Hi Ed!
(01)
Just a clarification on OWL 2 Full:
(02)
Ed Barkmeyer <edbark@xxxxxxxx> wrote:
(03)
>Sean Barker orignally wrote:
>
>> The statement was made that anything that could be written in EXPRESS
>> could be written in OWL.
>
>That statement is simply false.
>
>But it is first important to distinguish between OWL2/DL and OWL2/full.
>OWL2/DL is an extended description logic language, which greatly limits
>what it can express. OWL2/full is harder to characterize; it has
>essentially the full power of RDF, which allows you to express almost
>anything, but tells you nearly nothing about how to reason about it.
(04)
OWL 2 Full is precisely characterized as the language that has as its syntax
the RDF Abstract Syntax [1] and as its semantics the OWL 2 RDF-Based
Semantics [2]. The term "OWL 2 Full" is defined in Sec. 2.1 of [2]. The OWL
2 RDF-Based Semantics is a semantic extension of the model-theoretic
semantics of RDFS [3]. OWL 2 Full is a continuation of the original
specification of OWL Full, as specified in Sec. 5.3 of [4].
(05)
Since the syntax of OWL 2 Full is the RDF Abstract Syntax, this means that
every RDF graph is a valid OWL 2 Full ontology. So you may say that one can
"express almost anything" in this /syntactic/ sense, as the only syntactic
restrictions for OWL 2 Full are those coming from the RDF spec, which is no
literals in subject position of RDF triples and no literals or blank nodes
in predicate position.
(06)
However, this does not mean that OWL 2 Full allows you to express anything
/semantically/. Just as RDFS does not allow you to express anything
semantically, although its semantics is, just as the semantics of OWL 2
Full, a semantics for arbitrary RDF graphs. The semantics of OWL 2 Full
allows you, for any given pair of RDF graphs G1 and G2, to exactly determine
whether G1 logically entails G2, or not. And *very* often, the answer will
be negative.
(07)
The caveat is that entailment checking in OWL 2 Full is computationally
undecidable. So there is, in theory, no "perfect" entailment checker that
would in every single case terminate for two given input graphs and tell you
the correct answer, being either "yes" (for a positive entailment) or "no"
(for a non-entailment).
(08)
Maybe this is what you mean when you say: "... but tells you nearly nothing
about how to reason about it". But I would consider this largely
exaggerated, since there are many at least partial implementation of OWL 2
Full around. These include all reasoners that implement the new OWL 2 RL/RDF
rules, as defined in Sec. 4.3 of [5], and which are characterized there as a
"partial axiomatization of the OWL 2 RDF-Based Semantics".
(09)
In any case, the semantics of OWL 2 Full still allows, in principle at
least, to find out in each case whether one given RDF graph entails another
one or not. In fact, proving OWL 2 Full entailment manually for given
concrete graph pairs is what I am doing more or less on a day-by-day base at
the moment. The foundation for this is essentially the set of
model-theoretic semantic conditions given in Chap. 5 of [2], together with
the semantic conditions listed in [3].
(010)
Best,
Michael
(011)
[1] <http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/>
[2] <http://www.w3.org/TR/2009/REC-owl2-rdf-based-semantics-20091027/>
[3] <http://www.w3.org/TR/2004/REC-rdf-mt-20040210/>
[4] <http://www.w3.org/TR/owl-semantics/rdfs.html>
[5] <http://www.w3.org/TR/2009/REC-owl2-profiles-20091027/>
(012)
--
Dipl.-Inform. Michael Schneider
Research Scientist, Information Process Engineering (IPE)
Tel : +49-721-9654-726
Fax : +49-721-9654-727
Email: michael.schneider@xxxxxx
WWW : http://www.fzi.de/michael.schneider
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FZI Forschungszentrum Informatik an der Universität Karlsruhe
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(013)
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