However: Ontologies are Hard!

• Hard to implement a full ontology management system
  • may be superfluous for some applications
• Hence the "onion" model of increasingly complex specs:
  • no property expressions or datatypes in RDF Schemas
  • not all set operators, restricted cardinality in OWL Lite
  • full expressive power in OWL Full (no computational guarantee)
    • classes can themselves be instances
    • classes of properties can be defined
    • statements on properties are possible
• "Lite" < "DL" < "Full", but not completely true for RDFS
  • RDFS is "almost" a subcategory
  • not all RDFS statements are valid in DL...
  • ...but they are for Full
• Applications may take what they really need!