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Re: [Fwd: OWL reasoning with rules]

From: Tanel Tammet <tammet@staff.ttu.ee>
Date: Wed, 04 Oct 2006 00:12:05 +0300
Message-ID: <4522D225.1050609@staff.ttu.ee>
To: axel@polleres.net
CC: matthew.williams@cancer.org.uk, semantic-web@w3.org

Axel Polleres wrote:
>> I have noticed that many of the attempts at OWL reasoning using rules
>> run into problems on dealing with OWL-DL.
>> As I understand it, this is because OWL reasoning gets complex when we
>> include things like nominals and some cardinality.
> Yes, rules and OWL DL are not straightforward compatible due to 
> decidability issues.

I am not sure what you mean by compatibility here.

It is certainly true that once you make your logical formulas (in OWL DL,
some rule language, etc: does not really matter) sufficiently complex,
you will no longer have an algorithm capable of deciding the formula.

Prop logic is decidable, predicate logic is not. Description logics walk 
the fine line,
trying to be relatively expressive while still being decidable. OWL full is
over the line: it is expressive enough to be undecidable. Any nontrivial
rule language is also undecidable. Clearly, if you add an undecidable rule
language to something, the end result is still undecidable.

When people speak about "rule languages", it is typically unclear
what they mean: is it pure predicate logic, a subset of latter, does
it contain nonmonotonic logic, does it contain closed world assumption?
They all behave hugely differently.

On one hand, this decidability stuff is all basic logic and algorithm
theory: owl and specific rule languages are in no special position
here. On the other hand - from the practical standpoint - decidability
is not that important in itself: we have much more important issues
where we behave miserably, like existence of clear semantics
(rdfs semantics is a horrible mess, and owl - inevitably - likewise),
ease of understanding (all our stuff is hugely complex when
compared to, say, sql or prolog), efficiency on large datasets
(none present when compared to sql engines or, say, swi prolog)

            Tanel Tammet
Received on Tuesday, 3 October 2006 21:12:17 UTC

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