Just require URLs

John Cowan cowan at locke.ccil.org
Mon Jun 14 16:16:46 BST 1999

W. Eliot Kimber wrote:

> [For this reason, XML is *wrong* to require that notations use public
> IDs and not use system IDs, for example.]

Say what?  Notations *permit* the use of public ids without system ids,
since system ids are always URIs (where <!ENTITY URI "URL or URN">)
in XML.  It's entities that require URIs, and don't allow bare public ids.
> You either have only the name for the thing, in which case the name *is*
> the thing (because it's the only physical thing you've got) or you have
> the thing itself (the physical storage object the URI resolves to).

<sidetrack>Wow.  Nominalism redivivus: universals as flatus URis. :-)
But really that won't do.  One can live with only concrete things and
classes, but classes are not eliminable in favor of concrete things only,
nor are they just names: the number of classes, indeed, outruns the
number of names.</sidetrack>

> If the true intent of the namespace mechanism is that the URI *is* the
> namespace (in the sense defined above), then they have to *disallow*
> resolution of the URI, otherwise, you cannot reliably establish
> namespace identity because two different URIs could identify the same
> resource. If the only objects you have are the URIs, then you know that
> if two URIs are different that you have two different name spaces
> (because the namespace objects, the URIs, are different).  If namespace
> URIs can be resolved, then if you have two URIs, one of which you can
> resolve to a resource and one of which you cannot, you cannot know
> whether the two namespaces are the same or different because identity of
> the one whose URI you couldn't resolve has yet to be established (URIs
> cannot by themselves establish the identity of the resources they
> address).

This confuses the *possibility* of resolving a namespace URI with
the *non-necessity* of doing so.  I take the position of the
Namespace Rec to be that resolution is not useful for determining
identity (namespace identity = URI identity) but resolution *may*
provide properties of the namespace other than identity, because
of the principle of the indiscernibility of identicals: if two
namespace URIs are identical, the resources they mention are either
non-existent or identical likewise.  (The converse does not hold,
of course.)

The result is that two namespace URIs which resolve to the same
resource define two namespaces which share some properties but are
not identical.  

<sidetrack>It is not necessarily the case that URNs are resolvable
to URLs: the URN urn:isbn:0-671-79573-2 has a referent
(_The Gripping Hand_, by Larry Niven and Jerry Pournelle, 1st edn hc)
but 1) is not resolvable to an URL and 2) more strongly, there
does not exist an URL which would be a correct resolution of it.

John Cowan	http://www.ccil.org/~cowan		cowan at ccil.org
	You tollerday donsk?  N.  You tolkatiff scowegian?  Nn.
	You spigotty anglease?  Nnn.  You phonio saxo?  Nnnn.
		Clear all so!  'Tis a Jute.... (Finnegans Wake 16.5)

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