>To begin with the conclusion:
>JFS>> Similar problems occur with most other naming systems,
> >> but they become especially difficult when you're talking
> >> about abstract stuff like information.
>WK> Yes, and this is why the assumption behind semantic web
> > technologies that a URI should identify the same entity
> > on every occasion should be treated with highest suspicion.
>That is the problem. People need a content identifier, but
>what they are given is a location identifier. (01)
I wish you wouldn't keep saying this, John. Its
not true. A URI does not necessarily identify a
location. Its not intended to identify a location
(unless a location is the thing you are talking
about.) For a start, it can respond to a GET with
a redirect. There is an entire infrastructure of
caching, etc., which allows resources to be moved
without altering the identification relationship
between its URIs and it. Im not saying that the
Web is perfect, still less that it is always used
in recommended ways: but its not fair to the W3C
to imply that they are ignorant of the
location/content distinction. They have been
acutely aware of it for quite a while now, and
have done a lot of work to overcome exactly the
problem that you are pointing to. (02)
Any Internet identification scheme has to provide
*some* kind of pathway from the invocation of the
identifier to the thing identified. That is (I
presume) what the word 'identify' means in this
context. So if you want to say this makes it a
'location identifier', then indeed it is. But in
this sense it is *necessarily* a location
identifier, so its pointless to worry about how
to change that. (03)
>Some further comments on the preceding four points:
>JFS>> For example, a URI that points to a location uniquely
> >> identifies that location. But what happens with the
> >> following cases:
> >> 1. The contents at that location change over time.
>WK> I would say: nothing happens. If the URI points to a
> > location (given some arbitrary system of coordinates),
> > the change of the content of the location does not change
> > the location.
>That's the problem: nothing happens. But more often
>than not, the person who is looking for the information
>needs to know (a) whether the content has been changed,
>(b) why it has been changed, or (c) where the original
>information can be found. (04)
All of which can be done with URIs. Look at the
header of any W3C official document, for example,
which has 'this version', 'last version' and
'latest version' URIs. The 'latest version' URI
identifies a 'moving' resource; the others are
fixed to particular bit-images, but even those
are not addresses, but rather documents. In fact,
in many cases they have been moved from 'active'
storage to archives as the W3C reorganizes its
>JFS>> 2. The contents move to another location.
>WK> See above.
>Yes, all the problems of #1 are repeated.
>JFS>> 3. The contents are identical to the contents at
> >> many other locations.
>WK> Weird; what sort of identity do you have in mind?
> > Indiscernibility? Relative identity?
>Not weird at all. Indiscernible copies are abundant
>throughout the WWW, deliberately, accidentally, or
True. But, so? A copy is not *identical* to its master. It is a copy of it. (07)
>JFS>> 4. The contents can be interpreted in many different
> >> ways -- e.g., does the URI identify a web page or
> >> the service supported at that web site?
>WK> But here clearly you mean a URI that identifies the
> > content of a region, not the region. These should not be
> > confused.
>That's what I was trying to say. They are very commonly
>confused. What most people want is the content, but what
>they are given is a location whose connection with the
>content is tenuous at best. (08)
If people were to build websites which follow the
W3C good practices (all of which are published
and revised regularly), then what they are given
is an identifier of a 'thing' which may itself be
dynamic, but whose identity is not dependent on
its location. Of course, many people just build
websites in the easiest way, which is to add a
file locator to a base URI to make a new URI. I
confess that I do this myself quite often. (09)
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