:xxx rdf:_3 <ex:c> .
does not entail
_:xxx rdf:_2 _:yyy .
There is no way in RDF to 'close' a container, i.e. to assert that it contains only a fixed number of members. This is a reflection of the fact that it is always consistent to add a triple to a graph asserting a membership property of any container. And finally, there is no builtin assumption that an RDF container has only finitely many members.
RDF Collection Vocabulary 
rdf:List rdf:first rdf:rest rdf:nil 
RDF provides a vocabulary for describing collections, i.e.'list structures', in terms of headtail links. Collections differ from containers in allowing branching structure and in having an explicit terminator, allowing applications to determine the exact set of items in the collection.
As with containers, no special semantic conditions are imposed on this vocabulary
other than the type of rdf:nil
being rdf:List
. It
is intended for use typically in a context where a container is described using
blank nodes to connect a 'wellformed' sequence of items, each described by
two triples of the form
_:c1 rdf:first aaa .
_:c1 rdf:rest _:c2
where the final item is indicated by the use of rdf:nil
as the
value of the property rdf:rest
. In a familiar convention, rdf:nil
can be thought of as the empty collection. Any such graph amounts to an assertion
that the collection exists, and since the members of the collection can be determined
by inspection, this is often sufficient to enable applications to determine
what is meant. Note however that the semantics does not require any collections
to exist other than those mentioned explicitly in a graph (and the empty collection).
For example, the existence of a collection containing two items does not automatically
guarantee that the similar collection with the items permuted also exists:
_:c1 rdf:first <ex:aaa> .
_:c1 rdf:rest _:c2 .
_:c2 rdf:first <ex:bbb> .
_:c2 rdf:rest rdf:nil .
does not entail
_:c3 rdf:first <ex:bbb> .
_:c3 rdf:rest _:c4 .
_:c4 rdf:first <ex:aaa> .
_:c4 rdf:rest rdf:nil .
Also, RDF imposes no 'wellformedness' conditions on the use of this vocabulary, so that it is possible to write RDF graphs which assert the existence of highly peculiar objects such as lists with forked or nonlist tails, or multiple heads:
_:666 rdf:first <ex:aaa> .
_:666 rdf:first <ex:bbb> .
_:666 rdf:rest <ex:ccc> .
_:666 rdf:rest rdf:nil .
It is also possible to write a set of triples which underspecify a collection
by failing to specify its rdf:rest
property value.
Semantic extensions MAY
place extra syntactic wellformedness restrictions on the use of this vocabulary
in order to rule out such graphs. They MAY
exclude interpretations of the collection vocabulary which violate the convention
that the subject of a 'linked' collection of twotriple items of the form described
above, ending with an item ending with rdf:nil
, denotes a totally
ordered sequence whose members are the denotations of the rdf:first
values of the items, in the order got by tracing the rdf:rest
properties
from the subject to rdf:nil
. This permits sequences which contain
other sequences.
Note that the RDFS semantic conditions, described below, require that any
subject of the rdf:first
property, and any subject or object of
the rdf:rest
property, be of rdf:type rdf:List
.
The intended use for rdf:value
is explained
intuitively in the RDF Primer
document [RDFPRIMER]. It is typically
used to identify a 'primary' or 'main' value of a property which has several
values, or has as its value a complex entity with several facets or properties
of its own.
Since the range of possible uses for rdf:value
is so wide, it
is difficult to give a precise statement which covers all the intended meanings
or use cases. Users are cautioned, therefore, that the
meaning of rdf:value
may vary from application to application.
In practice, the intended meaning is often clear from the context, but may be
lost when graphs are merged or when conclusions are inferred.
RDF Schema [RDFVOCABULARY] extends RDF to include a larger vocabulary rdfsV with more complex semantic constraints:
RDFS vocabulary 
rdfs:domain rdfs:range rdfs:Resource rdfs:Literal
rdfs:Datatype rdfs:Class rdfs:subClassOf rdfs:subPropertyOf
rdfs:member rdfs:Container rdfs:ContainerMembershipProperty
rdfs:comment rdfs:seeAlso rdfs:isDefinedBy
rdfs:label 
(rdfs:comment
, rdfs:seeAlso
, rdfs:isDefinedBy
and rdfs:label
are included here because some constraints which
apply to their use can be stated using rdfs:domain
, rdfs:range
and rdfs:subPropertyOf
. Other than this, the formal semantics does
not assign them any particular meanings.)
Although not strictly necessary, it is convenient to state the RDFS semantics
in terms of a new semantic construct, a 'class', i.e. a resource which represents
a set of things in the universe which all have that class as the value of their
rdf:type
property. Classes are defined to be things of type rdfs:Class
,
and the set of all classes in an interpretation will be called IC.
The semantic conditions are stated in terms of a mapping ICEXT (for the Class
Extension in I) from IC to the set of subsets of IR. The meanings of
ICEXT and IC in a rdfinterpretation
of the RDFS vocabulary are completely defined by the first two conditions in
the table of RDFS semantic condiions, below. Notice that a class may have an
empty class extension; that (as noted
earlier) two different class entities could have the same class extension; and
that the class extension of rdfs:Class
contains the class rdfs:Class
.
An rdfsinterpretation of V is an rdfinterpretation I of (V union crdfV union rdfsV) which satisfies the following semantic conditions and all the triples in the subsequent table, called the RDFS axiomatic triples, which contain only names from (V union crdfV union rdfsV).
Since I is an rdfinterpretation, the first condition implies that IP
= ICEXT(I(rdf:Property
)).
These axioms and conditions have some redundancy: for example, all but one
of the RDF axiomatic triples can be derived from the RDFS axiomatic triples
and the semantic conditions on ICEXT, rdfs:domain
and rdfs:range
.
Other triples which must be true in all rdfsinterpretations
include the following:
rdfs:Resource rdf:type rdfs:Class .

Note that datatypes are allowed to
have class extensions, i.e. are considered to be classes, in RDFS. As illustrated
by the semantic condition on the class extension of rdf:XMLLiteral
,
the members of a datatype class are the values of the datatype.
This is explained in more detail in section 5 below.
The class rdfs:Literal
contains all literal values; however,
typed literals whose strings do not conform to the lexical requirements of their
datatype are required to have meanings
not in this class. The semantic conditions on rdfinterpretations
imply that ICEXT(I(rdf:XMLLiteral
)) contains all XML values of
welltyped XML literals.
The conditions on rdf:XMLLiteral
and rdfs:range
taken together make it possible to write a contradictory statement in RDFS,
by asserting that a property value must be in the class rdf:XMLLiteral
but applying this property with a value which is an illformed XML literal,
and therefore required to not be in that class: for example
<ex:a> <ex:p> "<notLegalXML"^^rdf:XMLLiteral
.
<ex:p> rdfs:range rdf:XMLLiteral .
cannot be true in any rdfsinterpretation; it is rdfsinconsistent.
The semantics given above is deliberately chosen to be the weakest 'reasonable' interpretation of the RDFS vocabulary. Semantic extensions MAY strengthen the range, domain, subclass and subproperty semantic conditions to the following 'extensional' versions:
<x,y> is in IEXT(I( 
<x,y> is in IEXT(I( 
<x,y> is in IEXT(I( 
<x,y> is in IEXT(I( 
which would guarantee that the subproperty and subclass properties were transitive and reflexive, but would also have further consequences.
These stronger conditions would be trivially satisfied when properties are
identified with property extensions, classes with class extensions, and rdfs:SubClassOf
understood to mean subset, and hence would be satisfied by an extensional semantics
for RDFS. In some ways the extensional versions provide a simpler semantics,
but they require more complex inference rules. The 'intensional' semantics described
in the main text provides for most common uses of subclass and subproperty assertions,
and allows for simpler implementations of a
complete set of RDFS entailment rules, described in section 7.3.
Although the semantic conditions on rdfsinterpretations include the intuitively
sensible condition that ICEXT(I(rdfs:Literal
)) must be the set
LV, there is no way to impose this condition by any RDF assertion or inference
rule. This limitation is due to the fact that RDF does not allow literals to
occur in the subject position of a triple, so there are severe restrictions
on what can be said about literals in RDF. Similarly, while properties
may be asserted of the class rdfs:Literal
, none of these can be
validly transferred to literals themselves.
For example, a triple of the form
<ex:a> rdf:type rdfs:Literal .
is consistent even though 'ex:a
' is a URI reference rather
than a literal. What it says is that I(ex:a
) is a
literal value, ie that the URI reference 'ex:a
'
denotes a literal value. It does not specify exactly which
literal value it denotes.
