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Re: Semantic Web and First Order Logic

first order logic rdf
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 

> 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



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