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Re: What do the ontologists want

From: pat hayes <phayes@ai.uwf.edu>
Date: Tue, 15 May 2001 17:02:59 -0500
Message-Id: <v04210110b72753d26192@[205.160.76.173]>
To: Graham Klyne <GK@ninebynine.org>
Cc: www-rdf-logic@w3.org
>At 10:44 AM 5/15/01 -0500, pat hayes wrote:
>>>'Jon says "The sky is blue."' ?
>>
>>It is logically exotic, if I may be forgiven the terminology, since 
>>it refers to a sentence by ostention, rather than by denotation. 
>>Actually, even this is relatively harmless, as long as you simply 
>>quote. However, in order to be useful, one usually has to be able 
>>to move between a quoted sentence or expression and the use of that 
>>sentence or expression as part of the language itself, and this 
>>'reflexion' is what is really arcane and tricky. Simple quotation 
>>is relatively harmless, I concede, but largely useless: it is 
>>simply a way of referring to character strings.
>>For example, would you want to be able to infer, from your example, 
>>that Jon said that the sky is blue? It doesnt follow from it, 
>>however, unless you provide some machinery to de-quote a quoted 
>>string. Seems to me that the latter is much more useful than the 
>>former: the relationship between an agent and the *content* of what 
>>they say is more interesting, usually, than the relationship 
>>between the agent and the *form of character strings* that they use 
>>(except of course when the mapping between form and meaning is 
>>central to the discussion, as it is in some legal settings; but 
>>again, this seems exotic for our purposes.) To refer to the content 
>>of an expression, however, one does not quote it: one simply uses 
>>it.
>
>Something that I very much wish to be able to do is something like this:
>
>  'Jon says "The sky is blue"'
>  'I believe Jon'
>=>
>  'I believe (the sky is blue)'

Careful, those are two different senses of 'believe'. You don't 
believe Jon is TRUE, right? You believe that what he SAYS is true. 
Write that out and your example looks more, er, logical:

Says(Jon,  (the sky is blue))
Says(Jon, ?p) implies ?p.
=>
(the sky is blue)

Notice that I havnt quoted the internal sentence, since there is no 
need to do so. Says(...) here is a modal operator, not a predicate on 
sentences.  If you want it to really be a predicate on sentences then 
the second assumption (about Jon's veracity) needs to be restated 
using a truth predicate:
Says(Jon, ?y) implies true(?y)
Notice that ?y here ranges over reified sentences - syntactic things 
- while ?p ranges over propositions - semantic things, the things 
that the ?y's refer to. 'true' maps the former to the latter. The 
inference then goes like this:
Says(Jon, "the sky is blue")
Says(Jon, ?y) implies true(?y)
=>
true("the sky is blue")
=>
the sky is blue

>Or:
>
>  'Jon says "The sky is blue"'
>  'My oracle says "Jon is reliable"'
>=>
>  'The sky is blue'
>
>I've deliberately not tried to state this rigorously, as I'd 
>probably miss the mark if I did.  I hope the general intent is 
>reasonably clear.
>
>Maybe there is a way of formulating this that doesn't rely on 
>logical exotica.  But it does seem to rely on some form of 
>"reflexion" -- a statement is used both as an object about which 
>other statements are made, and as an assertion in its own right.

To me, attribution seems better modelled as a modality. Certainly 
belief is best modelled that way. If you do it that way, you never 
need to use reification or reflexion, and the logic is... well, not 
transparent exactly, but reasonably well-behaved and thoroughly 
understood.

Pat Hayes

PS. There seems to be an implicit assumption in some of the RDF 
literature that the only two things to do with a sentence (triple) 
are to either assert it or to reify it, so any use that doesnt 
involve asserting a triple must reify it. This is just wrong. Logical 
notation is full of examples of sentences being used but not being 
asserted. The simplest is probably negation: when one writes (not P), 
P is being used (not mentioned or reified), but it is not being 
asserted: on the contrary, in fact. Now, it might be that RDF is 
incapable of making this distinction. So much the worse for RDF, if 
so.

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Received on Tuesday, 15 May 2001 18:03:01 UTC

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