The semantic conditions guarantee that any triple containing a simple literal object entails a similar triple with a blank node as object:
<ex:a> <ex:b> "10"
.
entails
<ex:a> <ex:b> _:xxx .
This means that the literal denotes an entity, which could therefore also be named, at least in principle, by a URI reference.
S rdfsentails E when every rdfsinterpretation of the vocabulary of S union E which satisfies every member of S also satisfies E. This follows the wording of the definition of simple entailment in Section 2, but refers only to rdfsinterpretations instead of all simple interpretations. Rdfsentailment is an example of vocabulary entailment.
Since every rdfsinterpretation is an rdfinterpretation, if S rdfsentails E then it rdfentails E; but rdfsentailment is stronger than rdfentailment. Even the empty graph has a large number of rdfsentailments which are not rdfentailments, for example all triples of the form
xxx rdf:type rdfs:Resource .
are true in all rdfsinterpretations of any vocabulary containing xxx.
An rdfsinconsistent graph rdfsentails any graph, by the definition of entailment; such 'trivial entailments' by an inconsistent set are not usually considered useful inferences to draw in practice, however.
RDF provides for the use of externally defined datatypes
identified by a particular URI reference. In the interests of generality, RDF imposes
minimal conditions on a datatype. It also includes a single builtin datatype
rdf:XMLLiteral.
This semantics for datatypes is minimal. It makes no provision for associating a datatype with a property so that it applies to all values of the property, and does not provide any way of explicitly asserting that a blank node denotes a particular datatype value. Semantic extensions and future versions of RDF may impose more elaborate datatyping conditions. Semantic extensions may also refer to other kinds of information about a datatype, such as orderings of the value space.
A datatype is an entity characterized by a set of character strings called lexical forms and a mapping from that set to a set of values. Exactly how these sets and mapping are defined is a matter external to RDF.
Formally, a datatype d is defined by three items:
1. a nonempty set of character strings called the lexical space of d;
2. a nonempty set called the value space of d;
3. a mapping from the lexical space of d to the value space of d, called the lexicaltovalue mapping of d.
The lexicaltovalue mapping of a datatype d is written as L2V(d).
In stating the semantics we assume that interpretations are relativized to a particular set of datatypes each of which is identified by a URI reference.
Formally, let D be a set of
pairs consisting of a URI reference and a datatype
such that no URI reference appears twice in the set, so that D can be regarded
as a function from a set of URI references to a set of datatypes: call this
a datatype map. (The particular URI references must be mentioned explicitly
in order to ensure that interpretations conform to any naming conventions imposed
by the external authority responsible for defining the datatypes.) Every datatype
map is understood to contain <rdf:XMLLiteral
, x> where
x is the builtin XML Literal datatype whose lexical and value spaces and lexicaltovalue
mapping are defined
in the RDF Concepts and Abstract
Syntax document [RDFCONCEPTS].
The datatype map which also contains
the set of all pairs of the form <http://www.w3.org/2001/XMLSchema#
sss,
sss>, where sss is a builtin datatype named sss
in XML Schema Part 2: Datatypes
[XMLSCHEMA2] and listed in the following
table, is referred to here as XSD.
The other builtin XML Schema datatypes are unsuitable for various reasons,
and SHOULD NOT
be used: xsd:duration
does not have a welldefined value space (this may be corrected in later revisions
of XML Schema datatypes, in which case the revised datatype would be suitable
for use in RDF datatyping); xsd:QName
and xsd:ENTITY
require an enclosing XML document context; xsd:ID
and xsd:IDREF
are for cross references within an XML document; xsd:NOTATION
is not intended for direct use; xsd:IDREFS
,
xsd:ENTITIES
and xsd:NMTOKENS
are sequencevalued datatypes which do not fit the RDF datatype
model.
If D is a datatype map, a Dinterpretation of a graph G is any rdfsinterpretation I of V, where V contains the vocabulary of G, which satisfies the following extra conditions for every pair <aaa, x> in D:
if <aaa,x> is in D then I(aaa) = x 
if <aaa,x> is in D then ICEXT(x) is the value space of x and is a subset of LV 
if <aaa,x> is in D then for any typed literal "sss"^^ddd
with I(ddd) = x , if sss is in the lexical space of x then IL("sss"^^ddd) = L2V(x)(sss), otherwise IL("sss"^^ddd) is not in LV 
if <aaa,x> is in D then I(aaa) is in ICEXT(I(rdfs:Datatype )) 
The first condition ensures that I interprets the URI reference according to the datatype map provided. Note that this does not prevent other URI references from also denoting the datatype.
The second condition ensures that the datatype URI reference, when used as a class name, refers to the value space of the datatype, and that all elements of a value space must be literal values.
The third condition ensures that typed literals respect the datatype lexicaltovalue
mapping. For example, if I is an XSDinterpretation then I("15"^^xsd:decimal
)
must be the number fifteen. The
condition also requires that an illtyped literal, where the literal
string is not in the lexical space of the datatype,
not denote any literal value. Intuitively, such a name does not denote any value,
but in order to avoid the semantic complexities which arise from empty names,
the semantics requires such a typed literal to denote an 'arbitrary' nonliteral
value. Thus for example, if I is an XSDinterpretation, then all that can be
concluded about I("arthur"^^xsd:decimal
) is that it is not in LV,
i.e. not in ICEXT(I(rdfs:Literal
)). An illtyped literal
does not in itself constitute an inconsistency, but a graph which entails that
an illtyped literal has rdf:type rdfs:Literal
, or
that an illtyped XML literal has rdf:type rdf:XMLLiteral
, would
be inconsistent.
Note that this third condition applies only to datatypes in the range of D. Typed literals whose type is not in the datatype map of the interpretation are treated as before, i.e. as denoting some unknown thing. The condition does not require that the URI reference in the typed literal be the same as the associated URI reference of the datatype; this allows semantic extensions which can express identity conditions on URI references to draw appropriate conclusions.
The fourth condition ensures that the class rdfs:Datatype
contains
the datatypes used in any satisfying Dinterpretation. Notice that this
is a necessary, but not a sufficient, condition; it allows the class
I(rdfs:Datatype
) to contain other datatypes.
The semantic conditions for rdfinterpretations
impose the correct interpretation on literals typed by 'rdf:XMLLiteral'
.
However, a Dinterpretation recognizes the datatype to exist as an entity,
rather than simply being a semantic condition imposed on the RDF typed
literal syntax. Semantic extensions which can express identity conditions
on resources could therefore draw stronger conclusions from Dinterpretations
than from rdfsinterpretations.
If the datatypes
in the datatype map D impose disjointness conditions on their value spaces,
it is possible for an RDF graph to have no Dinterpretation
which satisfies it. For example, XML Schema defines the value spaces of xsd:string
and xsd:decimal
to be disjoint, so it is impossible to construct
a XSDinterpretation satisfying
the graph
<ex:a> <ex:b> "25"^^xsd:decimal .
<ex:b> rdfs:range xsd:string .
This situation could be characterized by saying that the graph is XSDinconsistent,
or more generally as a datatype clash. Note that it is possible to
construct a satisfying rdfsinterpretation
for this graph, but it would violate the XSD conditions, since the class extensions
of I(xsd:decimal
) and I(xsd:string
) would have a nonempty
intersection.
Datatype clashes can arise in several other ways. For example, any assertion that something is in both of two disjoint dataype classes:
_:x rdf:type xsd:string .
_:x rdf:type xsd:decimal .
or that a property with an 'impossible' range has a value:
<ex:p> rdfs:range xsd:string .
<ex:p> rdfs:range xsd:decimal .
_:x <ex:p> _:y .
would constitute a datatype clash. A datatype clash may also arise from the use of a particular lexical form, for example:
<ex:a> <ex:p> "2.5"^^xsd:decimal .
<ex:p> rdfs:range xsd:integer .
or by the use of an illtyped lexical form:
<ex:a> <ex:p> "abc"^^xsd:integer .
<ex:p> rdfs:range xsd:integer .
Datatype clashes are the only inconsistencies recognized by this model theory; note however that datatype clashes involving XML literals can arise in RDFS.
If D is a subset of D', then restricting interpretations of a graph to D'interpretations amounts to a semantic extension compared to the same restriction with respect to D. In effect, the extension of the datatype map makes implicit assertions about typed literals, by requiring them to denote entities in the value space of a datatype. The extra semantic constraints associated with the larger datatype map will force interpretations to make more triples true, but they may also reveal datatype clashes and violations, so that a Dconsistent graph could be D'inconsistent.
Say that an RDF graph recognizes a datatype URI reference aaa when the graph rdfsentails a datatyping triple of the form
aaa rdf:type rdfs:Datatype .
