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Rules are *not* Horn clauses!

From: Wagner, G.R. <G.R.Wagner@tm.tue.nl>
Date: Wed, 19 Sep 2001 11:43:52 +0200
Message-ID: <511BB18E82E9D11188230008C724064602D9DDBD@tmex1.tm.tue.nl>
To: "'Pat Hayes'" <phayes@ai.uwf.edu>, "Wagner, G.R." <G.R.Wagner@tm.tue.nl>
Cc: www-rdf-rules@w3.org
> I would say that the languages of queries and rules in Prolog 
> were *identical*: both consist of Horn clauses.

Please notice that Prolog rules (as well as any other practical
computational rule concept such as SQL views and OCL implications) 
are *not* Horn clauses! They are, for good reasons, more expressive, 
allowing for negation in their body/antecedent (and other things, 
such as quantifiers, as in the case of SQL views). 

We should consider the concept of a rule as a basic one that 
is not derived from that of a Horn clause (which happens to be 
a very restricted/simplified type of rule). The concept of a
Horn clause is tight to classical (two-valued) logic. But rules 
play a role in all kinds of formalism with a non-classical
logic semantics (such as intuitionistic, partial, temporal,
inconsistency-tolerant logic, etc.).

-Gerd


--------------------------------------------
Gerd Wagner        Email: G.Wagner@tm.tue.nl
http://tmitwww.tm.tue.nl/staff/gwagner

Eindhoven University of Technology
Faculty of Technology Management
Department of Information & Technology 
P.O. Box 513       
5600 MB Eindhoven    Tel: (+31 40) 247 26 17
The Netherlands      Fax: (+31 40) 243 26 12
  
Received on Wednesday, 19 September 2001 05:44:04 GMT

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