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Bullard, Claude L (Len) wrote:
> Is it true or false that the semantic web
> limits the use of First Order Logic
RDF is less expressive than FOL, yes. N3 is closer as it has things
like quantifiers. OWL has 3 levels to cater for how much you want
ranging from 'I could hack that together' to 'this may never be
implemented fully'.
This concern about decidability on the semantic web comes from
description logics work. DLs are carefully designed to allow engines
to be tractable and scalable when it comes to generating results;
the downside is that it makes some things awkward to say and
enforces constraints based on what you can expect a reasoning engine
to be able to deal with, whether or not you care about reasoning
engines. IMVHO,if you force content providers and developers to chow
down on a DL for a length of time, they'll ask for a scripting hook.
Which kind of defeats the point of using a logic to begin with.
Pat Hayes has written (a very nice read) on how DLs square with the
semantic web [1]:
[[[
The semantic web doesnt need all these DL guards and limitations,
because it doesn't need to provide the industrial-quality guarantees
of inferential performance. Using DLs as a semantic web content
markup standard is a failure of imagination: it presumes that the
Web is going to be something like a giant corporation, with the same
requirements of predictability and provable performance. In fact (if
the SW ever becomes a reality) it will be quite different from
current industrial ontology practice in many ways. It will be far
'scruffier', for a start; people will use ingenious tricks to scrape
partly-ill-formed content from ill-structured sources, and there is
no point in trying to prevent them doing so, or tutting with
disapproval. But aside from that, it will be on a scale that will
completely defeat any attempt to restrict inference to manageable
bounds. If one is dealing with 10|9 assertions, the difference
between a polynomial complexity class and something worse is largely
irrelevant. And, further, almost all of this content will be
extremely simple and shallow, seen from a logical perspective.
Worrying about the complexity class of the few intricate ontologies
on the web is like being obsessed with the quality of the salt in a
supermarket.
]]]
> I realize that FOL has the undecidability
> problem, but is sufficient for everyday
> reasoning and is the most widely used
> logic in business.
I believe that would be EC logic as used in relational databases and
Prolog. Otherwise the most widely used 'business logic' is whatever
you can get to run in a middleware.
> I realize that is controversial and is
> deliberately so. I am wondering if the
> semantic web is somewhat over-engineered.
It's hardly engineered at all Len, that's the problem ;)
Bill de hÓra
--
Propylon
www.propylon.com
[1]
http://www.aifb.uni-karlsruhe.de/~sst/is/WebOntologyLanguage/hayes.htm
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