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Re: Universal Quantification

From: <ewallace@cme.nist.gov>
Date: Wed, 5 May 2004 15:07:05 -0400 (EDT)
Message-Id: <200405051907.PAA12971@clue.msid.cme.nist.gov>
To: stephen.rhoads@db.com
Cc: www-rdf-logic@w3.org



stephen.rhoads@db.com wrote:
>Can someone please explain the meaning and consequences of the following 
>in layman's terms:
>
>"Also notice that the correspondence of owl:allValuesFrom with the 
>universal quantifier means that an owl:allValuesFrom constraint for a 
>property P is trivially satisfied for an individual that has no value for 
>property P at all. To see why this is so, observe that the 
>owl:allValuesFrom constraint demands that all values of P should be of 
>type T, and if no such values exist, the constraint is trivially true."

It is an easy mistake for a non-logician to assume that 
owl:allValuesFrom implies owl:minCardinality 1 (or owl:someValuesFrom).  
After all, intuitively shouldn't allValuesFrom just be a more restricted 
version of someValuesFrom? The text quoted above makes clear that this is 
not the case.  If you want to say that any instance of some specified class 
must have some property values of a certain type AND only values of that 
type, then you must explicitly make both restrictions.

An example:

:Quote1
    a owl:Class ;
    rdfs:subClassOf
	[ a owl:Restriction ;
	  owl:onProperty :effectivePeriod ;
	  owl:allValuesFrom :Duration 
	] .

is not equivelant to

:Quote2
    a owl:Class ;
    rdfs:subClassOf
	[ a owl:Restriction ;
	  owl:onProperty :effectivePeriod ;
	  owl:allValuesFrom :Duration 
	] ;
    rdfs:subClassOf
	[ a owl:Restriction ;
	  owl:onProperty :effectivePeriod ;
	  owl:someValuesFrom :Duration 
	] .

In a Quote1 instance, the presence of Duration value is optional while it is 
mandatory in a Quote2 instance.

-Evan

Evan K. Wallace
Manufacturing Systems Integration Division
NIST
ewallace@nist.gov
Received on Wednesday, 5 May 2004 15:07:16 UTC

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