Greetings. This is a heads-up message to let y'all know what Guha and I have been up to this last week or so, and to ask for the WGs co-operation. We should be able to give you all a much more complete version in time for the Bristol F2F, including draft documents in a couple of days.

Currently, each web language (RDF, RDFS, DAML+OIL, OWL,...) has its own semantics and its own syntax. The various groups have succeeded in mapping all the various syntaxes into RDF triples, because this requirement was chartered; but the various model theories have not been aligned so successfully because, frankly, that is more difficult to do, and unexpected problems of 'layering' have arisen and have proved very hard to resolve. However, this situation seems peculiar when looked at from a logical perspective, since these various languages are all pretty much sub-languages of a very well-understood 'textbook' logic with a very simple and  uncontroversial semantics, which is classical first-order logic. So Guha has proposed that we make this intuitive mapping into a part of the formal specifications, in an effort to produce some overall coherence to the various web languages in use now and in the future.  (This idea is based on earlier work in AI in the 1980s, when an explosion of alternative knowledge representation formalisms were being invented, and in spite of their various pragmatic or philosophical goals, the relationships between them were clarified by mapping them all into first-order logic. Even those aspects which did not map into FOL - and there were some, particularly to do with nonmonotonicity - were clarified by the exercise, in fact. Guha and I were both influential in that effort: me by mapping frame languages and 'procedural' Krep languages into FOL, he by mapping CYC into FOL.)

So, the proposal is to write two new documents and to slightly modify the RDF MT document. The modifications to the MT will amount to little more than a revised introduction which points out that the other two documents, taken together, amount to an alternative way of describing RDF semantics which is entirely formally equivalent to this one, but may have certain advantages for some developers. (There is also a related proposal to allow unasserted triples, in a way that overcomes the nonmonotonicity issue that the 'owl:Dark' proposal raised. Details of that will follow in later messages.)

The other two documents are (1) a description of the overall technique, including a description of the semantic language Lbase, a model theory for it and a general discussion of how to use it to provide semantics for other languages) , and (2) a document using this technique to give a semantics for RDF, and relating that to the RDF MT, proving that they are equivalent. Another way to think about this second document is that it provides an alternative way of describing the RDF MT which will be accessible to anyone with a background in logic, and which will also provide machine-checkable renderings of RDF meaning in a general-purpose framework which allows RDF formal content to be connected to, for example, DAML+OIL and OWL formal content in a uniform framework.

Guha and I are writing the first of these in any case, but we believe that it would be very useful and beneficial in the long run if the 'official' account of RDF also incorporated the second document, which we should have drafted in a few days. I would like therefore to place this on the agenda for the F2F. If there are any questions that people want answered before the F2F, feel free to email.

Since the Lbase RDF(S) semantics and the RDF(S) MT align exactly (you get the same interpretations and the same entailments either way) it would be possible to merge the two documents into a single document which gave both versions of RDF semantics in parallel. This was Guha's original idea; but after trying to do it, my own preference is to have two documents, for several reasons. At this stage it is rather traumatic to make such a major change to the MT document, and would probably take longer than writing a new document from scratch; the parallel development is likely to be confusing to readers; the Lbase semantics requires one to understand a more complex model theory than the RDF MT alone does; and the two-document presentation allows the MT document to continue to be phrased as a kind of intro-to-MT for non-logicians, while the RDF-in-Lbase document can be written more tersely for readers who are already logic-savvy. (It will also have much shorter proofs of the lemmas, by the way, since it can appeal to established results like Herbrand's theorem and compactness.) I hope the WG approves of this decision, which (in spite of the extra document in the spec) I think provides the widest utility to the largest number of readers and at the same time provides a firm link between the RDF spec (which once the WG has finished with it will be cast in stone) and what will, we hope, become the standard way to relate content between different semweb languages.

We hope to have a draft version of both documents (largely assembled from cut/paste of existing text) by early in the week.

I apologize that the re-drafting of the MT document has been delayed by this. The likeliest prediction is now Tuesday. Since the changes will all be intro. text, minor editorials and bug-fixing, I hope this will not pose any severe problems for the F2F.

Pat Hayes