- From: Smith, Michael K <michael.smith@eds.com>
- Date: Mon, 29 Apr 2002 08:57:24 -0500
- To: Jeremy Carroll <jjc@hplb.hpl.hp.com>, www-webont-wg@w3.org
Jeremy, Re your response to http://lists.w3.org/Archives/Public/www-webont-wg/2002Apr/0200.html > I comment on whether <false> is a useful RDF graph, extending > the analysis. There is a really low-level variance in viewpoint here, one that I have had previously with other WG members. When we talk about the syntax of OWL and RDF, I take that to mean the grammar that defines the sentences that the semantics will provide a meaning for. The whole point of a syntax is to identify a subset the universal set of character strings that we are willing to provide a semantic interpretation for. The abstract syntax is an abstract version of the same thing. You argue that: > I see your first graph as having exactly the same status in > OWL as any other OWL contradictory graph: e.g. > > a owl:differentIndividualFrom a . I would argue that x rdf:_1 a x rdf:_2 OWL:NIL x rdf:_3 b is quite different from a owl:differentIndividualFrom a The second is clearly an OWL statement (albeit translated to triple notation). Asking OWL to interpret the ill-formed OWL list with 'b' after 'OWL:NIL' is like asking the RDF semantics to provide an interpretation for y rdf:type rdf:type z This is not an RDF triple. It has no interpretation in the RDF MT. It is simply not a statement in RDF. Just as the list, '(a OWL:NIL b)' is not an OWL syntactic component. We can't say it in OWL. And are thus not obligated to try to provide a semantic interpretation for it. > I think of these as simply false (in OWL). In terms of the > mappings T and TINV this is special. TINV of either of your > RDF graph above yields (owl)false. Then T((owl)false) is > (rdf)false. This is the only way of getting falsity in RDF, > by using a layer with the concept of contradiction. Neither T nor TINV have anything to do with truth or falsity. They take one syntax to the other and back. > In terms of Mike's discussion of: > > >SEMANTIC PROPERTY 1. > > > If RDF-ENTAILS(c,d) then OWL-ENTAILs(TINV(c),TINV(d)) > > and > > >SEMANTIC PROPERTY 2. > > > If RDF-ENTAILS(T(a),d) then OWL-ENTAILS(a,TINV(d)) > > I think my analysis gives practically the same results, although I support > (SP1) & (SP2) rather than just (SP2). The difference is in the cases where > Mike's analysis does not allow (SP1) my analysis says TINV(c) = false, and > (SP1) holds trivially. > > It's a different way of looking at it; quite probably not important. In a previous message http://lists.w3.org/Archives/Public/www-webont-wg/2002Apr/0320.html I gave a concrete example of why Semantic Property 1 cannot possibly hold if we support one interpretation of dark elements. So I would argue that there is at least a potentially important difference here. I believe (without proof) that if TINV may in principle drop RDF information that does not have an interpretation in OWL, that there will always be a way to defeat Sematic Property 1. Remember the high level language/Assembler comparison. In assembler we have information about where in memory values are laid out. We can say with certainty, given a particular memory layout, that if we step past the end of the memory containing array A (by exceeding the array size with an index) then we will overwrite the memory representing B. We could actually prove, in a language that did not guarantee array bounds checks, that the assembler version of a[a.size] := 1 would set b to 1, assuming 0 based indices. We cannot prove this at the higher level, we don't have the information that we need. - Mike Michael K. Smith EDS Austin Innovation Centre 98 San Jacinto, #500 Austin, TX 78701 512 404-6683
Received on Monday, 29 April 2002 09:59:45 UTC