The semantic conditions for rdfsinterpretations
require the builtin datatype URI reference 'rdf:XMLLiteral'
to
be recognized.
If every recognized URI reference in a graph is the name of a known datatype,
then there is a natural datatype map
DG which pairs each recognized URI reference to that known datatype (and 'rdf:XMLLiteral
'
to rdf:XMLLiteral
). Any rdfsinterpretation
I of that graph then has a corresponding 'natural' DGinterpretation which is
like I except that I(aaa) is the appropriate datatype
and the class extension of rdfs:Datatype
is modified appropriately.
Applications MAY
require that RDF graphs be interpreted by Dinterpretations
where D contains a natural datatype map of the graph. This amounts to treating
datatyping triples as 'declarations' of datatypes
by the graph, and making the fourth semantic condition into an 'iff' condition.
Note however that a datatyping triple does not in itself provide the information
necessary to check that a graph satisfies the other datatype semantic conditions,
and it does not formally rule out other interpretations, so that adopting this
requirement as a formal entailment principle would violate the general
monotonicity lemma.
S Dentails E when every Dinterpretation of the vocabulary of S union E which satisfies every member of S also satisfies E. This follows the wording of the definition of simple entailment in Section 2, but refers only to Dinterpretations instead of all simple interpretations. Dentailment is an example of vocabulary entailment.
As noted above, it is possible that a graph which is consistent in one vocabulary becomes inconsistent in a semantic extension defined on a larger vocabulary, and Dinterpretations allow for inconsistencies in an RDF graph. The definition of vocabulary entailment means that an inconsistent graph will entail any graph in the stronger vocabulary entailment. For example, a Dinconsistent graph Dentails any RDF graph. However, it will usually not be appropriate to consider such 'trivial' entailments as useful consequences, since they may not be valid entailments in a smaller vocabulary.
Given a set of RDF graphs, there are various ways in which one can 'add' information to it. Any of the graphs may have some triples added to it; the set of graphs may be extended by extra graphs; or the vocabulary of the graph may be interpreted relative to a stronger notion of vocabulary entailment, i.e. with a larger set of semantic conditions understood to be imposed on the interpretations. All of these can be thought of as an addition of information, and may make more entailments hold than held before the change. All of these additions are monotonic, in the sense that entailments which hold before the addition of information, also hold after it. We can sum up this in a single lemma:
General monotonicity lemma. Suppose that S, S' are sets of RDF graphs with every member of S a subset of some member of S'. Suppose that Y indicates a semantic extension of X, S Xentails E, and S and E satisfy any syntactic restrictions of Y. Then S' Yentails E.
In particular, if D' is a datatype map, D a subset of D' and if S Dentails E then S also D'entails E.
The following tables list some inference patterns which capture some of the various forms of vocabulary entailment, and could be used as a guide for the design of software to check RDF graphs for RDF and RDFS entailment. Implementations may be based on applying the rules forwards, or treating the conclusion as a pattern to be recognized in the consequent of a proposed entailment and searching backwards for an appropriate match with other rule conclusions or the proposed antecedent. Other strategies are also possible.
The rules are all stated with the form add a triple to a graph when it contains triples conforming to a pattern, and they are all valid in the following sense: a graph entails (in the appropriate sense listed) any larger graph that is obtained by applying the rules to the original graph. Notice that applying such a rule to a graph amounts to forming a simple union, rather than a merge, with the conclusion, and hence preserves any blank nodes already in the graph.
These rules all use the following conventions: uuu stands for any URI reference or blank node identifier, vvv for any URI reference or literal, aaa, bbb, etc., for any URI reference, and xxx, yyy etc. for any URI reference, blank node identifier or literal.
The interpolation lemma in Section 2 characterizes simple entailment in terms of instances and subgraphs. Being a subgraph requires no explicit formulation as an inference rule on triples, but one can give rules which infer generalizations from instances, for example:
The terminology 'allocated to' means that the blank node must have been created by an earlier application of the specified rules on the same URI reference or literal, or if there is no such blank node then it must be a 'new' node which does not occur in the graph. This rather complicated condition ensures that the resulting graph, obtained by adding the new blanknode triples, has the original graph as a proper instance and that any such graph will have a subgraph which is the same as one which can be generated by these rules: the association between introduced blank nodes and the URI reference or literal that they replace provides the instance mapping. For example, the graph
<ex:a> <ex:p> <ex:b> .
<ex:c> <ex:q> <ex:a> .
could be expanded as follows
_:x <ex:p> <ex:b> .
by se1 using a new _:x
which is allocated to ex:a
by se2 using the same
<ex:c> <ex:q> _:x . _:x
allocated
to ex:a
by se2 using a new
_:x <ex:p> _:y . _:y
which is
allocated to ex:b
but it would not be correct to infer
** _:x <ex:q> <ex:a> .
** by se2 (** since
_:x
is not allocated to ex:c
)
Applying these rules to a graph will produce a graph which is simply entailed by the original. These rules will not generate all graphs which have the original graph as an instance, which could include arbitrarily many blanknode triples all of which instantiate back to the original triples. In order to be able to generate all such graphs, the rules must be modified to generate nonproper instances, for example by allowing new blank nodes to be allocated to existing blank nodes. Appendix A describes such a modification.
Rule Name  if E contains  then add 
rdf1  xxx aaa yyy . 
aaa rdf:type rdf:Property . 
rdf2  xxx aaa lll 
xxx aaa where _:nnn is a blank node identifier allocated to lll by this rule. 
Applying these and the earlier rules to a graph produces a graph which is rdfentailed by the original. Note also that the RDF axiomatic triples are valid in all rdfinterpretations, so these rules can be applied to them as well as to any triples in the graph.
These rules are complete in the following sense:
RDF entailment lemma. S rdfentails E if and only if there is a graph which can be derived from S plus the RDF axiomatic triples by the application of the simple entailment rules and RDF entailment rules and which simply entails E. (Proof in Appendix A)
Rule Name  If E contains:  then add: 

rdfs1  xxx aaa lll where lll is a plain literal (with or without a language tag). 
xxx aaa where 
rdfs2  aaa 
xxx rdf:type zzz . 
rdfs3  aaa 
uuu rdf:type zzz . 
rdfs4a  xxx aaa yyy . 
xxx rdf:type rdfs:Resource . 
rdfs4b  xxx aaa uuu . 
uuu rdf:type rdfs:Resource . 
rdfs5  aaa 
aaa rdfs:subPropertyOf ccc . 
rdfs6  xxx rdf:type rdf:Property . 
xxx rdfs:subPropertyOf xxx . 
rdfs7  aaa 
xxx bbb yyy . 
rdfs8  xxx 
xxx rdfs:subClassOf rdfs:Resource . 
rdfs9  xxx 
aaa rdf:type yyy . 
rdfs10  xxx rdf:type rdfs:Class . 
xxx rdfs:subClassOf xxx . 
rdfs11  xxx 
xxx rdfs:subClassOf zzz . 
rdfs12  xxx rdf:type rdfs:ContainerMembershipProperty . 
xxx rdfs:subPropertyOf rdfs:member . 
rdfs13  xxx rdf:type rdfs:Datatype . 
xxx rdfs:subClassOf rdfs:Literal . 
These rules are complete in the following sense:
RDFS entailment lemma. If S is rdfsconsistent, then S rdfsentails E if and only if there is a graph which can be derived from S plus the RDF and RDFS axiomatic triples by the application of the simple, RDF and RDFS entailment rules and which simply entails E. (Proof in Appendix A)
The restriction to rdfsconsistent antecedents is required in order to rule out cases where an rdfsinconsistent set of graphs rdfsentails any graph, including graphs which are syntactically unrelated to the antecedent. Those cases are not covered by this lemma. In order to be complete in this case, a set of rules would need a clear syntactic criterion for recognizing inconsistency, and a rule which allowed the inference of any triple from a graph containing such a contradiction. In the case of RDFS, the appropriate syntactic signal of an inconsistency could be the derivation of a graph containing a subgraph of the following form:
xxx rdfs:subClassOf rdfs:Literal .
bbb rdfs:range
xxx .
aaa rdfs:subPropertyOf
bbb .
yyy aaa lll .
where lll is an illtyped XML literal.
The RDFS rules are somewhat redundant. All but one of the RDF axiomatic triples can be derived from the rules rdfs2 and rdfs3 and the RDFS axiomatic triples, for example; and rule rdfs1 subsumes cases of rule se1 where vvv is a plain literal.
