Re: Semantics, in particular DAML+OIL semantics

On August 12, Bob MacGregor writes:
> 
> From: "Ian Horrocks" <horrocks@cs.man.ac.uk>
> To: <www-rdf-logic@w3.org>
> Sent: Monday, August 12, 2002 03:51
> Subject: Semantics, in particular DAML+OIL semantics
> > ...
> > For example, when we parse an OWL ontology we may find that instead of
> > using the familiar subClassOf property, it contains lots of statements
> > like "Person foo Animal". If we allow statements in the ontology to
> > constrain the meaning of the syntax, then we may be able do deduce
> > that foo is equivalent to subClassOf, and that this is therefore a
> > meaningful OWL statement. The reasoning required for this deduction
> > may be extremely complex. It may even be IMPOSSIBLE to be sure that we
> > have derived the complete syntactic meaning of an OWL ontology
> > (because the language is undecidable).
> > 
> > Another example. In OWL, transitive properties cannot be used in
> > cardinality restrictions. If we allow inference to be used to deduce
> > that a property is a transitive property, then when we parse an OWL
> > ontology we can't be sure that it is valid until we have checked that
> > none of the properties used in cardinality constraints can be deduced
> > to be transitive. Again, the required reasoning may be very complex,
> 
> So you seem to be saying that OWL can't allow assertions about
> classes and properties (like "Person foo Animal") except for a few 
> built-in predicates, and this is because there is a messy interaction
> with the 'transitive' property.  

No. I said that messy interactions with transitive property is AN
EXAMPLE of the nasty things that can happen when we apply reasoning to
the syntax of the language. The main point is that it becomes
impossible to separate parsing from reasoning, and may even make it
impossible to determine the complete syntactic meaning of an ontology.


> My guess is that the utility of second-order
> assertions/statements exceeds the utility of the 'transitive' property by a couple
> orders of magnitude.

You seem to be confusing my argument about HOL with my argument about
the syntax of the language being within the domain of discourse.

>   That RDF allows second-order statements is
> one of its cardinal virtues.

One of its cardinal sins I would say. It seems inappropriate to me that
a language that is supposed to be the foundation for a hierarchy of
languages with increasing expressive power should already be
second-order.

>  On the other hand, I work with rule-based
> systems (instead of DL systems), and in all cases transitivity appears as an
> *emergent* property (derivable from the rules) rather than something
> that is explicitly asserted.

This is in fact very close to what we have with transitive
properties. The problem comes when the language is so powerful that
you can infer the existence of rules, change the meaning of existing
rules or even change the syntax of rules so that what initially
appeared to be a syntactically well (ill) formed rule becomes ill
(well) formed.

>  The appearance here is that we are catering to
> a marginal set of applications, and ruling out large classes of useful
> ones.  If indeed HOW syntax and certain OWL properties (like transitivity) are
> incompatible from a practical/simplicity standpoint, it would be preferable to drop those
> properties and to keep the second-order assertions.  I would be interested to know
> which properties we would need to toss out in order to achieve this effect.

It is difficult to answer this as you again seem to be mixing up the
arguments about HOL and reasoning with language syntax.

Ian

> 
> Regards, Bob
>

Received on Wednesday, 14 August 2002 05:09:46 UTC