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Re: Logic and Using The Semantic Web Toolbox

From: pat hayes <phayes@ai.uwf.edu>
Date: Mon, 11 Dec 2000 18:13:24 -0600
Message-Id: <v04210101b65b0adac114@[205.160.76.246]>
To: Ora Lassila <daml@lassila.org>
Cc: www-rdf-logic@w3.org
Hi Ora
You wrote:
>pat hayes wrote:
> > I understood that the central role of
> > reification in RDF arose from a perception that such importation of
> > external concepts was going to be done by the use of reification
> > itself: that reification provided some kind of general-purpose
> > mechanism for semantic extensibility.
>
>This is not the case. Who gave you that idea? (or maybe I just do not
>understand what you mean :-).

I was left with this impression after various conversations with 
Stefan Decker and Dan Connelly, from reading the W3C documents 
concerning RDF,  Tim Berners-Lee's writings, and from subsequent 
discussion concerning the contraints on DAML which arise from its 
having to be RDF-compliant, and from reading various discussions by 
various people on the RDF email archives. For example, Stefan and Dan 
have both, at different times, seemed to be saying that negation 
(andindeed all of KIF) could be imported into RDF by the use of 
reification; I believe that Stefan has claimed that by virtue of 
having reification, RDF in effect *already contains* all of KIF.

But I may have misunderstood what they were saying, and they are not 
responsible, of course, for any of my misunderstandings.

> > If this is not the case, there
> > seems to be little reason to have it in the language (?).
>
>We did have (and still do) applications which need reification, given
>that it is the only vehicle we have (in RDF) that allows us to
>distinguish between something been asserted about an object vs.
>something been asserted about a statement.

Ah, I discern a ray of hope. Am I correct, then, in thinking that by 
asserting a reified description of a triple (that is, the four 
triples which describe it as being a triple plus giving its three 
components), one is NOT thereby also asserting the triple itself? 
That would greatly simplify the semantics of RDF, certainly.

But if this is correct, then what exactly IS asserted by the 
assertion of a reified description? If it is asserting that the 
described triple exists, then there is literally no point in saying 
that. It would be like requiring elephants to go around with labels 
saying they existed. Look, consider the triple being described by the 
reification, and ask whether that triple exists (in the sense 
required by the semantics of reification, whatever that is.) Either 
it does or it doesn't. If it doesn't, then the reified assertion 
(that is, the assertion of the reified description) is simply false; 
and if it does, what is the point of asserting that something exists 
when this fact is evidently true, from the simple fact of it being 
present? (Or, if the existence referred to is of a more Platonic 
kind, by the simple fact of it being grammatical.)

A metalanguage may be useful to talk about the expressions of a 
language, but only when it is possible to use it to say something 
nontrivial about them. As I understand RDF, every single atomic 
component of any complex expression is asserted; so it is impossible 
to every assert, for example, a conditional (if A then B) in RDF, 
since the very act of writing this down would entail asserting A as 
well as asserting B, which means in effect that the only form of 
propositional combination is conjunction. Given this, there wouldnt 
seem to be any useful extra functionality to be obtained from the use 
of a reified description, since all it can do is to either be 
conjoined to the thing whose existence it is asserting (in which case 
it is redundant) or not to be (in which case it is false.)

Perhaps an actual example of the intended use of reification would 
help make this clearer. You referred to some: can you cite or point 
to where they are described?

Pat Hayes

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Received on Monday, 11 December 2000 19:11:48 GMT

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