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Re: ISSUE 5.19 proposal to resolve

From: Ian Horrocks <horrocks@cs.man.ac.uk>
Date: Thu, 5 Dec 2002 16:49:34 +0000
Message-ID: <15855.33694.323579.155299@galahad.cs.man.ac.uk>
To: Jim Hendler <hendler@cs.umd.edu>
Cc: Guus Schreiber <schreiber@swi.psy.uva.nl>, WebOnt WG <www-webont-wg@w3.org>

On November 21, Jim Hendler writes:
> 
> At 12:16 PM +0100 11/21/02, Guus Schreiber wrote:
> >SSUE 5.19 - Classes as instances
> >
> >I propose to resolve this issue in the following way:
> >[snip]
> 
> this works for me
> 
> >
> >[NOTE: Changes are marked with tags <add> and <del>, resp. standing
> >for add or delete this piece of text.]
> 
> But I have some suggestions for the below
> 
> >
> >
> >THE SPECIES OF OWL
> >
> >OWL is in fact a set of three, increasingly complex languages.
> 
> For political as well as technical reasons I think it is VERY 
> important that we as a group try to never
>   i. Call this three langauges (they are named subsets of ONE language 
> called OWL) - this is important because we are chartered to create 
> one langauge only
>   ii. use the term "complex" and OWL in the same sentence -- DL isn't 
> more complex to use than LITE or less complex than Full - we designed 
> these based on functional needs and guesses as to implementation 
> complexity and implementor needs.  Also, users may construe "complex" 
> to mean hard to use, as opposed to computationally complex (i.e. OWL 
> Full is not harder to use than OWL Lite, but it could have 
> computational ramification)
>   iii. there are a large number of people in the world who think DL 
> means "digital library" - I'd like to suggest the first use of OWL DL 
> in each document needs to make it clear DL means Description Logic, 
> not Dig Lib.
> 
> In light of these two things, I would like to suggest the following 
> wording as an ammendment to Guus' proposal
> 
> replace the line "OWL is in fact..." with:
> 
> The OWL langauge provides two specific subsets that we believe will 
> be of great use to implementors and language users.  OWL Lite was 
> designed for easy implementation and to provide users with a 
> functional subset that will get them started in the use of OWL.  OWL 
> Description Logic (OWL DL) was designed to support the existing 
> Description Logic business segment and to provide a language subset 
> that has desirable computational properties for reasoning systems. 
> The complete OWL language (called OWL Full to distinguish it from the 
> subsets) also contains some powerful features for users that are 
> available in many database and knowledge representation systems, but 
> which violate the constraints of description logic reasoners.  OWL 
> Full and its two language subsets have been designed for maximal 
> compatibility with RDF and RDF Schema

As I mentioned during the last teleconf, I don't like/disagree with
the emphasis on DL reasoners in the penultimate sentence - OWL Full
also violates constraints imposed by many other kinds of
reasoner. Instead, we could simply say that OWL Full does not have
desirable computational properties for reasoning systems (I presume
that saying that OWL Full does not has undesirable computational
properties for reasoning systems would be vetoed by the marketing
department).

> >* Owl Lite has been defined with the intention of creating a simple
> >   language that will satisfy users primarily needing a classification
> >   hierarchy and simple constraint features. For example, while it
> >   supports cardinality constraints, it only permits cardinality values
> >   of 0 or 1. For these reasons, it should be simpler to provide tool
> >   support for Owl Lite than its more complex relatives.
> >
> >* OWL DL includes the complete OWL vocabulary, interpreted under a
> >   number of simple constraints. Primary among these is type
> >   separation. Class identifiers cannot simultaneously be properties or
> >   individuals. Similarly, properties cannot be individuals. OWL DL is so
> >   named due to its correspondence with description logics.
> >   <add>
> >   OWL DL is expected to be supported by reasoning tools based on
> >   description logic.
> >   </add>
> >
> >* OWL Full includes the complete OWL vocabulary, interpreted more
> >   broadly than in OWL DL, with the freedom provided by RDF. In OWL
> >   Full a class can be treated simultaneously as a collection of
> >   individuals (the class extension) and as an individual in its own
> >   right (the class intension).
> >   <del>
> >   Another significant difference from OWL
> >   DL is that a DatatypeProperty can be marked as an
> >   InverseFunctionalProperty. These are differences that will be of
> >   interest to the advanced user. This document does not describe the
> >   use of these features.
> >   </del>
> >
> ><add>
> >RDF users "upgrading" to OWL should consider whether they require OWL
> >Full. This decision mainly depends on how much they want to keep using
> >the metamodelling facilities of RDF Schema (i.e. defining classes of
> >classes).
> 
> I like that sentence, but...
> 
> 
> >  The price you have to pay for OWL FULL when compared to OWL
> >DL is that reasoning support is less predicatable. For more
> >information about this issue see the <ref>semantic document</ref>.
> ></add>
> 
> ...i nstead of being more predictable (what is unpredictable about a 
> database key?)  Can we say instead something like
> 
> When using OWL FULL, compared to OWL DL, the language user is not 
> necessarily provided with the computational benefits of Description 
> Logics, which are desirable in many applications.

Again, Description Logics are being "blamed" for the restrictions in
OWL DL. While this is true for some of them, others are there simply
to maintain a direct correspondence between OWL DL and conventional
FOL.

Regards, Ian

[snip]
Received on Thursday, 5 December 2002 11:42:29 GMT

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