The outputs of these rules will often trigger others. These rules will propagate
all rdf:type
assertions in the graph up the subproperty and subclass
heirarchies, reasserting them for all superproperties and superclasses. rdfs1
will generate type assertions for all the property names used in the graph,
and rdfs3 together with the last RDFS
axiomatic triple will add all type assertions for all the class names used.
Any subproperty or subclass assertion will generate appropriate type assertions
for its subject and object via rdfs2 and
rdfs3 and the domain and range assertions
in the RDFS axiomatic triple set. The rules will generate all assertions of
the form
xxx rdf:type rdfs:Resource .
for every xxx in V, and of the form
xxx rdfs:subClassOf rdfs:Resource .
for every class name xxx; and several more 'universal' facts, such as
rdf:Property rdf:type rdfs:Class .
The stronger extensional semantic conditions described in
Section 4.1 sanction further entailments which are not covered by the RDFS
rules. The following table lists some entailment patterns which are valid in
this stronger semantics. This is not a
complete set of rules for the extensional semantic conditions. Note that
none of these rules are rdfsvalid; they apply only to semantic extensions which
apply the strengthened extensional semantic conditions described in
Section 4.1. These rules have other consequences, eg that rdfs:Resource
is a domain and range of every property.
Rules ext5ext9 follow a common pattern; they reflect the fact that the strengthened
extensional conditions require domains (and ranges for transitive properties)
of the properties in the rdfV and rdfsV vocabularies to be as large as possible,
so any attempt to restrict them will be subverted by the semantic conditions.
Similar rules apply to superproperties of rdfs:range
and rdfs:domain
.
None of these cases are likely to arise in practice.
ext1  xxx 
xxx rdfs:domain zzz . 
ext2  xxx 
xxx rdfs:range zzz . 
ext3  xxx rdfs:domain yyy . zzz rdfs:subPropertyOf xxx . 
zzz rdfs:domain yyy . 
ext4  xxx rdfs:range yyy . zzz rdfs:subPropertyOf xxx . 
zzz rdfs:range yyy . 
ext5  rdf:type rdfs:subPropertyOf zzz
. zzz rdfs:domain yyy . 
rdfs:Resource rdfs:subClassOf yyy . 
ext6  rdfs: subClassOf rdfs:subPropertyOf
zzz . zzz rdfs:domain yyy . 
rdfs:Class rdfs:subClassOf yyy . 
ext7  rdfs:subPropertyOf rdfs:subPropertyOf
zzz . zzz rdfs:domain yyy . 
rdf:Property rdfs:subClassOf yyy . 
ext8  rdfs: subClassOf rdfs:subPropertyOf
zzz . zzz rdfs:range yyy . 
rdfs:Class rdfs:subClassOf yyy . 
ext9  rdfs:subPropertyOf rdfs:subPropertyOf
zzz . zzz rdfs:range yyy . 
rdf:Property rdfs:subClassOf yyy . 
In order to capture datatype entailment in terms of assertions and entailment rules, the rules need to refer to information supplied by the datatypes themselves; and to state the rules it is necessary to assume syntactic conditions which can only be checked by consulting the datatype sources.
For each kind of information which is available about a datatype, inference rules for information of that kind can be stated, which can be thought of as extending the table of RDFS entailment rules. These should be understood as applying to datatypes other than the builtin datatype, the rules for which are part of the RDFS entailment rules. The rules stated below assume that information is available about the datatype denoted by a recognized URI reference, and they use that URI reference to refer to the datatype.
The basic information specifies, for each literal string, whether or not it is a legal lexical form for the datatype, i.e. one which maps to some value under the lexicaltovalue mapping of the datatype. This corresponds to the following rule, for each string sss that is a legal lexical form for the datatype denoted by ddd:
rdfD1  ddd 
aaa ppp _:xxx where _:xxx is a blank node identifier allocated to "sss"^^ddd by this rule. 
Suppose it is known that two lexical forms sss and ttt map to the same value under the datatype denoted by ddd; then the following rule applies:
rdfD2  ddd 
aaa ppp "ttt"^^ddd . 
Suppose it is known that the lexical form sss of the datatype denoted by ddd and the lexical form ttt of the datatype denoted by eee map to the same value. Then the following rule applies:
rdfD3  ddd 
aaa ppp "ttt"^^eee . 
In addition, if it is known that the value space of the datatype denoted by ddd is a subset of that of the datatype denoted by eee, then it would be appropriate to assert that
ddd rdfs:subClassOf
eee .
but this needs to be asserted explicitly; it does not follow from the subset relationship alone.
Assuming that the information encoded in these rules is correct, applying these and the earlier rules will produce a graph which is Dentailed by the original.
The rules rdfD2 and 3 are essentially substitutions by virtue of equations
between lexical forms. Such equations may be capable of generating infinitely
many conclusions, e.g. it is possible to add any number of leading zeros to
any lexical form for xsd:integer
without it ceasing to be a correct
lexical form for xsd:integer
. To avoid such correct
but unhelpful inferences, it is sufficient to restrict rdfD2 to cases which
replace a lexical form with the canonical form for the datatype in question,
when such a canonical form is defined. In order not to omit some valid entailments,
however, such canonicalization rules should be applied to the conclusions as
well as the antecedents of any proposed entailments, and the corresponding rules
of type rdfD3 would need to reflect knowledge of identities between canonical
forms of the distinct datatype.
In particular cases other information might be available, which could be expressed using a particular RDFS vocabulary. Semantic extensions may also define further such datatypespecific meanings.
These rules allow one to conclude that any wellformed typed literal of a recognized
datatype will denote something in the class rdfs:Literal
.
aaa ppp "sss"^^ddd .
ddd rdf:type rdfs:Datatype .
aaa ppp _:xxx .
(by rule rdfD1)
_:xxx rdf:type ddd .
(by rule rdfs11)
ddd rdfs:subClassOf rdfs:Literal .
(by rule rdfs9)
_:xxx rdf:type rdfs:Literal .
The rule rdfD1 is sufficient to expose some cases of a datatype clash, by a chain of reasoning of the following form:
ppp rdfs:range ddd .
aaa ppp "sss"^^eee .
aaa ppp _:xxx .
(by rule rdfD1)
_:xxx rdf:type eee .
_:xxx rdf:type ddd .
(by rule rdfs3)
These rules do not provide a complete set of inference principles for Dentailment,
since there may be valid Dentailments for particular datatypes which depend
on idiosyncratic properties of the particular datatypes, such as the size of
the value space (eg xsd:boolean
has only two elements, so anything
established for those two values must be true for all literals with this datatype.)
In particular, the value space and lexicaltovalue mapping of the XSD datatype
xsd:string
sanctions the identification of typed literals with plain literals without language
tags for all character strings which are in the lexical space of the datatype,
since both of them denote the Unicode character string which is displayed in
the literal; so the following inference rule is valid in all XSDinterpretations.
Here, 'sss' indicates any RDF string in the lexical space of xsd:string
.
xsd 1a  uuu aaa "sss". 
uuu aaa "sss"^^xsd:string
. 
xsd 1b  uuu aaa "sss"^^xsd:string . 
uuu aaa "sss". 
Again, as with the rules rdfD2 and rdfD3, applications may use a systematic replacement of one of these equivalent forms for the other rather than apply these rules directly.
Empty Graph Lemma. The empty set of triples is entailed by any graph, and does not entail any graph except itself.
Proof. Let N be the empty set of triples. The semantic conditions on graphs require that N is true in I, for any I; so the first part follows from the definition of entailment. Suppose G is any nonempty graph and s p o . is a triple in G, then an interpretation I with IEXT(I(p)) = { } does not satisfy G but does satisfy N; so N does not entail G. QED.
This means that most of the subsequent results are trivial for empty graphs, which is what one would expect.
Subgraph Lemma. A graph entails all its subgraphs.
Proof. Obvious, from definitions of subgraph and entailment. If the graph is true in I then for some A, all its triples are true in I+A, so every subset of triples is true in I. QED
Instance Lemma. A graph is entailed by all its instances.
Proof. Suppose I satisfies E' and E' is an instance of E. Then for some mapping A on the blank nodes of E', I+A satisfies every triple in E'. For each blank node b in E, define B(b)=I+A(c), where c is the blank node or name that is substituted for b in E', or c=b if nothing was substituted for it. Then I+B(E)=I+A(E')=true, so I satisfies E. But I was arbitrary; so E' entails E. QED.
Merging lemma. The merge of a set S of RDF graphs is entailed by S, and entails every member of S.
Proof. Obvious, from definitions of entailment and merge. All members of S are true if and only if all triples in the merge of S are true. QED.
