W3C home > Mailing lists > Public > w3c-rdfcore-wg@w3.org > July 2001

Re: A use case for anon nodes - action from telecon

From: pat hayes <phayes@ai.uwf.edu>
Date: Mon, 23 Jul 2001 21:49:07 -0700
Message-Id: <v0421011fb782abf8ab78@[130.107.66.237]>
To: Graham Klyne <Graham.Klyne@Baltimore.com>
Cc: w3c-rdfcore-wg@w3.org
>At 09:40 AM 7/20/01 -0400, Dan Brickley wrote:
>>I think part of the problem here is our natural tendency to take a
>>commonsense reading of what FOL "there exists" means, ie. reading
>>existential quantifier as taking about some form of "existence in the
>>world". If we try to take a strong reading of "there exists" we'll be
>>bouncing into a whole family of (what I understand to be) fairly well
>>known puzzles: how do we talk about pictures that depict Unicorns,
>>future events that may not come to pass etc.
>
>Dan,
>
>I think you've touched an important point here.  After I posted my 
>comments, I also thought about Unicorns.  I think this is leading to 
>the need for some kind of "reference without assertion" -- 
>reification, or whatever.
>
>However, I do believe that "there exists" in logic does actually 
>mean that something satisfying the associated description does 
>indeed exist.  Without this, much of the maths I learned would 
>collapse.

This is a well-known tarpit, indeed, but I think it is orthogonal to 
the anonymous-nodes issue and can be ignored for now. There isnt any 
logical ambiguity about what 'exists x..' means in a model theory, 
but if one wants to both have 'things' that are imaginary (unicorns, 
etc.) in the universe of discourse (which we probably do, since who 
can be sure that someone else believes in what we believe in?), *AND 
ALSO* have the existential quantifier mean 'really exists', ie where 
unicorns don't really exist, then one gets into the tar-pit in 
spades. The result is called 'free logic' and I would strongly urge 
that we do not try to make RDF a free logic. Please!!  Free logics 
are horrible. There is no gurantee that any name is being used to 
speak of something that 'really exists' , so all URI's will need to 
have anonymous nodes backing them up to stop them being imaginary, 
for example.

There is no need to go to a free logic. We can just have 'really 
exists' be a property of things in the universe, if we need to make 
the distinction. (Some philosophers don't like doing this, because of 
something that Kant said, but to hell with them: it works.) If we are 
making the distinction, then you just have to remember that 'exists 
X...' means something like 'one of the things we talk about' rather 
than 'one of the REAL things..'; but you get used to that quite 
quickly. It's usually harmless since you know that many things are 
always real, eg sellers of truckfuls of roses.

Pat

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Received on Tuesday, 24 July 2001 00:49:01 EDT

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