- From: Jeremy Carroll <jjc@hplb.hpl.hp.com>
- Date: Mon, 22 Apr 2002 17:28:11 +0100
- To: "Pat Hayes" <phayes@ai.uwf.edu>
- Cc: <www-webont-wg@w3.org>
We had what I found to be a helpful discussion after the RDF Core telecon on dark triples. It made me reconsider what are the drivers for the problem. Previously I posted the following Problem Statement [1], and had a positive ack from Pat. >Problem statement: > >Layering on top of RDF involves using graph syntax to encode OWL concepts. >If such graph syntax is asserted then the RDF model theory assigns meaning >to such syntax specifically denoting things in the domain of discourse that >correspond to the syntactic expressions of OWL. Given this, an OWL model >theory with appropriate entailments (in which the theory of classes in OWL >is recognisable as a subtheory of some well known set theory) is difficult >and problematic to construct. I have a feeling that this does not do justice to the divide between two different schools of thought, roughly of "computer scientists" vs "mathematicians" (or perhaps "hackers" vs "theoreticians"). At some level we seem to touch on one of those religious wars which DAML+OIL delicately balances. Using a langauge as its own metalanguage is known to be impossible. Hence, the theoretically minded don't even try. Hackers see it as a challenge :). RDF M&S follows the hackers' tradition. In as much as RDF metatheory can be encoded in RDF, RDF M&S tries. In fact, it seems to go too far. e.g. the notion of reification is a mess in M&S. The model theory for RDF (edited by Pat) superbly captures RDF M&S intent, by having properties that have special semantics according to the theory, within the model, also behaving as normal properties. This is unusual from the point of view of language design, particularly mathematical language design. A type statement in a language is typically very different from a user-defined property; and the ability to take a subproperty of the type property (in RDFS) is surprising. Thus RDFS uses the technique of having "things in the domain of discourse that correspond to the [special] syntactic expressions". My view is that the RDF Model Theory shows that, when done well, this technique does allow a significant portion of the metalanguage to be encoded in the language; which may make it easier to extend. This also satifies the hacker community "eat your own dog food" mantra. Essentially I am now reading the problem statement as the WebOnt WG being unhappy with that game plan because: - there is theoretical risk of inconsistency etc. (Illustrated by the Patel-Schneider paradox) - the existensability is not viewed as valuable (I would not be surprised if some members see it as a liability) I think the original paragraph [2] from our f2f which called out "entailments" as an issue, was probably misleading. I do not believe that identifying the desired entailments and a solution like [3] will meet the needs of Pat and Peter. I am currently seeing them as needing a clear separation between OWL syntax and OWL semantics; which is fundamentally obstructed by the use of asserted triples. DAML+OIL appears to be at a very delicate compromise between these two opposed positions. The axiomatics semantics is on my reading consistent with encoding language significant syntactic structures in the semantics. The model theory, in contrast, seems to go from the graph syntax to the model, without going through the image of the graph syntax in the model. There is no daml:Ontology for DAML+OIL (although the axiomatic semantics suggests that there could be one). Jeremy [1] Jeremy's summary of Problem http://lists.w3.org/Archives/Public/www-webont-wg/2002Apr/0128.html [2] Amsterdam paragraph http://lists.w3.org/Archives/Public/www-webont-wg/2002Apr/0164.html [3] A non-dark triples "solution" http://lists.w3.org/Archives/Public/www-webont-wg/2002Apr/0155.html
Received on Monday, 22 April 2002 12:28:53 UTC