- From: Michael Schneider <schneid@fzi.de>
- Date: Tue, 8 May 2012 10:38:58 +0200
- To: "Evain, Jean-Pierre" <evain@ebu.ch>
- CC: 'Ivan Herman' <ivan@w3.org>, Bijan Parsia <bparsia@cs.man.ac.uk>, Ian Horrocks <ian.horrocks@cs.ox.ac.uk>, "public-owl-wg@w3.org" <public-owl-wg@w3.org>, "Public-Rif-Wg (E-mail)" <public-rif-wg@w3.org>, "Peter F. Patel-Schneider" <pfpschneider@gmail.com>, Sandro Hawke <sandro@w3.org>

Hi Jean-Pierre! Am 08.05.2012 09:25, schrieb Evain, Jean-Pierre: > I understand the point which is being made being what is useful for reasoning or not. Replace "useful for reasoning" by "required for the well-definedness of the semantics" of OWL 2 or RIF! It is a basic technical requirement for the specifications of these languages that for every syntactically well-formed expression (aka an OWL 2 ontology or a RIF rule set), the semantic meaning can be determined by mathematical means. At a minimum, for OWL 2 and RIF, this means that it can always be determined whether an input ontology is satisfiable or not, or whether one given ontology entails another given one or not. Only in the cases of OWL 2 DL and its profiles, it is an additional requirement (by design) that there are reasoning procedures that are able to do these determinations in an automated way for all input, because these languages are required to be computationally decidable. But having a well-defined semantics is always needed. Clearly, if there are ontologies for which it cannot uniquely be deduced (mathematically) whether they are satisfiable or not, a reasoner cannot give the "right" reasoning result for them, because it cannot then be determined whether it's answer is right or not, or just one correct answer out of many. To illustrate this problem, take the case of xsd:duration in its definition as of the time of finalizing OWL 2, where each literal of xsd:duration would essentially denote a pair (m, s) consisting of a certain number m of month plus a certain number s of seconds. Let there be two such durations: d1 := (2, 0) d2 := (1, 30*24*60*60) Now, depending on what is meant by "a month", these two durations can represent either (i) the same value (if a month has 30 days), or (2) d1 can be greater than d2 (if a month has 31 days), or (3) d1 is smaller than d2 (if a month has, say, 28 days = 4 weeks). I may well have missed a precise definition of "a month" in the (newest version of the) XSD spec, in which case the above example may be void. But if not, then it is clear that any OWL 2 (+xsd:duration) ontology for which the question of satisfiability depends on whether the above two durations are the same or not, or which of them is greater, does not have a uniquely defined semantic meaning. An example for the need of being able to determine whether equality between two duration values holds or not would be an ontology with data enumerations consisting of duration values (denoted by "d1" and "d2", as defined above, but in a real ontology one would use their correct literal form, of course): :D a rdfs:Datatype ; owl:oneOf ( d1 ) . :dp a owl:DatatypeProperty ; rdfs:range :D . :s :dp d2 . This set of axioms should be satisfiable if and only if d2 = d1, because only in this case, the object d2 of the property assertion (last statement) would denote an instance of the singleton datatype :D = {d1}. But if it cannot be determined whether d2 equals d1 or not, then it cannot be determined whether the axiom set is satisfiable or not. An example for the need of comparison of two durations (greater or lesser than) could be constructed from the use of OWL 2 datatype restrictions. > But does that mean that all other information is garbage? Everyone can say everything about everything. But an OWL 2 or RIF or whatever language specification with a formal semantics at its core would, if not well-defined, IMO count as just that: garbage. :-) Best, Michael -- ......................................................... Dipl.-Inform. Michael Schneider Research Scientist, IPE / WIM FZI Forschungszentrum Informatik Haid-und-Neu-Str. 10–14 76131 Karlsruhe, Germany Tel.: +49 721 9654-726 Fax: +49 721 9654-727 michael.schneider@fzi.de www.fzi.de ......................................................... Forschungszentrum Informatik (FZI) an der Universität Karlsruhe Stiftung des bürgerlichen Rechts Stiftung Az: 14-0563.1 Regierungspräsidium Karlsruhe Vorstand: Dipl. Wi.-Ing. Michael Flor, Prof. Dr. Ralf Reussner, Prof. Dr. Rudi Studer, Prof. Dr.-Ing. J. Marius Zöllner Vorsitzender des Kuratoriums: Ministerialdirigent Günther Leßnerkraus .........................................................

Received on Tuesday, 8 May 2012 08:39:45 UTC