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Re: Some basic questions about OWL-Full

From: Pat Hayes <phayes@ihmc.us>
Date: Mon, 22 Oct 2007 15:56:33 -0500
Message-Id: <p0623090ac342b8d82143@[192.168.1.2]>
To: "Michael Schneider" <schneid@fzi.de>
Cc: "Owl Dev" <public-owl-dev@w3.org>

>Hi!
>
>OWL-Full has ever been a complete mystery to me, ....

....

It is really quite simple. Take OWL/RDF and think of it as an RDF 
'extension'. The RDF and OWL vocabulary (and associated 
constructions) have to satisfy all the RDF and OWL semantic 
conditions stated in the specs. OK, that is all there is to OWL-Full. 
It does not constrain the form of an RDF graph and it does not impose 
any syntactic conditions on how the OWL vocabulary is used. It does 
however insist that however it gets used, the meanings of this 
vocabulary must satisfy the semantic conditions imposed upon it. It 
does not recognize distinctions like that between 
class/individual/property and between object/datatype classes or 
properties. In this, it follows RDF and RDFS, since the RDF semantics 
(and the ISO Common Logic semantics) allows any name to denote any 
'type' of thing, or indeed to denote several of them at once.

Notice that I did not mention the OWL 'abstract syntax' at all. At 
the insistence of the Manchester members of the working group, the 
OWL spec is stated with the abstract syntax as primary, and the 
mapping into RDF described almost as an afterthought, a 'projection' 
from the real language to an alien notation. This is not the right 
way to think about OWL-Full, and not how it was conceived. It is 
designed to fit into a picture where RDF is primary and more complex 
languages are created by adding special vocabularies to RDF with 
associated semantic conditions imposed on their meanings. This is how 
RDFS is described, for example, and OWL-Full is in the same 
tradition. In fact, OWL-Full was created in response to a claim made 
and reiterated several times in the WG, that a language as complex as 
OWL was inherently incompatible with RDF, and that the RDF encoding 
therefore should be abandoned. Echoes of that debate can still be 
heard in some parts of the world.

This gives a rather different perspective to several contentious 
issues. From the Manchester view, some parts of OWL/RDF are genuine 
OWL assertions, while others are simply artifacts of the syntactic 
embedding from the abstract syntax. There is absolutely no such 
distinction in OWL-Full. Again, some RDF graphs are considered by DL 
thinkers to be 'assertions about the logical syntax' or to 'change 
the meaning of the logical syntax'; but neither claim is true when 
seen from the perspective of OWL Full itself. No OWL/RDF assertion 
can change the semantics of, say, rdf:type or of owl:Restriction, as 
these semantics are written into the semantic specification.

What one can do is *add to* the meaning of such terms by imposing 
extra, axiomatically stated, conditions, ie by writing axioms. This 
can, in some cases, have some peculiar consequences; but they are not 
incoherent or illogical, just, well, peculiar. (OWL-DL also allows 
for some very peculiar conclusions arising from its insistence on 
extensional readings of classes and properties.) But it also allows 
for some very useful generalizations which have potential uses in 
real ontologies.

Anyway, I hope this helps.

Pat
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Received on Monday, 22 October 2007 20:56:50 GMT

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