This means that, as noted in the text, a set of graphs can be treated as a single graph when discussing satisfaction and entailment. This convention will be adopted in the rest of the appendix, where a reference to an interpretation of a set of graphs, a set of graphs entailing a graph, and so on, should be understood in each case to refer to the merge of the set of graphs, and references to 'graph' in the following can be taken to refer to graphs or to sets of graphs.
An interpretation I is a subinterpretation of J, written I<<J, when the vocabulary of I is a subset of that of J and there is a mapping k, called a projection mapping, from (IR_{I} union IP_{I}) to (IR_{J} union IP_{J}) which satisfies the following conditions:
if x is in IR_{I} then k(x) is in IR_{J};
if x is in IP_{I} then k(x) is in IP_{J};
if x is a string or pair of strings then k(x)=x;
if x is a URI or literal then J_{}(x) = k(I(x));
if <x,y> is in IEXT_{I}(z) then <k(x),k(y)> is in IEXT_{J}(k(z)) .
Clearly, << is reflexive and transitive (by composing the projection mappings.) The subinterpretation relation between interpretations is the dual of the entailment relation between sentences, as shown by the following lemma:
Subinterpretation Lemma. If I << J and I satisfies E then J satisfies E.
Proof. Let k be the projection mapping of I into J. Since I satisfies E there is a mapping A from the blank nodes of E to IR_{I }such that [I+A] satisfies every triple
s p o.
in E, i.e. <[I+A](s),[I+A](o)> is in IEXT_{I}(I(p)); hence,
<k([I+A](s)),k([I+A](o))> is in IEXT_{J}(k(I(p))).Define the mapping B by B(x)=k(A(x)); then [J+B](x)=k([I+A](x)) for any uriref, literal or blank node x, so the above can be written:
<[J+B](s),[J+B](o)> is in IEXT_{J}(J(p))
i.e. [J+B] satisfies the triple; so [J+B] satisfies every triple in E, so J satisfies E. QED
The proof of the subsequent lemmas uses a way of constructing an interpretation of a graph by using the lexical items in the graph itself. (This was Herbrand's idea; we here modify it slightly to handle XML literals and rdf:type appropriately.) Given a nonempty graph G, the Herbrand interpretation of G, written Herb(G), is the interpretation I defined as follows. LV is the set of all required literal values (wellformed XML literals in G, character strings and pairs of character strings and language tags); IR is LV plus all names and blank nodes which occur in a subject or object position in a triple in G; IP is the set of URI references which occur in the property position of any triple; IS is the identity mapping on the vocabulary of G, IL maps all typed literals into themselves, and IEXT is defined by: <s,o> is in IEXT(p) just when there is a triple in the graph of the form s p o . Clearly Herb(G)+B, where B is the identity map on blank nodes in G, satisfies every triple in G, by construction, so Herb(G) satisfies G.
Herbrand interpretations treat URI references and typed literals (and blank nodes) in the same way as simple literals, i.e. as denoting their own syntactic forms. Of course this may not be what was intended by the writer of the RDF, but the lemma shows that any graph can be interpreted in this way. This therefore establishes a useful result:
Satisfaction Lemma. Any RDF graph has a satisfying interpretation. QED
Herbrand interpretations have some very useful properties. The Herbrand interpretation of a graph is a 'minimal' interpretation, which is 'just enough' to make the graph true; and so any interpretation which satisfies the graph must in a sense agree with the Herbrand interpretation; and of course any interpretation which does agree with the Herbrand interpretation will satisfy the graph. Taken together and made precise, these observations provide a way to characterize entailment between graphs in terms of Herbrand interpretations.
Herbrand lemma. I satisfies E if and only if Herb(E) << I.
Proof. Write H=Herb(E).
Suppose I satisfies E, then the interpretation mapping I itself defines a projection mapping from H into IR; for H(a)=a for any name in E, and so I(H(a))=I(a); and <s,o> is in IEXT_{H}(p) just in case the triple s p o
.
is in E; since I satisfies E, <I(s),I(o)> = <I(H(s)),I(H(o))> must be in IEXT_{I}(I(p)), i.e. IEXT_{I(H)}(I(H(p)), which is the condition for I to be a projection from H to I; so H<<I.Now suppose H << I. H satisfies E by construction, so I satisfies E by the subinterpretation lemma. QED
The following is an immediate consequence:
Herbrand entailment lemma. S entails E if and only if Herb(S) satisfies E.
Proof. Suppose S entails E. Herb(S) satisfies S, so Herb(S) satisfies E.
Now suppose Herb(S) satisfies E. If I satisfies S then Herb(S) << I; so I satisfies E. But I was arbitrary; so S entails E.
QED
The syntactic properties of Herbrand interpretations can be described in terms of instances and subgraphs. Say that a graph E' is connected to a graph E if some instance of E' is a subgraph of E. In particular, a ground graph is connected to E just when it is a subgraph of E, a ground triple is connected just when it is in the graph. Graphs which are connected to E are entailed by E, by the subgraph and instance lemmas; but for all others, there is a way to arrange the world so that they are false and E true.
In particular, if E' is not connected to E then Herb(E) does not satisfy E'; for suppose that it did, then for some mapping B from the blank nodes of E' to the blank nodes and vocabulary of E, Herb(E)+B satisfies E', which means that for every triple
s p o .
in E', the triple
[Herb(E)+B](s) p [Herb(E)+B](o) .
occurs in E, by definition of Herb(E). But the set of these triples is an instance of E', by construction; so E' is connected to E.
This provides an exact correspondence between separability and Herbrand interpretations:
Herbrand separation lemma. Herb(E) satisfies E' if and only if E' is connected to E. QED
Putting the separation and entailment results together, it is obvious that S entails E if and only if E is connected to S. This is simply a restatement of the:
Interpolation Lemma. S entails E if and only if a subgraph of S is an instance of E. QED.
This lemma can be used as the basis for several different mechanical processes for checking simple entailment. One way to characterize entailment syntactically is in terms of inference rules which apply to a set S and can be used to derive E just when S entails E. For example, a slight modification of the simple entailment rules described in section 7.1 will suffice, where we allow new blank nodes to be allocated to existing blank nodes as well as to URI references and literals:
These rules allow blank nodes to proliferate, producing highly nonlean graphs; they sanction entailments such as
ex:a exp ex:b .
ex:a ex:p _:x .
by xse1 with _:x allocated
to ex:b
ex:a ex:p _:y .
by xse1 with _:y allocated to _:x
_:z ex:p _:y .
by xse2 with _:z allocated to ex:a
_:u ex:p _:y .
by xse2 with _:u allocated to _:z
_:u ex:p _:v .
by xse1 with _:v allocated to _:y
It is easy to see that if S is an instance of E then one can derive E from S by applying these rules in a suitable sequence; the rule applications required can be discovered by examination of the instance mapping. So, by the interpolation lemma, S entails E iff one can derive (a graph containing) E from S by the application of these rules. However, it is also clear that applying these rules naively would not result in an efficient search process, since the rules will not terminate and can produce arbitrarily many redundant derivations of equivalent triples.
The general problem of determining simple entailment between arbitrary RDF graphs is decideable but NPcomplete. This can be shown by encoding the problem of detecting a subgraph of an arbitrary directed graph as an RDF entailment, using blank nodes to represent the graph (observation due to Jeremy Carroll.)
The following are direct consequences of the interpolation lemma:
Anonymity lemma. Suppose E is a lean graph and E' is a proper instance of E. Then E does not entail E'.
Proof. Since E' is a proper instance and E is lean, E' is not connected to E. Therefore E does not entail E' QED
Monotonicity Lemma. Suppose S is a subgraph of S' and S entails E. Then S' entails E. (Special case of general monotonicity lemma)
Compactness Lemma. If S entails E and E is a finite graph, then some finite subset S' of S entails E.
Proof. By the interpolation lemma, a subgraph S' of S is an instance of E; so S' is finite, and S' entails E. QED
Although this result is trivial for simple entailment, it becomes progressively less trivial in more elaborate semantic extensions.
General monotonicity lemma. Suppose that S, S' are sets of RDF graphs with every member of S a subset of some member of S'. Suppose that Y indicates a semantic extension of X, S Xentails E, and S and E satisfy any syntactic restrictions of Y. Then S' Yentails E.
Proof. This follows simply by tracing the definitions. Suppose that I is a Yinterpretation of S'; then since Y is a semantic extension of X, I is an Xinterpretation; and by the subgraph and merge lemmas, I satisfies S; so I satisfies E.
QED
RDF entailment lemma. S rdfentails E if and only if there is a graph which can be derived from S plus the RDF axiomatic triples by the application of the simple entailment rules and RDF entailment rules and which simply entails E.
