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from the RDF model theory: Re: [httpRange-14] What do HTTP URIs Identify?

From: Jonathan Borden <jonathan@openhealth.org>
Date: Fri, 2 Aug 2002 16:38:46 -0400
Message-ID: <087f01c23a64$9bff0fc0$0a2e249b@nemc.org>
To: Bill de hÓra <dehora@eircom.net>, "'Bullard, Claude L \(Len\)'" <clbullar@ingr.com>, <www-tag@w3.org>

Bill de hÓra wrote:

>... I don't believe this axiom is tolerable for the scale the
> Semantic Web wants to work on, any more than backlinks were tolerable
> for the current Web, and it will need to be softened or qualified. I
> know Joshua Allen and Tim BL (at least!) aren't going to appreciate that
> belief, but I don't see any way past it, short of calling everyone who
> breaks it an idiot, or radically reducing what we'll be able to say, or
> insisting that everyone is really talking about the same thing when they
> use a URI, no matter what they were saying, which is what I understand
> Jonathan Borden was saying in another post (personally I find that very
> bizarre, but since very little Jonathan says is bizarre, it I probably
> just didn't get it). On the other hand if we accept a priori ambiguity,
> we can get on with the job of vastly reducing it...
>

I can see how what I am saying may be counterintuitive, so let me try to
give it more intuitive sense, nah that would be too hard, instead let me try
to be very precise in how these things are defined and see if I can transit
this more precise definition to you so that even if it isn't intuitive, you
can see that it makes sense.

If we start with the definition: A URI identifies a resource, and understand
URIs to be 'universal' identifiers, then knowing _nothing else_ we are left
with a 1:1 mapping of a URI to some conceptual entity called a "resource".
Typically we represent resources as nodes on a graph. Since the URI
identifies the resource in a 1:1 fashion, we can use the URI as the label
for the node in the graph.

Now assume we have this huge Semantic Web containing all sorts of arcs
between nodes, the nodes still are identified by URIs -- in fact the node
really is just a URI with an ellipse drawn around it. So what is a
"concept", well since we are using graph terminology for our Semantic Web, a
"concept" might be a node or arc or some collection of nodes and arcs. Let's
say that a "concept" is a node on the Semantic Web -- well then yes when two
people use the same URI they are referring to the same concept -- unless you
use some other mechanism to label nodes on the Semantic Web it really can't
be any other way.

If two people using the same URI might be referring to _different concepts_,
then the model that you would need would someone need to represent a concept
as, for example, a node (which is unlabelled) and has a property called "my
URI" and that this property is not uniquely identifying i.e. it would not be
an "inverse functional property" in OWL parlance (oh I hope i've got this
new OWL terminology correct :-)). Well you _could create_ such a
representation but that wouldn't be the way that RDF, for example, models
the world.

From the RDF model theory http://www.w3.org/TR/rdf-mt/ (1.2)

"Urirefs are treated as logical constants, i.e. as names which denote
something (the things are called 'resources', but no assumptions are made
about the nature of resources.)
...
Similarly, the model theory given here has no special provision for tracking
temporal changes. It assumes, implicitly, that urirefs have the same meaning
whenever they occur.To provide an adequate semantics which would be
sensitive to temporal changes is a research problem which is beyond the
scope of this document.
"

Hope this helps.

Jonathan
Received on Friday, 2 August 2002 16:45:10 UTC

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