- From: Dan Connolly <connolly@w3.org>
- Date: Mon, 24 Nov 2003 17:59:53 -0600
- To: Drew McDermott <drew.mcdermott@yale.edu>
- Cc: www-rdf-rules@w3.org
On Mon, 2003-11-24 at 16:27, Drew McDermott wrote:
> [Dan Connolly]
> I've found cwm's log:includes and log:notIncludes mechanisms
> http://www.w3.org/2000/10/swap/doc/Reach
> useful for building a number of applications. I've tried
> to figure out what it corresponds do in other systems, but
> I'm not having much luck. It can't really be all that
> novel, can it?
>
> Maybe other people thought about it, got nervous, and backed off!
Hmm... perhaps, but I'm getting the impression from elsewhere
in this thread that such mechanisms are old-hat.
> It's hard to give a precise semantics to log:includes because you're
> using the embedded formula in a twilight-zone way:
yes, twilight-zone is the feeling I mostly get when I try to
reconcile cwm's design with the logic literature I read, but...
>
> Here's one of your examples:
>
> this log:forAll :x.
> {:x :homePage log:includes { :x a :Vegetarian }}
> => { :x a :Vegetarian}.
>
> The variable :x is quantified over objects,
... actually, the best guess I have for N3 semantics
is that :x is quantified over terms.
> but the intended meaning
> of {A log:includes B} appears to be that B is found as an element of
> the document A.
log:includes corresponds quite nicely to the 'simple entailment'
notion from the RDF semantics. i.e. F log:includes G iff
G can be obtained from F by erasure and existential introduction.
(F and G range over formulas).
> If that means that the formula B literally occurs as
> a triple in A, then we have to explain how to build a formula from an
> object and a formula with a variable to be bound to that object.
>
> I'm guessing, from the context, that the "semantic :homepage" of a
> person is the assertion (a big conjunction, presumably) obtained by
> parsing his or her webpage.
yes.
> So the rule is actually not about
> triples, but instead means:
>
> Forall :x
> If the assertion made by :x's home page
> entails that :x is a vegetarian
> then :x is a vegetarian
>
> How is "entails" defined?
In this case, as per RDF semantics. Existential-conjunctive logic.
> I believe there are problems with allowing
> a predicate like this in a language (did Montagu write some papers on
> this?),
Yes, I'm still working my way thru various citations that
have been thrown my way...
> but even putting the possibility of paradox aside, don't we
> have the problem that entailment includes the use of rules just like
> this one?
not unless we say so explictly, using log:conclusion.
(hmm... is that covered in the tutorial? I don't
see it...)
> We can't define entailment without a big fixed-point
> construction of the sort beloved of nonmonotonic logicians. Does the
> CWM inference engine attempt to embody that construction?
I'm not sure. I don't think so.
> Of course we get apparent weirdness such as
>
> Fred's SW homepage:
> {Sally :homePage log:notIncludes {Fred a :Vegetarian}}
> => (Fred a :Carnetarian}
>
> Sally's SW homepage:
> {Fred :homePage log:includes {Fred a :Carnetarian}}
> => {Fred a :Vegetarian}
>
> where :Vegetarian and :Carnetarian are disjoint. But we can just say
> the two pages have multiple fixed points, and therefore imply nothing
> about what Fred eats.
Yeah, I don't see much of a problem from that example.
>
> -- Drew
--
Dan Connolly, W3C http://www.w3.org/People/Connolly/
Received on Monday, 24 November 2003 18:59:55 UTC