Proof (Note: the following informative proof is incomplement. A corrected proof is anticipated in a future version of this document.) To show 'if' here is trivial: one has only to check that the RDF entailment rules are rdfvalid, which is left as an exercise for the reader. To establish 'only if' requires more work, however.
If S or E is empty then the result follows trivially; so suppose they are both nonempty.
Define the rdfVclosure of a graph G, rdfclos(V, G), to be the graph gotten by adding all the RDF axiomatic triples which contain any vocabulary from (V union crdfV) to G and then applying the simple entailment rules and RDF entailment rules in all possible ways until the graph is unchanged. Define the combined vocabulary vocab(S) union vocab(E) to be W. We will show that rdfclos(W, S) satisfies the conditions of the lemma.
Let H be the Herbrand interpretation of rdfclos(W, S). It is sufficient to show that there is an rdfinterpretation H' of W with H'<<H; for in that case since S rdfentails E then H' satisfies E; and if I is any simple interpretation of W which satisfies rdfclos(S) then H <<I by the Herbrand lemma, so H' <<I, so I satisfies E; so rdfclos(S) simply entails E.
H' is constructed from H by adjusting the definition of IP and the interpretation of welltyped XML literals and the class extension of
rdf:XMLLiteral
; in other respects H' and H are identical (so that for any other URI reference uuu, I_{H'}(uuu)=I_{H}(uuu)=uuu.) The intuition behind this construction is that the Herbrand interpretation of literals is replaced by a genuine literal value, which is then projected onto the blank node which was introduced into the closure, and allocated to the literal, by the appropriate existential generalization rule. To be precise, defineIP_{H'} = the set of nodes which occur in rdfclos(W, S) as the subject of a triple of the form
xrdf:type rdf:Property .
LV_{H'} = LV_{H} plus all XML literal values of welltyped XML literals in W;
IEXT_{H'}(
rdf:type
) = IEXT_{H}(rdf:type
) union {<IL_{H'}(x),rdf:XMLLiteral
>: x a welltyped XML literal in W};and for each welltyped XML literal x in W, define IL_{H'}(x) to be the XML value of x.
In other respects H' is defined as H.
To see that H'<<H, define k by mapping XML literal values to their corresponding welltyped XML literals, blank nodes introduced by inference rules to the symbols to which they are allocated by the rule, and otherwise the identity mapping.
To show that k is a projection mapping two cases must be examined in detail.
A URI aaa is in IP_{H'} just when a triple
aaardf:type rdf:Property .
is in rdfclos(W,S) ; so either this triple was in S, or it was introduced by rule rdf1 and S contains a triple x aaa y.
; either way, aaa is in IP_{H}, so k maps IP_{H'} to IP_{H}.If <x,y> is in IEXT_{H'}(z) then either it is also in IEXT_{H}(z) or else <x,y> = <IL_{H'}(lll),
rdf:XMLLiteral
> where lll is a wellformed XML literal in W. Suppose lll occurs in W in a triple
xxx aaa lll.
in W; then <xxx, lll> is in IEXT_{H}(aaa) , and <xxx, _:nnn> is in IEXT_{H'}(aaa) and <_:nnn,rdf:XMLLiteral
> is in IEXT_{H'}(rdf:type
), where _:nnn is allocated to lll by rule rdf2. By construction, k(IL_{H'}(lll))=k(_:nnn)=lll and k(aaa)=aaa, k(xxx)=xxx, and k(rdf:XMLLiteral
)=rdf:XMLLiteral
; so k is a projection.Note that this requires that blank nodes introduced by rules are mapped appropriately to their allocated names.
H' satisfies the first two RDF semantic conditions by construction. To see that it satisfies the third, observe that IL_{H'}(lll)=IL_{H}(lll)=lll when lll is an illtyped XML literal, so IL_{H'}(lll) is not an XML literal value and is not in LV_{H'}, and that since no triple in W can cause a pair <lll, x> to occur in any extension in the Herbrand interpretation, and no entailment rule can impose any such condition, that therefore it will not occur in any extension in H'. So the third RDF semantic condition is satisfied and H' is an RDF interpretation, as required.
QED
RDFS entailment lemma. If S is rdfsconsistent, then S rdfsentails E if and only if there is a graph which can be derived from S plus the RDF and RDFS axiomatic triples by the application of the simple, RDF and RDFS entailment rules and which simply entails E.
Proof (Outline) (Note: the following informative proof is incomplement. A corrected proof is anticipated in a future version of this document.) Again, to show 'if' it is sufficient to show that the RDFS entailment rules are rdfsvalid, which is again left as an exercise; and again, the empty cases are trivial.
The proof of 'only if' is similar to that used in the previous lemma, and similar constructions and terminology will be used, except that the RDFS closure, rdfsclos(V,G), is the graph gotten by adding the appropriate RDF and RDFS axiomatic triples to G, and then applying the simple, RDF and RDFS entailment rules until the graph is unchanged. Let H be the Herbrand interpretation of rdfsclos(W,S) and H' be constructed from H as in the previous proof, except that the extension of
rdf:type
is more complicated:IEXT_{H'}(rdf:type) = IEXT_{H}(rdf:type) union
{<IL_{H'}(x),rdf:XMLLiteral
>: x a welltyped XML literal in W}union
{<IL_{H'}(x),rdfs:Literal
>: x a plain literal in W}union
{<IL_{H'}(x), y>: x a literal in W and rdfclos(W,S) contains a graph of the form:aaa
rdfs:subPropertyOf
bbb.
bbbrdfs:range
zzz.
zzzrdfs:subClassOf
y.
www aaa x.
} .H' satisfies the first two RDF semantic conditions by construction. On the RDFS semantic conditions H and H' are indistinguishable, and the argument can be made case by case, matching the RDFS semantic conditions with the appropriate entailment rules. The form of the argument is illustrated with two examples.
Consider the first RDFS semantic condition. Suppose <x,y> is in IEXT_{H}(
rdfs:domain
) and <u,v> is in IEXT_{H}(x); then by the minimality of the Herbrand interpretation, rdfsclos(W,S) must contain triplesx
rdfs:domain
y.
u x v
.
so by rule rdfs2, it must also contain the triple
u
rdf:type
y.
so by the construction of H, IEXT_{H}(
rdf:type
) contains <u,y>, i.e. u is in ICEXT_{H}(y).In some cases, the rule derivation involves several steps and some of the RDF and RDFS axiomatic triples; for example, to show that H satisfies the seventh RDFS semantic condition
If <x,y> is in IEXT(I( rdfs:subClassOf
)) then x and y are in IC and ICEXT(x) is a subset of ICEXT(y)note that <x,y> is in IEXT_{H}(
rdfs:subClassOf
) only if rdfsclos(W,S) containsx
rdfs:subClassOf
yand then the following are all in rdfsclos(W,S):
rdfs:subClassOf rdfs:domain rdfs:Class .
(RDFS axiomatic triple)
xrdf:type rdfs:Class .
(rule rdfs2)
rdfs:subClassOf rdfs:range rdfs:Class .
(RDFS axiomatic triple)
yrdf:type rdfs:Class .
(rule rdfs3)so x and y are both in IC_{H}; and that if z is in ICEXT_{H}(x) then rdfsclos(W,S) must contain a triple
z
rdf:type
x.
so it must also contain a triple
z
rdf:type
y.
by rule rdfs9so by the construction of the Herbrand interpretation, <z,y> is in IEXT_{H}(
rdf:type
), i.e. z is in ICEXT_{H}(y).The other RDFS semantic conditions can be checked similarly. The correspondence between rule rdfs3 and the RDFS semantic condition on
rdfs:range
relies on the extra conditions on H' described above to handle the case where the entity in the object position is a literal, which is not covered by the rule itself. In this case, the closure contains assertions which mirror the semantic conditions but are stated in the triples using the blank node allocated to the literal by the appropriate rule: rdf2 for welltyped XML literals, rdfs1 for other typed literals and rule se1 for all other literals. Note that this construction relies on the projection mapping respecting blank node allocation.To show that H'<<H, define k as in the previous proof. The argument proceeds similarly to that used in the previous lemma, using rules rdfs1and se1 to cover the extra cases.
It remains to show that H' satisfies the third RDF semantic condition. Using the minimality of the Herbrand universe and the definition of k, a survey of the RDFS semantic conditions shows that this condition fails just when the graph constrains an illtyped XML literal value to be in LV_{H'}, and that by the above construction, rdfsclos(S,W) must be rdfsinconsistent, so S is rdfsinconsistent, contrary to hypothesis: to the condition is satisfied.
