W3C home > Mailing lists > Public > www-tag@w3.org > July 2003

Re: New issue - Meaning of URIs in RDF documents

From: Tim Berners-Lee <timbl@w3.org>
Date: Thu, 24 Jul 2003 14:40:05 -0400
Cc: www-tag@w3.org
To: pat hayes <phayes@ihmc.us>
Message-Id: <3E03306B-BE06-11D7-B294-000393914268@w3.org>


On Tuesday, Jul 22, 2003, at 12:31 US/Eastern, pat hayes wrote:

> [...] And the answer is, there isn't anything to say.  Just by using 
> the same URIs, any two agents who swap or communicate some RDF have 
> *already* agreed to use those URIs to "mean the same thing" in all the 
> senses required for proper communication. The syntactic coincidence of 
> vocabulary by itself is enough to do this, given the model theory: we 
> don't need to add any extra conditions or principles.

First of all, I am much relieved that what I was trying to say has been 
reclassified from "crazy" to so obvious that it "doesn't need to be 
said"!  There's that communication thing happening again.

So yes, it is obvious to you who have internalized the RDF model 
theory.   But it needs to be said in a wider context, because those who 
use RDF more peripherally or do not use it at all have actually 
considered quite deliberately using the same URI for distinct things.

Much of the architecture document sometimes seems like stating what's 
obvious to some, to the great surprise or objection of some others.  We 
have a diverse set of cultures here.

>   That is the current SW design; names are thought of as being in 
> global use, so that whenever two or more agents use those names, their 
> assertions can all be merged; and then applying the model theory to 
> the merged ontology *does* treat the names as meaning the same thing, 
> automatically.

Absolutely

> Further, if some thing external to RDF somehow does magically fix the 
> referent of a URI, then as long as the RDF uses that URI, what the RDF 
> is saying will be about that thing; that also is already guaranteed by 
> the model theory.

Absolutely.

> So this problem is solved: no need to say or do anything else, no need 
> for further axioms or principles. The purely syntactic fact of using 
> the same URIs is enough to capture the required degree of semantic 
> alignment, without any further stipulations.

Given the RDF model theory.  Note though that other non-RDF systems may 
and do use URIs.  So the principle can must be a general one of web 
architecture.

> Another way to put the same point: how could two agents not mean the 
> same thing, when using the same URI? The only way, if you check the 
> model theory, would be for them to both somehow distinguish the 
> URI-when-used-by-A from the URI-when-used-by-B, perhaps by some kind 
> of McCarthy/Guha style 'context logic' mechanism,
> or by 'separating' the vocabulary in the way that OntoMerge does. Ie, 
> to treat them like two different names in the semantics. But our basic 
> SW design doesn't have any provision for doing this: we just assume 
> that all tokens of a URI are the same URI, and hence mean the same 
> thing, wherever they occur, anywhere on the planet at any time.

> That is not, note, the same as saying that all URIs must denote only 
> one thing (one resource), or still less that the agents need to or can 
> possibly know what that thing is, or have any kind of access to it. It 
> is saying that whatever the URI might denote, the agents have all 
> agreed, as it were, ahead of time that what they are saying to one 
> another is about that.  But it might be easier to just say "they agree 
> URIs mean the same thing".

So long as we say "mean" it is obvious, and "denote" is is wrong.
Ok. I can live with that, when talking to you at least.

Tim

PS: (I had personally preferred "identify" for when we talk about what 
does a symbol mean, and keep "mean" for what a document or message 
means.   In the latter sense there is a sense of information transfer, 
a reduction in the number of interpretations believed by the receiver, 
a possibility of expressing the meaning in different languages.    In 
the first case there is non of that sort of meaning, as one symbol 
doesn't make a sentence in RDF at least.)

(Your second point in second mail, which I am sending to RDF Interest 
as it is not, as you say, a TAG proposed issue let a long a TAG issue)
Received on Thursday, 24 July 2003 14:39:55 GMT

This archive was generated by hypermail 2.2.0+W3C-0.50 : Thursday, 26 April 2012 12:47:19 GMT