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Re: Grist for layering discussion

From: Pat Hayes <phayes@ai.uwf.edu>
Date: Thu, 10 Jan 2002 10:59:14 -0600
Message-Id: <p05101011b8636d88ce8c@[65.212.118.208]>
To: Aaron Swartz <me@aaronsw.com>
Cc: <www-archive@w3.org>
>Hi there,
>
>Jim Hendler forwared your discussion about why DAML can't be layered atop
>RDF. I found it very interesting (and I think I even understood it!) but I
>had one comment to make. I agreed with what you wrote entirely...
>
>You wrote:
>
>>  Just so you don't think Im just a naysayer: I don't think it will be
>>  hard to jury-rig a quick fix that will enable work to go ahead. For
>>  example, if we can introduce a simple distinction into RDF between
>>  triples that assert and triples that encode then that will avoid the
>>  immediate problem.  Ive put a hook into the MT to do this. It's ugly
>>  but it works. But if we just ignore this issue, then it is going to
>>  rise up and bite us very soon, so we do need to do something.
>
>...however, it was my understanding that this was the purpose of
>reification. TimBL writes:
>
>[[[
>The RDF model currently is that of an (unordered) set of assertions. We will
>demonstrate that this remains all that is needed to represent the new
>langauge features. Every new feature can be introduced as a new RDF
>property. However, we will see that this is an impractical way of actually
>processing information, as it involves using RDF indirectly to describe the
>parts of a statement instead of making it directly. This process (called
>reification) is described in the RDF Model & Syntax document.
>]]]
>  - http://www.w3.org/DesignIssues/Toolbox
>
>Does this solve both the reification and the layering problem?

Im not sure what the 'reification problem' is, but reification does 
not solve the layering problem. Or rather, it introduces problems of 
its own that make it impossible to solve the layering problem this 
way. Or, well, to be excruciatingly precise, its really not clear 
what reification actually means, but *if* it means what the RDF M&S 
document seems to say it means, then it introduces those problems.

The reason why is that (according to the M&S), what reification means 
is that the reified assertion exists. That is, the reification 
asserts that a piece of syntax exists. Now, while that indeed does 
not assert that piece of syntax, it also doesn't assert whatever that 
piece of syntax was supposed to encode in the 'upper' language (in my 
example, if you reify the triples encoding a rdfn:notAll, what that 
reification means in RDF is that some set of triples exists; but that 
still doesn't assert the content of the rdfn:notAll, and what it does 
assert is not entailed by that content. What we want, rather, is some 
way to make those encoding triples "invisible" to RDF; they shouldn't 
assert *anything*.

Now, Dan C. tells me that this way of understanding reification is 
wrong (which is why reification isn't in the MT yet) , and that there 
is a better way to understand it. Maybe that will enable reification 
to solve the layering problem, but as I don't yet grok this 
alternative way of understanding reification, I can't yet say....

>(I hope you don't mind that I've cc-ed www-archive on this.)

Not at all.

Pat Hayes
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Received on Thursday, 10 January 2002 14:09:16 GMT

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