John F. Sowa wrote:
> Patrick,
>
> > Yes, but would you agree that there is no one
> > universal space-time coordinate system such that
> > any physical object could have two or more unique
> > identifiers in different coordinate systems?
>
> There is no problem with having multiple naming systems
> if there is a one-to-one mapping between them.
>
> For example, there's a one-to-one mapping between
> rectangular coordinates (x,y,z) and polar coordinates
> with a radial distance r and two angles (assuming, of
> course, that you treat angles modulo 360 degrees as
> identical). For some applications, one coordinate
> system might be better than the other, but you can
> always transform one system to the other. (01)
Theoretically, yes. In many real-life cases there would be a loss of
informations, since real (possibly irrational) numbers may be involved,
and practical systems have limits on their representation of numbers.
(But this is an implementational detail.) (02)
> But there are many issues about systems for which
> there is no unique mapping and for which there is
> no clear idea of what exactly is being identified.
>
> 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. (03)
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. (04)
> 2. The contents move to another location. (05)
See above. (06)
> 3. The contents are identical to the contents at
> many other locations. (07)
Weird; what sort of identity do you have in mind? Indiscernibility?
Relative identity? (08)
> 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? (09)
But here clearly you mean a URI that identifies the content of a region,
not the region. These should not be confused. But I agree that URIs
supposed to identify a resource are often heavily underspecified as to
what they actually identify. Now, if the content of a region moves to
another location, preserving its identity, then a URI pointing to the
previous location no longer traces the content (but note: if it
identified the location, it did not identify its content.) (010)
>
> Similar problems occur with most other naming systems,
> but they become especially difficult when you're talking
> about abstract stuff like information. (011)
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. (012)
Regards,
Wacek (013)
>
> John
>
>
>
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--
Wacek Kusnierczyk (015)
------------------------------------------------------
Department of Information and Computer Science (IDI)
Norwegian University of Science and Technology (NTNU)
Sem Saelandsv. 7-9
7027 Trondheim
Norway (016)
tel. 0047 73591875
fax 0047 73594466
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