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Re: owl:sameAs - Is it used in a right way?

From: Pat Hayes <phayes@ihmc.us>
Date: Wed, 27 Mar 2013 13:04:31 -0500
Cc: public-semweb-lifesci@w3.org, Oliver Ruebenacker <curoli@gmail.com>
Message-Id: <75C84966-1745-4FDC-BDCF-78DB50771557@ihmc.us>
To: David Booth <david@dbooth.org>

On Mar 27, 2013, at 8:37 AM, David Booth wrote:

> Hi Oliver,
> On 03/25/2013 04:02 PM, Oliver Ruebenacker wrote:
>>      Hello David,
>>   We agree that there are different interpretations. But you haven't
>> shown that the boundaries between interpretations are graphs
>> boundaries (others, including me, think that each interpretation is
>> global).
> I don't know what you mean by "boundaries between interpretations".
> An interpretation may be applied to any graph or statement to determine its truth value (or to a URI to determine the resource to which it is bound in that interpretation).
> The notion of a graph boundary is purely a matter of convenience and utility.  A graph can consist of *any* set of RDF triples.  If you wanted, you could apply an interpretation to a graph consisting of three randomly selected triples from each RDF document on the web, but it probably wouldn't be very useful to do so, because you probably would not care about the truth value of that graph.  We generally only apply an interpretation to a graph whose truth value we care about.
> An interpretation corresponds to the *use* of a graph.  Suppose I have a graph that "ambiguously" uses the same URI to denote both a toucan and its web page, without asserting that toucans cannot be web pages:
>   @prefix : <http://example/>
>   :tweety a :Toucan .
>   :tweety a :WebPage .
> When a conforming RDF application takes that RDF graph as input, assumes it is true, and produces some output such as "Tweety is a toucan", in effect the application has chosen a particular interpretation to apply to that graph.  In effect, the choice of interpretation causes the app to produce that particular output.  For example, the app might categorize animals into species, choosing an interpretation that maps :tweety to a kind of bird.  But a different conforming RDF application that only cares about web page authorship might take that *same* RDF graph as input and choose a different interpretation that maps :tweety to a web page, instead outputting "Tweety is a web page".  In effect, the app has chosen an interpretation that is appropriate for its purpose.
> If the graph had also asserted :Toucan owl:disjointWith :WebPage, then the graph cannot be true under OWL semantics, and the graph (as is) would be unusable to both apps.
>>   That makes me wonder whether you consider it in conformance with the
>> specs to choose different boundaries?
>>   For example, would you consider it conforming to apply a different
>> interpretation to each statement? Or how about a different
>> interpretation for each node of a statement? Do you see anything in
>> the specs against doing so?
> Sure it is in conformance with the spec.  An interpretation can be applied to any graph or any RDF statement.  And certainly you could determine the truth value of N different statements according to N different interpretations.  But would it be useful to do so?  Probably not.  Furthermore, if two statements are true under two different interpretations, that would not tell you whether a graph consisting of those two statements would be true under a single interpretation.
> OTOH, it *is* useful to apply different intepretations to different graphs, and one reason is that you may be using those graphs for different applications, each app in effect applying its own interpretation.  But the fact that those graphs may be true under different interpretations does *not* tell you whether the merge of those graphs will be true under a single interpretation.
> The RDF Semantics spec only tells you how to compute the truth value of one <interpretation, graph> pair at a time, but you can certainly apply it to as many <interpretation, graph> pairs as you want -- in full conformance with the intent of the spec.

Not with the *intent*, even if I have to concede that it does conform to the letter. The intention of the spec is to describe a model-theoretic semantics for RDF and RDF extensions. What you are doing, David, is not model theory semantics and does not describe any useful notion of interpretation. 


>  This is the same as if I define a function f of two arguments, such that f(x,y) = x+y, that function definition only tells you how to compute f(x,y) for one pair of numbers at a time, but you can certainly apply it to as many pairs as you want, without in any way violating the intent of f's definition.
> David

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Received on Wednesday, 27 March 2013 18:05:03 UTC

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