A Pragmatic Theory of Reference for the Web

Dan Connolly, MIT Computer Science and Artificial Intelligence Laboratory (CSAIL)
IRW 20006
Edinburgh, Scotland
23 May 2006


A PDF version of this paper, formatted per WWW2006 author guidelines is generated, along with a LaTeX version of the main text and a BibTex version of the bibliography, using some tools for Transforming XHTML to LaTeX and BibTeX. I intend to modify this XHTML version to suit XHTML submission instructions presently.

The World Wide Web is a global information universe[BL92]. Links are used to represent all sorts of subtle, rich, and and even ambiguous references. One one web page, we might find...

I saw a great <a
>movie starring Sean Connery</a>

... while another says...

The <a
>IMDB page on "The Name of the Rose"</a>
is a great source of information.

Meanwhile, the Web Architecture document[webarch] says, By design a URI identifies one resource. Which does http://www.imdb.com/title/tt0091605/ identify, then, the movie, or the page about the movie? The pragmatic answer is: it doesn't matter that much, provided the community of people making the links and serving the information agree (to a reasonable extent) on a set of terms and their meanings.

Human readers are quite robust when it comes to understanding puns and ambiguously indirect references, but computers are not; in a C program, the difference between *p and **p is the difference between a useful computational result and a crash. Even for human readers, there are limits. If visiting http://www.imdb.com/name/nm0000225/ with a web browser showed Christian Slater's photo and filmography, and a writer used that address to refer, indirectly, to Sean Connery, readers would likely feel that Grice's Maxim of Manner[Gr89], Avoid ambiguity, had been violated.

The design of the Web of documents we have today is the result of taking the simplest features of hypertext designs from 15 to 20 years ago, adding globally scoped Uniform Resource Identifiers (URIs), and relaxing link consistency constraints. The social dynamics of the Web include lots of people agreeing to just a few design constraints in order to get a significant return on their investment, whether from reading or writing or both.

By analogy, the Semantic Web involves starting with simple database and logic designs and using URIs for column names and symbol terms. Which constraints need relaxing and which social norms will result in exponential growth are still open questions.

The W3C Technical Architecture Group (TAG) is chartered to document and build consensus around principles of Web architecture. This paper gives a pragmatic theory of reference in the form of some principles established by the W3C TAG and some personal conjectures about issues that are still open.

An analysis of httpRange-14

One of the most intensely debated TAG issues is httpRange-14: What is the range of the HTTP dereference function?. The dicussion of the issue is almost all publicly recorded, but following it is challenging, not only because of the quantity, but because of the diverse backgrounds of the participants, leading to much miscommunication.

Perhaps a formal analysis is an effective way to summarize. For example, the simple logical statement that http://purl.org/dc/elements/1.1/title is an rdf:Property was a matter of some dispute.

The Dublin Core Metadata Initiative (DCMI) publishes a schema http://purl.org/dc/elements/1.1/ that says:

  <rdfs:label xml:lang="en-US"
  <!-- ... details elided ... -->

Or, equivalently, using turtle[turtle] notation:

@prefix rdf:
@prefix rdfs:
  <http://www.w3.org/2000/01/rdf-schema#> .
@prefix dc:

dc:title rdf:type rdf:Property.

Meanwhile, Tim Berners-Lee argued that HTTP URIs (without #) should be understood as referring to documents, not cars. To formalize this position, we'll use some string manipulation properties from N3[N3CWM2003] and set aside the prohibition against literal subjects, going just beyond turtle into N3. Also, we use the log:uri property, which is similar to the name quoting function in KIF[KIF].

@prefix dc:
@prefix log:
  <http://www.w3.org/2000/10/swap/log#> .
@prefix str:
  <http://www.w3.org/2000/10/swap/string#> .

  log:uri "http://purl.org/dc/elements/1.1/title".
  str:startsWith "http:".
  str:notMatches "#".

We can use OWL[OWL] to formalize that something is a document and not a car, once we choose, for the purpose of this discussion, URIs for the concepts of documents and cars:

@prefix owl:
@prefix tbl:

tbl:Document owl:disjointWith tbl:Car.

Berners-Lee also argued that properties are not documents:

  owl:disjointWith tbl:Car, rdf:Property.

To state his position generally, we add an N3 rule:

  owl:disjointWith tbl:Car, rdf:Property.

 ?X log:uri [
   str:startsWith "http:";
   str:notMatches "#"
 ] }
 => { ?X a tbl:Document }.

