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Re: Two questions about bagid

From: pat hayes <phayes@ai.uwf.edu>
Date: Wed, 12 Feb 2003 10:46:39 -0600
Message-Id: <p05111b00ba702970e735@[10.0.100.86]>
To: Seth Russell <seth@robustai.net>
Cc: www-rdf-comments <www-rdf-comments@w3.org>

>pat hayes wrote:
>
>>
>>>On Tue, 11 Feb 2003, Tim Berners-Lee wrote:
>>>
>>>>  Another question is about the semantics.
>>>>  I understand that an id  :i  on the statement  { :superman :can :fly}
>>>>  should generate the statements
>>>>
>>>>  :i a rdf:Statement;  rdf:subject :superman;   rdf:predicate :can;
>>>>  rdf:object :fly.
>>>>  :superman :can :fly.
>>>>
>>>>  and I might want to use this to generate attribution:
>>>>
>>>>  :lois :thinks :i.
>>>>
>>>>  This would suggest that an owl reasoner (say) that knows that :superman
>>>>  and
>>>>  :clarkekent are daml:equivalent should be able to infer that
>>>>
>>>>  :i a rdf:Statement;  rdf:subject : clarkekent;   rdf:predicate :can;
>>>>  rdf:object :fly.
>>>>  : clarkekent :can :fly.
>>>>  :lois :thinks :i.
>>>>
>>>>  This seems counterintuitive, as one would expect it allow one to
>>>>  conclude
>>>>  that the modified statement is due to the original source.
>>>>  Some form of quoting around the subject, predicate
>>>>  and object would seem necessary.
>>>
>>
>>Right. This arises from the way that RDF handles reification. The 
>>obvious, and simplest, way to understand the above would be one 
>>where the reification :i refers to the triple itself, ie to its 
>>syntactic form, in effect quoting it.  Then the equality 
>>substitution would not be valid, since even though :clarkekent = 
>>:superman, the *triples* expressing the propositions that Clarke 
>>can fly, and that Superman can fly, are distinct piece of syntax. 
>>But this way of understanding reification was rejected by the WG in 
>>favor of one where the reification is understood to refer to the 
>>things that the original triple referred to, so that for example 
>>the subject of the reified triple is not the subject NODE of the 
>>triple, but the thing that node refers to, which is that same 
>>flying guy, no matter what name you use to refer to him by.
>
>Huh .... How did :clarkkent and :superman suddenly end up refering 
>to the same object?

By virtue of the (hypothesised)  daml:equivalence (or if you prefer, 
owl:sameIndividialAs) which is known to the reasoner to hold.

>  That fact is not in the example you site, nor in the triples that 
>you have specified above.

Er,,, yes, it is.

>  It is rather in our background knowledge of the story.   So my 
>interpertation of  the triples "clarkekent can fly" and "superman 
>can fly" is that we have two flying guys .. not just the one in your 
>interpertation.

You have two names which are known to co-denote.

>  Now if someone actually asserted some dumb thing like "clarkekent 
>daml:equivalentTo superman" they would be destroying the story. 
>What am I missing?

Apparently, that they DID assert this dumb thing.

>
>This might be called a de-re rather than a de-dicto interpretation 
>of reification. This allows equality substitution, but it does not 
>allow a reification to be coherently used as a de-dicto object of a 
>psychological modality like 'thinks' or 'believes'.
>
>Where is it written that we *can* do substitutions of equivalent 
>names in RDF reified statements and preserve the authors meaning ?

In the DAML (or OWL) semantic conditions.

>
>>
>>We could have gone either way on this. But we can't go both ways at once.
>>
>>>  >
>>>
>>>>  I have been guilty of ignoring this rather complicated bit of the spec,
>>>>  and wonder whether others have done the same.
>>>>
>>>>  A developer.
>>>
>>>
>>>I've had this conversation with Danbri before. The owl reasoner you
>>>posit has superman and clarkkent denoting the same thing (ie, it applies
>>>an interpretation that a comic reader would agree with). Strictly
>>>speaking, from the comic reader's point of view (ie, in that
>>>interpretation) the conclusion is correct: Lois thinks that the person
>>>denoted by "Clark Kent" can fly, which he can, 'cause he's super.
>>>
>>>Lois wouldn't reason using the same interpretation, so her conclusions
>>>would be different.
>>
>>
>>Right. This is the classical de-re/de-dicto distinction. Sentences 
>>like 'thinks' ('believes', 'knows', etc.) which express a 
>>relationship between a cognitive agent and a proposition can always 
>>be read in one of two ways, depending on whether the statement of 
>>the proposition in the assertion itself is supposed to be a factual 
>>statement of the way things actually are (de re), or a statement 
>>that the agent themselves would be willing to assent to (de dicto). 
>>And, notoriously, these are often not the same, since if the 
>>agent's beliefs are factually wrong, they would often be inclined 
>>to deny sentences or claims  which express propositions that they 
>>in fact believe, but which use names in ways that are denied by 
>>their false beliefs.  The net result of all this is that modal 
>>logics are referentially opaque, ie a name occurring inside a modal 
>>context (or in a reified triple being used as the de-dicto object 
>>of a modal property) cannot be understood in the same sense as a 
>>name outside such a context; you can't use equality reasoning on it 
>>safely. So if you want to use names transparently (and we have 
>>effectively mandated this use in RDF reification now) then you 
>>can't use psychological modalities as simple properties.
>>
>>In a nutshell, :thinks isn't a relationship between an agent and an 
>>RDF reification, so it can't be an RDF property.
>
>I find that hard to live with.  So, if the WG had chosen that 
>reified statements were to be interperted de-dicto, then ":thinks" 
>could have been a RDF property?

Yes, but then there would have been no particular connection between 
the reified triple and any content that you might find by de-reifying 
it. In the present set-up, the reified triple is required to mean 
what it would mean if you de-reified it. It refers to the 
proposition, not to the surface syntax.

>  In which case, if the WG wanted that RDF reified statements to be 
>considered statings, and since substituting equivalent names into 
>statings do not necessarily preserve the author's  intended meaning, 
>then imho the WG goofed.

Well, its (literally) impossible to give a coherent interpretation of 
reification which satisfies everyone. We had to choose one, and we 
chose the one that seemed to support the existing use cases that 
people felt strongly about.

Pat
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Received on Wednesday, 12 February 2003 11:46:44 GMT

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