- From: Pierre-Antoine Champin <swlists-040405@champin.net>
- Date: Mon, 02 Oct 2006 11:18:28 +0200
- To: Yoshio Fukushige <fukushige.yoshio@jp.panasonic.com>
- CC: semantic-web@w3.org
Yoshio Fukushige a écrit : > Hello, > > I've seen many documents saying "OWL FULL is undecidable" ... (a) > > But which of the following does (a) mean?: > > (1) "There is no system which is decidable where there is a Class, say ClassA, > that is an instance of another Class." > > or > > (2)"There is at least one system which is undecidable where/because there is a Class, say ClassA, > that is an instance of another Class. > i.e. Not every system with a vocabulary in OWL FULL is decidable." Short answer: (2), because RDFS is decidable, and definitely allows ClassA to be an instance as well. Long answer: "OWL FULL is undecidable" is actually a short (and unprecise) for "the problem of entailment is undecidable in OWL FULL", i.e. there is no algorithm which can tell us *for sure* (i.e. sound and complete) whether an arbitrary OWL FULL graph is a consequence of another arbitrary OWL FULL graph. That does not mean that it is impossible to do anything with OWL FULL. Pellet is able to make some inferences whith OWL FULL ontologies; there is just no guarantee on that. This can neverthess be useful in some situations. Note also that extended expressiveness (like the ability to make a class an instance of another class) is not the only/main reason of the undecidability of OWL FULL: another problem is that some expressions in OWL FULL have *no* formal semantics at all. E.g. [ a owl:Restriction ; owl:onProperty :child ; owl:minCardinality 2 ; owl:minCardinality 4 ] is valid in OWL FULL. Reject the intuition telling you that it is the class of things having between 2 and 4 children, because it is *not* what that means. It could as well mean that any instance having at least 2 children is entailed to also have at most 4 children (because the two possible restrictions described by that construct are then bound to be equivalent). So there are several possible intuitive meanings to that, the editors of OWL didn't choose any. hope this helps pa
Received on Monday, 2 October 2006 09:18:42 UTC