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   Re: [xml-dev] Semantic Web and First Order Logic

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Thomas B. Passin wrote,
> I do not think that ordinary business applications today come close
> to using full FOL.  Mostly things run on ordinary assertions (this
> would cover relational databases), and sometimes on rulesets. 
> Ordinary markup languages are EC - Existential Conjunctive - only one
> sector of FOL - and they are almost always enough for these kinds of
> uses.

I disagree.

Actually, I'll go further. Even full first-order logic is too 
restrictive to express many trivial inferences straightforwardly. For 

  There's a 5% surcharge on primary coloured widgets
  x is a red widget
  red is a primary colour


  There's a 5% surcharge on x

This is easy to express in second-order logic, or in first-order logic 
plus set theory (the "plus set theory" bit means that you can forget 
about decidability). Not so easy in plain first-order logic.

Expressive power, decidablity ... pick one.




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