This position is quite clearly inconsistent with the DCMI schema. Berners-Lee was unable to persuade a critical mass of the TAG to accept this position. The utility of the dublin core vocabulary was apparent and the argument against its use of hashless HTTP URIs included few practical consequences. Plus, the constraint seems to encroach on the very important principle of opacity of URIs. Any general-purpose algorithm for finding out the nature of a resource starting from only its URI is a constraint on how URIs are minted.

Further, Mark Baker asserted the right to assign any meaning at all to URIs that he owns. In particular, he said that http://markbaker.ca/ denotes his very self. To capture this position, we will borrow from the FOAF[FOAF] vocabulary:

  a foaf:Person;
  foaf:name "Mark Baker".

This is also inconsistent with Berners-Lee's position, since Berners-Lee held that foaf:Person is disjoint with tbl:Document.

Meanwhile, the DCMI showed some willingness to cooperate with those who hold that RDF Properties and web pages are disjoint; they arranged for HTTP redirections [RFC2616], rather than 200 OK repsonses, in reply to GET requests to http://purl.org/dc/elements/1.1/title.

Let's introduce some terms for discussing the HTTP protocol. To dereference http://site.example/path, it's typical to make a TCP connection to port 80 of site.example and send a GET /path request. If the reply is...

200 OK
content-type: text/plain

hello world.

Then we'll say that:

@prefix http:
@prefix mime:

_:reply1 a http:OKResponse;
  http:about <http://site.example/path>;
  mime:body "hello world.";
  mime:content-type "text/plain".

It is stipulated by all parties in the httpRange-14 discussion that in this case, the hello world. body of type text/plain is a representation of http://site.example/path. We can state the general rule as:

@prefix w:

_:m a http:OKResponse;
  http:about ?R;
  mime:body ?BYTES;
  mime:content-type ?TYPE.
} => {
?R w:representation [
  mime:content-type ?TYPE;
  mime:body ?BYTES ].

When asked if tbl:Document included the targets of http POST messages, Berners-Lee said yes, and agreed, to some extent that the that the term document is misleading. The TAG coined the term Information Resource. The term is not completely defined, but the 15 Jun 2005 decision of the TAG to address httpRange-14 says:

The TAG provides advice to the community that they may mint http URIs for any resource provided that they follow this simple rule for the sake of removing ambiguity:

We can state this formally as:

?M a http:OKResponse;
  http:about ?R.
} => { ?R a w:InformationResource }.

We consider it implicit in this decision that it applies to the case of a dereferencing an ftp: URI as well, so the following single triple states the position of the TAG quite concisely with respect to the terminology developed so far:

  rdfs:domain w:InformationResource.

Since DCMI does not claim that dc:title is a w:InformationResource, Berners-Lee is able to endorse their claim that dc:title is an rdf:Property (for example, in his tabulator implementation, announced in [BL05]) while maintaining his position that rdf:Property is disjoint from w:InformationResource.

Note that the TAG has not taken a position on whether w:InformationResource intersects with rdf:Property.They do say, Other things, such as cars and dogs [...] are resources too. They are not information resources, however [...] which strongly suggests that w:InformationResource is disjoint with foaf:Person. Since Mark Baker's server responds with ordinary 200 OK replies when asked about http://markbaker.ca/, we have:

  a w:InformationResource.

foaf:Person owl:disjointWith w:InformationResource.

So we are have an inconsisitency between the definition of w:InformationResource and Mark Baker's claim that http://markbaker.ca/ is a foaf:Person. While anyone can say anything about anything[RDFC], there are consequences to making disagreeable claims.

The value of agreement

Consider a formalized movie database, where a request to GET http://fmdb.example/title/tt0091605/ gives:

  dc:title "The Name of the Rose";
  film:rating 7.7;
  film:star <#nm0000225>.

  dca:agentName "Christian Slater".

And suppose John Doe wants a widely-understood identifier for Christian Slater so that he can use it in a description of a photo that Jim took of Christian. Jim writes, in http://photohost.example/jim/photo20.ttl:

  dc:title "Smile, Christian!";

Clearly, then, a SPARQL query[SPARQL] over the merge of those documents of the form...

SELECT ?photo, ?name
 ?photo foaf:depicts [
   dca:agentName ?name

...will return:

name "Christian Slater"

But suppose, meanwhile, that Jim does not think The Name of the Rose merits 7.7 out of 10 stars. A SPARQL query over the same merge of those documents of the form...

