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At 11:08 AM 4/26/2002 -0400, Mark Baker wrote:
>On Fri, Apr 26, 2002 at 09:25:20AM -0500, Bullard, Claude L (Len) wrote:
> > A black hole.
>
>Could have lots of different representations;
>
>- a picture (xray)
>- its location
>- its Schwarzschild radius
And each of these would be a representation, not the black hole itself. In
fact, if all of these representations existed, each would have a different
identity.
>It obviously can't identify itself, but anybody can identify it for
>themselves. For example;
>
>http://lenbullard.com/xml-dev-black-hole/
That's an identifier, not the property of identity. Just writing that
string did not give your system any magical ability to touch a black hole.
In most computer systems, identity is a mapping of some symbol (an address
in RAM, a table name and ID in a database, a persistent identifier, a URI)
and a representation of an instance (an object, a row in a table, a
persistent object, a web resource). The notion is that a given identifier
always maps onto "the same thing".
Of course, "the same thing" is vaguely defined if we allow the
representation of an instance to change.
Does the resource identified by this URL have identity?
http://www.weather.com/weather/local/27703
Or how about the resource identified by this URL:
http://www.time.gov/timezone.cgi?Eastern/d/-5/java
There are at least two consistent ways to say that it does have identity.
The first is to say that the identity is in the mapping itself, and
whatever is returned by the mapping is the resource associated with the
identifier. If I click on http://www.weather.com/weather/local/27703
several times and get the home page of Amazon.com the first time, get a
treatise on Wittgenstein's paradox of identity with the second click, and
get the weather in Borneo with my third click, then all of these are "the
same thing" as defined by the mapping. By definition, there is no such
thing as a bug in code that returns the resource associated with a URI.
Another consistent way to say that an Internet resource has identity is to
introduce human judgment. The system I described in the previous paragraph
does not correctly support the identity of resources because some person or
group of people say it is returning different resources. First let's set up
our jury - pick one or more of the following: Colin Powell, God, the
members of XML-Dev, the QA department for my development team, an Arthur
Andersen auditor moonlighting as a Playboy model. Now let's ask our jury to
click on the same URL twice and tell us if they are the same or different.
For instance, given http://www.time.gov/timezone.cgi?Eastern/d/-5/java, if
the two resources returned are both web pages representing times in the
given time zone, they might reply that two web pages both represent the
same thing, the current time in the EST time zone, but observed at
different times. If the two resources include a description of books
offered by Amazon and a treatise on Wittgenstein's paradox of identity, the
jury might say they are different. How do they know if they are the same or
different? Don't ask me, ask the jury.
Incidentally, it's worth mentioning Wittgenstein's paradox of identity [1]:
Wittgenstein says (Tractatus Logico-Philosophicus, 5.5303):
'Roughly speaking, to say of two things that they are identical is
nonsense, and to say of one thing that it is identical with itself
is to say nothing at all.' If identity is a relation it must hold
either between two distinct things or between a thing and
itself. To say that A is the same as B, when A and B are distinct,
is bound to be false; but to say that A is the same as A is to
utter a tautology.
Wittgenstein dealt with this paradox by a notational trick that is not
unlike URIs[1]:
Anything useful that is said by means of 'is the same as' can be
said by a sentence containing a repeated expression. Thus instead
of saying 'The author of the Iliad was the same as the author of
the Odyssey' we can say, repeating the 'x', 'For some person, x,
both x wrote the Iliad and x wrote the Odyssey', and for 'Florence
is the same as Firenze' 'For some city, x, both x is called
Florence and x is called Firenze'.
>A GET could return any of the above.
But this can be genuinely confusing. Consider the following:
http://www.objects.com/monalisa
Does a GET return a painting, a JPEG of the painting, a biography of the
woman, an art historian's description of the painting, an oldies song? Are
all of these things just representations of a woman, or are they separate
things with identies of their own?
This has caused me problems when querying RDF, since it is not unusual for
assertions to be about completely different "things" in my mind, while
using the same URI. I was surprised by a query telling me that
http://www.objects.com/monalisa was a JPG image of a given size who was
married to an Italian nobleman and hung in the Louvre.
Jonathan
[1] http://www.xrefer.com/entry/552371
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