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Re: Two questions about bagid

From: pat hayes <phayes@ai.uwf.edu>
Date: Tue, 11 Feb 2003 18:17:36 -0600
Message-Id: <p05111b41ba6f325672d5@[10.0.100.86]>
To: Tim Berners-Lee <timbl@w3.org>, Jan Grant <Jan.Grant@bristol.ac.uk>
Cc: RDF Core <w3c-rdfcore-wg@w3.org>, www-rdf-comments <www-rdf-comments@w3.org>

>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. 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'.

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.

Pat
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Received on Tuesday, 11 February 2003 19:17:53 GMT

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