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Re: [ontolog-forum] OWL and lack of identifiers

To: "John F. Sowa" <sowa@xxxxxxxxxxx>
Cc: "[ontolog-forum]" <ontolog-forum@xxxxxxxxxxxxxxxx>
From: Pat Hayes <phayes@xxxxxxx>
Date: Sun, 15 Apr 2007 22:20:55 -0500
Message-id: <p0623090dc24896909230@[]>
>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 
website.    (05)

>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
>illegally.    (06)

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)

Pat    (010)

>John Sowa
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