


From: John Black <JohnBlack@deltek.com>
Date: Thu, 7 Oct 2004 09:52:31 -0400
Message-ID: <CBEA695878CA104ABC6E74C6B1769275542700@DLTKVMX2.ads.deltek.com>
To: <Patrick.Stickler@nokia.com>, <r.newman@reading.ac.uk>, <rhoadsnyc@mac.com>

> From: Patrick.Stickler@nokia.com


> The requesting client is not getting back a resource. It
> is getting back a representation. And a given representation
> can serve as the representation of more than one resource.
> A representation can also be the representation of itself. 
> I.e. a representation is the atomic component of the web, and
> corresponds to a octet stream returned in the response to a
> request. If the request URI denotes a representation, then
> the representation returned is a representation of that
> representation (resource) denoted by the request URI, and
> thus the representation of a representation is always a 
> bit-equal copy of itself.
> So, the representation of a chair may be an binary encoded
> image of the chair, yet that image is also a resource, and 
> its representation is itself. Thus, the same representation
> is the representation of two resources: the chair, and the
> image (itself) and thus two GET requests with distinct 
> request URIs denoting distinct resources can both return
> the very same representation.
> And, insofar as the semantics of the web machinery is
> concerned, even when one gets back a representation of
> a representation (such that one could fairly conclude
> that one has gotten back the actual resource) one has
> not actually gotten back the resource, only its 
> representation.   

In my opinion, a better term for "representation of itself" is 
"replication". It is lossless replication. I think of it as a 
subclass of representation so as to fit in with the AWWW. So I 
would say that the class of representations can be usefully 
partitioned into those that are lossless replications and those 
that are not. When a URI as symbol denotes a resource that 
can be losslessly replicated by using that URI as retrieval 
path we have a perfect name, a perfect reference, because it 
creates perfect common knowledge of the denotation of that 
name. Any one who knows the name, knows (algorithmically) the 
resource denoted by the name. This reminds me of TimBL's position 
on the http range issue. Because of these properties of URIs for 
resources that can be losslessly replicated, i.e., HTTP URL URIs, 
they do seem to deserve special recognition. 

Now the concept of bit-equal copy fits with the concept 
of URI as retrieval path, but doesn't apply to URI as symbol. 
And in RDF semantics URIs are names, not retrieval paths.
The denotation of a name is a bit of knowledge, and as such has 
properties of knowledge, not properties of representation 
retrieval. The difference between HTTP URL URIs as names for 
resources that can be losslessly replicated and those URIs 
that name resources that cannot be losslessly replicated is 
a difference in knowledge. The semantic equivalence between two 
instances of interpretation of the denotation of the URI is exact 
in the former case and it is approximate in the latter case. 

It is like the difference between meeting someone at a 
conference wearing her name on a name tag and looking at a 
picture or reading a description of the person denoted by 
that name. At the conference, the denotation of the name 
is right there in person behind the name for all to know.
Any other ways of representing or describing the denotation 
of that person's name will always create approximations of 
the knowledge that exists when the name is thus attached to 
the person. 

John Black

> Cheers,
> Patrick
Received on Thursday, 7 October 2004 13:52:35 GMT
