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Honda exists? (was:rdf as a base for other languages)

From: pat hayes <phayes@ai.uwf.edu>
Date: Fri, 1 Jun 2001 15:24:18 -0500
Message-Id: <v0421010fb73da7792755@[205.160.76.212]>
To: Sandro Hawke <sandro@w3.org>
Cc: www-rdf-logic@w3.org
I didnt notice this:

>Perhaps another possible confusion is around existence: if I describe
>to you that x is a blue Honda Civic with license plate 9948JI , does
>that mean that such a car exists?    (I think not.)

Well, in normal logic, it wouldnt make sense to say that ?x is 
anything, since that would have a free variable. You would need to 
bind the variable with a quantifier. If you were to say that
(exists (?x)((blue-Honda-Civic ?x) & (Licence-plate ?x '9948JI))
  then yes, you would be saying that a thing exists that satisfied the 
statement. Same if you were to assert that
(blue-Honda-Civic A) & (Licence-plate A '9948JI)
where 'A' is a unique name (skolem constant).

However, there are versions of logic where the use of the name 
wouldnt assert the existence ("free logics"). They were designed to 
let you talk about unicorns, Santa, etc., and there are all kinds of 
variations on this theme. Sounds like you would be happier with a 
free logic. An alternative is to just say that 'exists' means a kind 
of mathematical existence as a kind of possibility, and have a 
predicate ("isReal" or some such) to assert that the thing really, 
actually, exists. But then you have to make sure that you use this 
consistently, which tends to get a bit tedious when you have a lot of 
data about real things. (Jerry Hobbs uses this trick, he has a entire 
framework where there are 'eventualities' which are scenarios that 
might possibly be the case, and some of them are 'actualized', so 
that to say not-P, you assert the actualization of the eventuality of 
the denial of the eventuality of P.)

Pat Hayes

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Received on Friday, 1 June 2001 16:24:30 GMT

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