- From: pat hayes <phayes@ai.uwf.edu>
- Date: Wed, 16 May 2001 19:54:19 -0500
- To: Dan Brickley <danbri@w3.org>
- Cc: Jonathan Borden <jborden@mediaone.net>, <www-rdf-logic@w3.org>
Hi Dan > > >pat hayes wrote: > >[...] > > So it has no semantics at all? I doubt if that is really what you > > mean, since then it would have no point to it, as far as I can see. > > Certainly the RDF literature seems to *want* RDF to actually mean > > something. > >I consider RDF content to be meaningful largely by proxy: RDF >piggybacks on URIs, which are meaningful by virtue of >a quasi-mystical social/legal process involving the devolution of naming >rights (via various URI schemes) to individuals and organisations. I'm not >sure how this aspect of meaning (ie. theories of reference) fits with the >DAML+OIL formalisms (the Model-Theoretic and the Axiomatic semantics). The >DAML+OIL documents provide (very useful) rules for inferring >DAML+OIL expressions from other DAML+OIL expressions, but are rather >quiet on the grounding of those structures in social / legal reality. Logic has little, if anying, to say about social/legal reality, indeed. (If you want to get involved with that area, seems to me that y'all ought to be recruiting lawyers and sociologists to be in RDF-Core..?) >http://www.daml.org/2001/03/model-theoretic-semantics.html says "This >document ignores all aspects of naming" while appealing to a notion of >DAML+OIL semantic structures being _true_ ("A semantic structure ><AD,IC,IO,IR> is a model for the DAML+OIL ontology if the constraints > resulting from the mappings from the ontology are true in the >structure."). That is true *in the structure*, notice. Do not read that as meaning 'true' simpliciter, ie true in the real world. Logical semantics - model theory - does not purport to be able to say anything much about the actual, real, world. Which is entirely appropriate, since its purpose is to analyse the validity of inference processes, and that depends not on what is true (in this world) but on what could possibly be true, given the semantic rules of the language. The fact that P can be inferred correctly from (P & Q) is entirely independent of what P and Q *actually* mean: they could mean naything, and it would still be valid. It would be valid even if P were false. One way to think about this is to regard the logical semantics (the model theory) as a process by which meaning is carried from small, atomic, basic expressions - names, typically - and transferred to large, complex, expressions. (In case Uche Ogbuji ever reads this, I should maybe explain that by "expression" here I mean to refer not just to surface syntax, but any structured representation of propositional content. KIF expressions are LISP Sexpressions, for example, rather than strings, and McCarthy's abstract syntax provides a very abstract characterisation of logical syntax independently of the details of surface rendering. RDF triples for example are 'expressions' in this sense.) It defines the rules of meaning composition. All the machinery of inferential logic depends on how this composing of meaning is done, and all the issues of designing inference engines, strategies for determining meanings, and so on, depend on it also. The validity of the inference machinery is usually independent of the base meanings, which is just as it should be, of course: one wouldnt usually say that the particular referents of names are a matter for *logical* analysis. So there is usually no particular role in logical langauge design for the details of the 'base' meanings. Thus the model-theoretic semantics for DAML just says that an interpretation is defined by three mappings from the basic symbols to three classes of entities. It says almost nothing about those entities or those mappings: not because that is unimportant, but in order that the theory can be applied as widely as possible. Whatever the names denote, this is saying, the grammatical/logical machinery of the language will work in the same way. So I think that your concerns - which seem to be entirely focussed on the base meanings, the referents of the logical names, which here are URIs - and our concerns, are not opposed, but nicely complement each other. If we can find a way to put them together, we will have a much more powerful theory. >I find it difficult to understand how the truth >condidtions for DAML+OIL expressions can in practice be usefully reasoned >about by Web applications unless DAML+OIL acquires a theory of reference >/ naming. Without some specification that tells us what (if anything) a >DAML+OIL expression *refers to*, DAML+OIL content will be meaningless. No, since DAML+OIL largely talks about classes and their relationships. One can make true assertions about classes without knowing exactly what the classes contain, or even without exactly identifying the classes. For example, I might draw some conclusion about the class of all people who have at least two children who work for Boeing. I have no idea who is actually in this class, and I might not need to know that. Many representational languages are able to express useful information without fully identifying the things they are talking about. >Maybe I'm misreading the specs, but an account of meaning that ignores >naming / reference is missing something. (Maybe KIF has an attempt at >this? I should probably re-read the KIF docs...) KIF uses exactly the same approach to model theory. All logical and representational languages do, in fact. I agree something is missing, if you want an formal account of reference. But are you sure that you really do want that, especially since it seems to be embedded in a kind of a quasi-mystical social/legal cloud that seems likely to resist any kind of precise analysis? Let me ask you in return: what extra leverage would a theory of reference get you, compared to a model theory? In a word, what do you see it as being useful FOR? >I'm not complaining about DAML, and I'm not claiming that RFC 2396 (the >URI syntax spec) embodies an adequate theory of reference. Just that we're >in the same boat, in that there's an aspect to meaning (ie. reference) >that all these specs have a problem with. If they were expected to deal with it then they would have a problem, I agree. Also, this is not an aspect that logical semantic theories are likely to be much direct help with. You are right to point out that the model-theoretic semantics of DAML (or any other assertional langauge) says very little about reference. Reference and naming is not a topic that logic has much to say about: model theory treats names simply as symbols that denote, and says nothing about *how* they denote, or how their denotation is supposed to be determined. To get back to the orginal discussion, when I was talking about the semantics of RDF, I meant it in the sense of logical , perhaps 'structural' semantics: that is, given that the basic symbols denote something (by some means we need not go into for now), what determines the meanings of more complex RDF expressions? You refer to 'piggybacks', which is fine, but I want to know the piggybacking rules. Simple triples seem easy enough, but it isnt entirely clear whether or not relations can be objects (if so, the model theory required is going to be a bit more complicated) and it isnt clear what the anonymous nodes range over (I see they are like existential quantifiers, but what domain do those quantifiers range over? A lot turns on the answer, if that domain contains relations). (It is a bit worrying when one sees triples being used to encode things that seem to have no obvious relational interpretation, but never mind....) And when reification appears, it seems that we have to put the triples themselves into the domain. What *is* a triple, then? What kinds of properties does it have? Are the triples in the domain of quantification the same triples used in RDF syntax, or are they abstract things denoted by the RDF syntax? And so on... those are the kinds of questions that I want to see answered. (Its not that it is particularly hard to answer these questions, by the way, only that someone in a position of authority needs to provide them, just to keep things clear, and the current picture seems to be confused on just what the answers are. Some people have sharply clear views on some of these question, but others have different, sharply clear views.) Notice that the basic terms can denote anything (any 'resource'), in this discussion: this is orthogonal to a theory of reference. BTW, the utility of doing this stuff is not just to keep some crabby Floridian ex-logician happy. The model theory provides the mathematical machinery that enables people to make very precise statements of just how expressive the formalism is and isn't, for example. Pat --------------------------------------------------------------------- IHMC (850)434 8903 home 40 South Alcaniz St. (850)202 4416 office Pensacola, FL 32501 (850)202 4440 fax phayes@ai.uwf.edu http://www.coginst.uwf.edu/~phayes
Received on Wednesday, 16 May 2001 20:54:17 UTC