A rule language for the semantic web

1. A rule language for the semantic web Dieter Fensel, Lausanne, June 14, 2004 SDK cluster meeting on WSMO

2. Language Penal • This panel is about the discussion on the further development of OWL • OWL is not the end-point of the semantic web • OWL will be extended / complemented by languages that employ the potential of the Horn logic fragment of first order logic.

3. An example for this: SWRL SWRL: A Semantic Web Rule Language Combining OWL and RuleML W3C Member Submission 21 May 2004 This version: http://www.w3.org/Submission/2004/SUBM-SWRL-20040521/ Authors: Ian Horrocks, Network Inference Peter F. Patel-Schneider, Bell Labs Research, Lucent Technologies Harold Boley, National Research Council of Canada Said Tabet, Macgregor, Inc. Benjamin Grosof, Sloan School of Management, MIT Mike Dean, BBN Technologies

4. Language Panelist • We have the pleasure to have four world-leading experts here to discuss issues related to this. • Guus Schreiber, expert on conceptual modeling languages • Ian Horrocks expert on Description Logic • Michael Kiefer expert on rule languages • Dieter Fensel (panel chair) expert on scientific marketing

5. Six opinions of Dieter Fensel • DL is an interesting subset of 1st order logic • HL is an interesting subset of 1st order logic • Full 1st order logic is an interesting language • Pure logics are cumbersome tools for modeling • SWRL is crap • A useful rule language for the semantic web must follow a DL-minimalistic approach

6. (1) DL is interesting • because of its decidability/tractability properties • spoken for the man on the street it is the fragment of logic that can be computationally explored around the existential quantifier. • it is therefore worth to become standardized in OWL • besides the fact that I do not like at all OWL-Lite which is an overkill, • a smooth extension of RDFS would have been much more appropriate.

7. (2) HL is interesting • because of its decidability/tractability properties • Spoken for the man on the street it is the fragment of logic that can be computationally explored around the all quantifier • without function symbols it is decidable and • with minimal-model semantics function symbols, non-monotinicity of negation, and even transitive closure (an extension of first order logic) can be expressed. • It is therefore worth to become standardized by W3C, however, not in a way that destroy all features of rule languages by mixing it up too much with DL type of expressions.

8. (3) First order logic is interesting • In oposition to DLs and HLs it provides significant more flexibility in writing down required axioms. • This language is not fully mechanizable in terms ofreasoning support, however, • many interesting theorems can be proven and the theorem prover community has made significant progress over the last years. • First order language could define the common umbrella, where DLs and HLs are sublanguages and unified.

9. (4) Pure logics are a cumbersome tool for modeling • Therefore, extensions like Flogic that allow quantification over classes and attributes are that helpful. • W3C recommendations should care for this if they want to make their languages widely adopted.

10. (5) I strongly dislike SWRL • since it spoils the nice properties of rule languages for the price of defining the rule language as an extension of DLs. • SWRL is meaningless since it defines a syntactical restriction of 1st order logic without any computational justification. • SWRL is as undecidable and untractable as first order logic. • Therefore, it is meaningless to syntactically restrict first order logic in that way.

11. (6) A useful rule language for the semantic web must follow a different approach • It should emply the full power of the HL fragment and include DL features only in case they do not harm this language type. • Therefore, a HL language should be defined as an extension of RDFS and/or OWL-Lite (where OWL Lite is reduced to an actual lite subset of DL).

12. The floor is yours