SELECT ?photo ?title ?rating
 ?photo foaf:depicts _:who.
 [] dc:title ?title;
    film:star _:who;
    film:rating ?rating

...will return:

photo <http://photohost.example/jim/photo20>
title "The Name of the Rose"
rating 7.7

How can Jim reuse the information such as the name of the person depicted in his photo without implyling that the movie merits 7.7 out of 10 stars? One approach is to not merge the sources, but rather treat the fmdb data as a separate graph in the SPARQL dataset. The photo subject's name can be computed using a more explicit query:

FROM <http://photohost.example/jim/photo20.ttl>
FROM NAMED <http://fmdb.example/title/tt0091605/>
 ?photo, ?name WHERE
  { ?photo foaf:depicts ?who.
    GRAPH <http://fmdb.example/title/tt0091605/> {
      ?who dca:agentName ?name }.

This approach is akin to lifting between contexts[G91]. It is a workable apprach to integrating data from separate contexts, but it is clearly not as straightforward as merging. The cost of keeping contexts separate demonstrates that agreement is valuable. The providers of fmdb.example can lower Jim's cost to use their information if they publish film ratings that Jim agrees with.

Delegation, consent and causal chains

Jim could, of course, make up his own URI for the subject of his photo. But then Jim would have to maintain the information about the actor's name, the movies he starred in, and their titles, at his own cost. And if others in Jim's position did likewise, consumers of all this data would be able to correlate photo subjects only at a significant cost of dealing with URI aliases. We conjecture that overall utility for the community is maximized if we adopt a causal theory of reference[Kr80]. In particular:

  1. To mint a term in the community, choose a URI of the form doc#id and publish at doc some information that motivates others to use the term in a manner that is consistent with your intended meaning(s).
  2. Use of a URI of the form. doc#id implies agreement to information published at doc.

Justification of this conjecture is in progress[Co06], using terms such as intent and impact from the TAG discussion of extensibility and versioning in Edinburgh in September 2005.

Advice: Use hash URIs for properties and classes

If you want to write RDF schemas that are consistent with the TAG's position on httpRange-14, you have three options:

  1. Use the doc#id pattern as above.
  2. Set up HTTP redirects a la dc:title.
  3. Populate the intersection of w:InformationResource with rdf:Property.

The third option is like publishing movie reviews that people disagree with. The second option is more trouble than the first, unless the vocabulary you're describing is very large. So I advise the first option.

Fragments as sections vs. people

Some argue that Using # [in this way] makes it impossible to make assertions about parts of documents (e.g. Person A authored Section #3).[A04]. Indeed, this is a concern. Let's consider it formally, using FRBR[FRBR], [IFLA]. Suppose http://fansite.example/baseball is a little database of great baseball players, with some statistics on http://fansite.example/baseball#FredPatek, among others:

@prefix foaf:
@prefix baseball:

  rdfs:domain foaf:Person.

  baseball:average 0.250 .

These data might be published in RDF/XML:

    <foaf:Person rdf:ID="FredPatek">

And then someone might use the ID to say that the Fred Patek section was written by somebody named John Doe:

@prefix frbr:

 frbr:creator [ foaf:name "John Doe" ].

These two are inconsistent, since the domain of frbr:creator is frbr:Work, which is disjoint with foaf:Person, the domain of baseball:average.

So indeed, if we choose URIs of the form doc#id for people, it leads to inconsistencies with quite reasonable ontologies if we also use them as document section identifiers. I advise authors to choose one or the other for each fragment identifier they publish and be consistent.

In order for this to work with documents published both in RDF/XML and XHTML, the XHTML media type specifications may need to be ammended so that authors can opt out of the section-of-the-document meaning of fragment identifiers that they publish. For example, the profile attribute from section Meta data profiles of the HTML 4 specification[HTML4] seems like a reasonable opt-out signal.

Populating the intersection of w:InformationResource with foaf:Person, the way Mark Baker seems to, seems likely to conflict with useful and reasonable ontologies. I suggest adopting w:InformationResource rdfs:subClassOf frbr:Work as a practical constraint. The foaf:primaryTopic relationship seems particularly useful for relating web pages to things. Rather than..

<http://markbaker.ca/> a foaf:Person;
foaf:name "Mark Baker"

... I suggest:

<http://markbaker.ca/> foaf:primaryTopic
  [ foaf:Person; foaf:name "Mark Baker"].

