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Re: Predicates and Arcs vs Triples RE: use/mention and reification: rdf:predicate/subject/object

From: Graham Klyne <GK@NineByNine.org>
Date: Fri, 01 Jun 2001 11:08:46 +0100
Message-Id: <5.0.2.1.2.20010601105206.00a5aa50@joy.songbird.com>
To: pat hayes <phayes@ai.uwf.edu>
Cc: www-rdf-logic@w3.org
Thanks for helpful comments.

At 11:05 AM 5/31/01 -0500, pat hayes wrote:
>Yes (other than your use of 'valid', which has a different meaning in 
>logic. Terminological quibble only.)

Yup, I was a bit careless there.  (Although for a wider audience of 
developers, I think we're going to have to be able to present the essential 
structures without assuming familiarity with logic.)

[...]
>>I think you may be saying that, in order to attach meaningful semantics, 
>>certain values or sets of values that appear within the triple structure 
>>must be recognized as distinguished symbols in the syntax, in order to be 
>>able to say that they have specific associated semantics.
>
>Well, *something* needs to be indicating which parts of the triple-sets 
>are being used to indicate logical syntax and which are genuine 
>assertions. Using special values is one way to do it.

It seems to be the way in RDF:  the various things referred to as "rdf:..." 
all appear to be used to describe deeper constructs based on triples.

[...]
>>>That is what some of us find an unacceptable trade-off, as it uses a 
>>>huge burden of semantic expressivity (KIF is probably the only formalism 
>>>on the planet with a fully defined truth predicate, and even that has 
>>>never been used in practice by anyone, as far as I know, and is likely 
>>>to be eliminated from the new KIF standard) to purchase a tiny advantage 
>>>in interoperability (the property that any set of triples is 
>>>well-formed.) Also, we are pretty sure it is going to lead to problems 
>>>later: it is notoriously easy to produce paradoxes if used casually, it 
>>>renders any 'genuine' usage of reification semantically suspect, and so on.
>>
>>... but now you've lost me.
>
>Well, reification is almost unknown outside the RDF world; it is generally 
>considered a very exotic and rarely-used device, only put into a few 
>languages to keep the damned theoreticans happy, and so on; and so it 
>seems odd to see it being used at the very simplest level of the langauge 
>to do the simplest kind of propositional syntax. That is only an aesthetic 
>argument,  but it might help to explain the intensity of the feelings in 
>these discussions. The basic point is that there seems to be no rational 
>reason for using it, other than RDF being forced to use it in order to 
>avoid having any kind of syntactic structure above the lone triple, and 
>this in turn seems to be a matter of doctrine, with no rational basis for it.
>The 'problems' arise if we ever want to have genuine reification (as 
>opposed to reification being used to encode syntax), eg of the 'John said 
>"...."' variety. That had better not be interpreted as merely a syntactic 
>encoding, or the reasoners that really need to be able to handle 
>reification will not be able to distinguish reified encodings of 
>transparent contexts from genuinely opaque contexts, and will prodice 
>invalid conclusions (any with the careless use of negation, paradoxical 
>conclusions.)

I'll hazard an opinion that what you describe as genuine reification is 
handled in RDF by literals, without any built-in interpretation.  Thus, to 
distinguish between:

    John says "The sky is blue"
and
    John says-that (the sky is blue)

I think RDF aims to handle the latter, but not the former.  In the latter 
case, I think it is an RDF expression associated with John's utterance that 
can be interpreted, not the utterance itself.  The utterance must be 
expressed (interpreted?) as RDF before it can be "understood" by an RDF 
processor.

#g


------------
Graham Klyne
(GK@ACM.ORG)
Received on Friday, 1 June 2001 06:16:57 GMT

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