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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@[]>
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 

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.

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Received on Wednesday, 12 February 2003 11:46:44 UTC

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