Conclusions and Future Work

While the theory presented here gives little by way of rigorously justified theorems, we hope it gives a coherent and pragmatic approach to nagivating many issues of identity and reference in the Web.

We hope the formal analysis of the httpRange-14 discussion and decision, down to a single RDF triple, makes the issue clear without distorting the positions that it summarizes. We also hope it demonstrates the utility of RDF, turtle, and N3 as analytic tools.

The justification of the causal theory of reference is ongoing work. See, for example, Berners-Lee's Total Cost of Ontologies (TCO) argument in his ISWC 2005 presentation.

When considering which constraints need relaxing and which social norms will result in exponential growth of the Semantic Web, mechanisms for expressing trust seem to be critical. We are exploring approaches using quoting a la if http://weather.example/ny contains a formula of the form <ny#weather> nws:temp ?X then lift that claim, <ny#weather> nws:temp ?X into the knowledge base as a fact. We are exploring these quoting techniques along with digital signature and proof exchange as a general-purpose trust infrastructure. These explorations suggest that the law of the excluded middle, i.e that every formula is either true or false, should be relaxed in order to allow indirect self-reference. Constructive proofs seem more promising than those that use classical fist order reasoning[Co06b].


Kripke, Saul. 1980. Naming and Necessity. Cambridge, Mass.: Harvard University Press.
Grice, Paul, (1989), Studies in the Way of Words, Harvard University Press.
Guha, R. V. Contexts: A Formalization and Some Applications Stanford Computer Science Department STAN-CS-91-1399 1991 Stanford, California
Berners-Lee, Tim and Cailliau, Robert and Groff, Jean-François and Pollermann, Bernd World-Wide Web: The Information Universe in Electronic Networking: Research, Applications and Policy 1 2 (Meckler, Westport CT, USA, 1992) pages 74--82
Michael R. Genesereth and Richard E. Fikes Knowledge Interchange Format (KIF) Version 3.0 Reference Manual Logic-92-1, Computer Science Department, Stanford University, 1992, Stanford, California,
K.G. Saur IFLA Study Group on the Functional Requirements for Bibliographic Records. Functional Requirements for Bibliographic Records: Final Report. UBCIM Publications-New Series. Vol. 19, Munchen: , 1998.
Fielding, Roy and Gettys, Jim and Mogul, Jeff and Frystyk, Henrik and Masinter, Larry and Leach, Paul and Berners-Lee, Tim Hypertext Transfer Protocol -- HTTP/1.1 IETF RFC2616 June 1999
Le Hors, Arnaud and Raggett, Dave and Jacobs, Ian Editors, HTML 4.01 Specification , W3C Recommendation, 24 December 1999, http://www.w3.org/TR/1999/REC-html401-19991224 . Latest version available at http://www.w3.org/TR/html401 .
Edd Dumbill. Finding friends with XML and RDF 01 Jun 2002 IBM developerworks
Anonymous HashURI 2003-04-03 19:50:51
Tim Berners-Lee and Dan Connolly and Sandro Hawke. Semantic Web Tutorial Using N3 Budapest 2003
Dave Beckett. Turtle - Terse RDF Triple Language work in progress 2003-2006
Dean, Mike and Schreiber, Guus OWL Web Ontology Language Reference http://www.w3.org/TR/2004/REC-owl-ref-20040210/ W3C Recommendation 10 February 2004
Architecture of the World Wide Web, Volume One , Jacobs, Ian and Walsh, Norman, W3C Recommendation, 15 December 2004, http://www.w3.org/TR/2004/REC-webarch-20041215/ . Latest version available at http://www.w3.org/TR/webarch/ .
Richard Newman and Ian Davis. Expression of Core FRBR Concepts in RDF 2005
T. Berners-Lee Links on the Semantic Web 2005-12-30
Connolly, Dan Using RDF and OWL to model language evolution, Feb 2006
Connolly, Dan Investigating logical reflection, constructive proof, and explicit provability Feb 2006.
Seaborne, Andy and Prud'hommeaux, Eric SPARQL Query Language for RDF , W3C Candidate Recommendation (work in progress), 6 April 2006, http://www.w3.org/TR/2006/CR-rdf-sparql-query-20060406/. Latest version available at http://www.w3.org/TR/rdf-sparql-query/ .
Carroll, J. J. and Klyne, G. Editors Resource Description Framework (RDF): Concepts and Abstract Syntax , W3C Recommendation, 10 February 2004, http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/ . Latest version available at http://www.w3.org/TR/rdf-concepts/ .