QED
Skolemization is a syntactic transformation routinely used in automatic inference systems in which existential variables are replaced by 'new' functions  function names not used elsewhere  applied to any enclosing universal variables. While not itself strictly a valid operation, Skolemization adds no new content to an expression, in the sense that a Skolemized expression has the same entailments as the original expression provided they do not contain the new skolem functions.
In RDF, Skolemization simplifies to the special case where an existential variable is replaced by a 'new' name, i.e. a URI reference which is guaranteed to not occur anywhere else. (Using a literal would not do. Literals are never 'new' in the required sense.) To be precise, a Skolemization of E (with respect to V) is a ground instance of E with respect to a vocabulary V which is disjoint from the vocabulary of E.
The following lemma shows that Skolemization has the same properties in RDF as it has in conventional logics. Intuitively, this lemma shows that asserting a Skolemization expresses a similar content to asserting the original graph, in many respects. In effect, it simply gives 'arbitrary' names to the anonymous entities whose existence was asserted by the use of blank nodes. However, care is needed, since these 'arbitrary' names have the same status as any other URI references once published. Also, Skolemization would not be an appropriate operation when applied to anything other than the antecendent of an entailment. A Skolemization of a query would represent a completely different query.
Proof. sk(E) entails E by the interpolation lemma.
Now, suppose that sk(E) entails F where F shares no vocabulary with V; and suppose I is some interpretation satisfying E. Then for some mapping A from the blank nodes of E, I+A satisfies E. Define an interpretation I' of the vocabulary of sk(E) by: IR'=IR, IEXT'=IEXT, I'(x)=I(x) for x in the vocabulary of E, and I'(x)=[I+A](y) for x in V, where y is the blank node in E that is replaced by x in sk(E). Clearly I' satisfies sk(E), so I' satisfies F. But I'(F)=[I+A](F) since the vocabulary of F is disjoint from that of V; so I satisfies F. But I was arbitrary; so E entails F.
QED.
Antecedent (n.) In an inference, the expression(s) from which the conclusion is derived. In an entailment relation, the entailer. Also assumption.
Assertion (n.) (i) Any expression which is claimed to be true. (ii) The act of claiming something to be true.
Class
(n.) A general concept, category or classification. Something used primarily to
classify or categorize other things. Formally, in RDF, a resource of type
rdfs:Class
with an associated set of resources all of which
have the class as a value of the rdf:type
property.
Classes are often called 'predicates' in the formal logical
literature.
(RDF distinguishes class from set, although the two are often identified. Distinguishing classes from sets allows RDF more freedom in constructing class hierarchies, as explained earlier.)
Complete (adj., of an inference system). (1) Able to detect all entailments between any two expressions. (2) Able to draw all valid inferences. See Inference. Also used with a qualifier: able to detect entailments or draw all valid inferences in a certain limited form or kind (e.g. between expressions in a certain normal form, or meeting certain syntactic conditions.)
(These definitions are not exactly equivalent, since the first requires that the system has access to the consequent of the entailment, and may be unable to draw 'trivial' inferences such as (p and p) from p. Typically, efficient mechanical inference systems may be complete in the first sense but not necessarily in the second.)
Consequent (n.) In an inference, the expression constructed from the antecedent. In an entailment relation, the entailee. Also conclusion.
Consistent (adj., of an expression) Having an satisfying interpretation; not internally contradictory. (Also used of an inference system as synonym for Correct.)
Correct (adj., of an inference system). Unable to draw any invalid inferences, or unable to . See Inference.
Decideable (adj., of an inference system). Able to determine for any pair of expressions, in a finite time with finite resources, whether or not the first entails the second. (Also: adj., of a logic:) Having a decideable inference system which is complete and correct for the semantics of the logic.
(Not all logics can have inference systems which are both complete and decideable, and decideable inference systems may have arbitrarily high computational complexity. The relationships between logical syntax, semantics and complexity of an inference system continue to be the subject of considerable research.)
Entail (v.), entailment (n.). A semantic relationship between expressions which holds whenever the truth of the first guarantees the truth of the second. Equivalently, whenever it is logically impossible for the first expression to be true and the second one false. Equivalently, when any interpretation which satisfies the first also satisfies the second. (Also used between a set of expressions and an expression.)
Equivalent (prep., with to) True under exactly the same conditions; making identical claims about the world, when asserted. Entails and is entailed by.
Extensional (adj., of a logic) A setbased theory or logic of classes, in which classes are considered to be sets, properties considered to be sets of <object, value> pairs, and so on. A theory which admits no distinction between entities with the same extension. See Intensional.
Formal (adj.) Couched in language sufficiently precise as to enable results to be established using conventional mathematical techniques.
Iff (conj.) Conventional abbreviation for 'if and only if'. Used to express necessary and sufficient conditions.
Inconsistent (adj.) False under all interpretations; impossible to satisfy. Inconsistency (n.), any inconsistent expression or graph.
(Entailment and inconsistency are closely related, since A entails B just when (A and notB) is inconsistent, c.f. the second definition for entailment. This is the basis of many mechanical inference systems.
Although the definitions of consistency and inconsistency are exact duals, they are computationally dissimilar. It is often harder to detect consistency in all cases than to detect inconsistency in all cases.)
Indexical (adj., of an expression) having a meaning which implicitly refers to the context of use. Examples from English include words like 'here', 'now', 'this'.
Inference (n.) An act or process of constructing new expressions from existing expressions, or the result of such an act or process. Inferences corresponding to entailments are described as correct or valid. Inference rule, formal description of a type of inference; inference system, organized system of inference rules; also, software which generates inferences or checks inferences for validity.
Intensional (adj., of a logic) Not extensional. A logic which allows distinct entities with the same extension.
(The merits and demerits of intensionality have been extensively debated in the philosophical logic literature. Extensional semantic theories are simpler, and conventional semantics for formal logics usually assume an extensional view, but conceptual analysis of ordinary language often suggests that intensional thinking is more natural. Examples often cited are that an extensional logic is obliged to treat all 'empty' extensions as identical, so must identify 'round square' with 'santa clause', and is unable to distinguish concepts that 'accidentally' have the same instances, such as human beings and bipedal hominids without body hair. The semantics described in this document is basically intensional.)
Interpretation (of) (n.) A minimal formal description of those aspects of a world which is just sufficient to establish the truth or falsity of any expression of a logic.
(Some logic texts distinguish between a interpretation structure, which is a 'possible world' considered as something independent of any particular vocabulary, and an interpretation mapping from a vocabulary into the structure. The RDF semantics takes the simpler route of merging these into a single concept.)
Logic (n.) A formal language which expresses propositions.
Metaphysical (adj.). Concerned with the true nature of things in some absolute or fundamental sense.
Model Theory (n.) A formal semantic theory which relates expressions to interpretations.
(The name 'model theory' arises from the usage, traditional in logical semantics, in which a satisfying interpretation is called a "model". This usage is often found confusing, however, as it is almost exactly the inverse of the meaning implied by terms like "computational modelling", so has been avoided in this document.)
Monotonic (adj., of a logic or inference system) Satisfying the condition that if S entails E then (S + T) entails E, i.e. adding information to some antecedents cannot invalidate a valid entailment.
(All logics based on a conventional model theory and a standard notion of entailment are monotonic. Monotonic logics have the property that entailments remain valid outside of the context in which they were generated. This is why RDF is designed to be monotonic.)
Nonmonotonic (adj.,of a logic or inference system) Not monotonic. Nonmonotonic formalisms have been proposed and used in AI and various applications. Examples of nonmonotonic inferences include default reasoning, where one assumes a 'normal' general truth unless it is contradicted by more particular information (birds normally fly, but penguins don't fly); negationbyfailure, commonly assumed in logic programming systems, where one concludes, from a failure to prove a proposition, that the proposition is false; and implicit closedworld assumptions, often assumed in database applications, where one concludes from a lack of information about an entity in some corpus that the information is false (e.g. that if someone is not listed in an employee database, that he or she is not an employee.)
(The relationship between monotonic and nonmonotonic inferences is often subtle. For example, if a closedworld assumption is made explicit, e.g. by asserting explicitly that the corpus is complete and providing explicit provenance information in the conclusion, then closedworld reasoning is monotonic; it is the implicitness that makes the reasoning nonmonotonic. Nonmonotonic conclusions can be said to be valid only in some kind of 'context', and are liable to be incorrect or misleading when used outside that context. Making the context explicit in the reasoning and visible in the conclusion is a way to map them into a monotonic framework.)
Ontological (adj.) (Philosophy) Concerned with what kinds of things really exist. (Applied) Concerned with the details of a formal description of some topic or domain.
Proposition (n.) Something that has a truthvalue; a statement or expression that is true or false.
(Philosophical analyses of language traditionally distinguish propositions from the expressions which are used to state them, but model theory does not require this distinction.)
Reify (v.), reification (n.) To categorize as an object; to describe as an entity. Often used to describe a convention whereby a syntactic expression is treated as a semantic object and itself described using another syntax. In RDF, a reified triple is a description of a tripletoken using other RDF triples.
Resource (n.)(as used in RDF)(i) An entity; anything in the universe. (ii) As a class name: the class of everything; the most inclusive category possible.
Satisfy (v.t.), satisfaction,(n.) satisfying (adj., of an interpretation). To make true. The basic semantic relationship between an interpretation and an expression. X satisfies Y means that if the world conforms to the conditions described by X, then Y must be true.
Semantic (adj.) , semantics (n.). Concerned with the specification of meanings. Often contrasted with syntactic to emphasize the distinction between expressions and what they denote.
Skolemization (n.) A syntactic transformation in which blank nodes are replaced by 'new' names.
(Although not strictly valid, Skolemization retains the essential meaning of an expression and is often used in mechanical inference systems. The full logical form is more complex. It is named after the logician A. T. Skolem)
Token (n.) A particular physical inscription of a symbol or expression in a document. Usually contrasted with type, the abstract grammatical form of an expression.
Universe (n., also Universe of discourse) The universal classification, or the set of all things that an interpretation considers to exist. In RDF/S, this is identical to the set of resources.
Use (v.) contrasted with mention; to use a piece of syntax to denote or refer to something else. The normal way that language is used.
("Whenever, in a sentence, we wish to say something about a certain thing, we have to use, in this sentence, not the thing itself but its name or designation."  Alfred Tarski)
Valid (adj., of an inference or inference process) Corresponding to an entailment, i.e. the conclusion of the inference is entailed by the antecedent of the inference. Also correct.
Wellformed (adj., of an expression). Syntactically legal.
World (n.) (with the:) (i) The actual world. (with a:) (ii) A way that the actual world might be arranged. (iii) An interpretation (iv) A possible world.
(The metaphysical status of 'possible worlds' is highly controversial. Fortunately, one does not need to commit oneself to a belief in parallel universes in order to use the concept in its second and third senses, which are sufficient for semantic purposes.)
This document reflects the joint effort of the members of the RDF Core Working Group. Particular contributions were made by Jeremy Carroll, Dan Connolly, Jan Grant, R. V. Guha, Graham Klyne, Ora Lassilla, Brian McBride, Sergey Melnick, Jos deRoo and Patrick Stickler.
The basic idea of using an explicit extension mapping to allow selfapplication without violating the axiom of foundation was suggested by Christopher Menzel.
Peter PatelSchneider and Herman ter Horst found several major problems in earlier drafts, and suggested several important technical improvements.
Patrick Hayes' work on this document was supported in part by DARPA under contract #250722522.
Changes since the 5 September 2003 working draft.
The appendix describing the Lbase translation (formerly Appendix A) has been removed, with corresponding minor editoral changes to the text to refer to the Lbase Note. Editorial (wording and notation) changes made to the proof appendix for clarity (following comments by ter Horst) and also to correct errors found in earlier versions.
Some items (Complete, Decideable) and comments added to the glossary.
The rules in section 7 are no longer considered normative, and a comment has been added to the text concerning implementation strategies. The wording of the text in 7.1 concerning the simple entailment rules has been modified for clarity, in response to email comments by PatelSchneider, and a pointer added to new text in the Appendix describing and discussing a complete rule set for simple entailment.
Text has been added, with examples, to section 5 to better explain the concept of a datatype clash, following email suggestions by Horrocks and Pan, with corresponding minor wording changes made to section 7. Text has been added to 7.3 after the statement of the RDFS entailment lemma pointing out that it does not apply to inconsistent graphs, with a brief discussion; the proof of the lemma has been rewritten.
Character strings in RDF graphs SHOULD (not MUST) conform to normal form C; (WG decision Oct 3 2003) and the text in section 0.2 has been modified to reflect this.
Several minor errors (typos, broken links, redundant references) have been corrected.
Changes since the 23 January 2003 last call working draft.
The following changes do not affect the normative technical content
Many small changes to wording and document organization to improve clarity,
remove ambiguities, make definitions clearer, etc. and for conformity with other
RDF documents and W3C house style, following review comments by Lech. The definition
of 'graph merge' has been rewritten, following review comments by PatelSchneider
and Beckett. The background colors have been changed to avoid red/green confusion
and internal links highlighted with background color in the text. Some of the
lemmas have been restated more economically and the proofs rephrased for clarity.
The semantic conditions are now aligned exactly with the vocabularies, so that
RDF interpretations exactly constrain the rdf: vocabulary, etc.. Some of the
section numbers and titles have been changed to better reflect this realignment.
The term 'namespace' has been replaced by 'vocabulary' , cf pfps21.
The description of reification refers to rdf:ID
.
The informative appendices on Lbase and the proof appendix have been extensively rewritten and errors corrected, cf. pfps02. Some parallel changes have been made to the Lbase note. The text of the datatypes section been extensively rewritten following technical changes noted below; the older version was ambiguous and contained errors. The XSD datatypes suitable for RDF use are listed explicitly in the text, cf. pfps01.
Following email by PatelSchneider, the possibility of an rdfsinconsistency has been noted in the text (section 4, end), with an example, a comment added on trivial entailments in section 4.3, and the wording of the rdfs entailment lemma has been restricted to consistent antecedents (section 7.3). This corrects an omission and a consequent error in the text.
The following changes do not affect any entailments or test cases.
rdf:type rdf:List
triples for all sublists. (WG decision recorded
http://lists.w3.org/Archives/Public/w3crdfcorewg/2003May/0199.html)
The wording of the text and examples in section 3.2.3 have been modified to
suit.All the following changes cause changes to some entailments.
1. The treatment of language tags in literals has been simplified. Typed literals, including XML literals, no longer have associated language tags. (WG decision recorded http://lists.w3.org/Archives/Public/w3crdfcorewg/2003May/0138.html) There are therefore three types of literal: plain without a language tag, plain with a language tag, and typed literals.
XML literals are required to be in canonical form, and to denote entities which are distinct from any character string.(WG decision recorded http://lists.w3.org/Archives/Public/w3crdfcorewg/2003Jun/0156.html)
The chief effect of these decisions in this document is that XML literals can
be treated uniformly with other typed literals. However, the rdf semantic conditions
on rdfs:XMLLiteral
are stated without explicit reference to datatypes
in order to make it possible for a conforming RDF inference system to avoid
full datatyping. The accounts of rdfs:XMLLiteral
given for RDF,
and for RDF with datatyping, are in exact correspondence. This change is relevant
to the last call comments: pfps02,
pfps06,
pfps07.
2. The treatment of rdfs:subClassOf
and rdfs:subPropertyOf
has been changed. They are no longer required to satisfy 'iff' semantic conditions,
but only to be transitive and reflexive. This decision was taken as a result
of the felt need to support simple complete sets of inference rules for RDFS.
We are grateful to Herman ter Horst and Peter PatelSchneider for noticing the
complexities which arose from the older conditions, c.f. horst01
et. seq.. (WG decision recorded
http://lists.w3.org/Archives/Public/w3crdfcorewg/2003Jul/0025.html)
This has required changes to the RDFS entailment rules table. The older conditions are now explained in informative sections as 'extensional' conditions, and corresponding entailment rules discussed.
3. Plain literals, and literals typed with xsd:string
, both denote
character strings. (WG decision recorded
http://lists.w3.org/Archives/Public/w3crdfcorewg/2003Jul/att0393/rdfcore20030725minutes.txt
) The semantics now states explicitly that these are the coextensive in
XSD interpretations and describes a corresponding inference rule.
4. The account of datatyped interpretations has been stated relative to a 'datatype map' from URI references to datatypes. This change was introduced after email discussions with PatelSchneider, cf pfps08 and pfps09 et. seq.. The previous treatment was not adequate to support the intended entailments. (WG decision recorded http://lists.w3.org/Archives/Public/w3crdfcorewg/2003May/0199.html)
5. LV is required to be the class extension of rdfs:Literal
in
all RDFS interpretations, cf. pfps06.
This clarifies and rationalizes the treatment of rdfs:Literal
and supports the entailments noted in pfps10.
6. The rule rdfD4 has been removed: given the change mentioned in 2. above, it was no longer valid. The text notes that it is consistent to assert a subClass relation between datatypes in this case.