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Re: Possible new issue: Things with and without identity?

From: Stefan Eissing <stefan.eissing@greenbytes.de>
Date: Tue, 10 Sep 2002 14:09:12 +0200
Cc: Miles Sabin <miles@milessabin.com>, uri@w3.org
To: "Roy T. Fielding" <fielding@apache.org>
Message-Id: <1E8327EA-C4B6-11D6-9825-00039384827E@greenbytes.de>


Am Dienstag den, 10. September 2002, um 11:43, schrieb Roy T. Fielding:

>
> On Monday, September 9, 2002, at 08:44  PM, Miles Sabin wrote:
>> A discussion point from the rest-discuss list which is probably better
>> dealt with here,
>>
>>   http://groups.yahoo.com/group/rest-discuss/message/2465
>>
>> At issue is the first sentence of the informal definition of 
>> resource in
>> RFC 2396 1.1,
>>
>>   A resource can be anything that has identity.
>>
>> "that has identity" is redundant because *everything* has identity in
>> the only reasonably straightforward understanding of identity, ie. the
>> logical truth in all but the most obscure formal systems that,
>>
>>   (Vx) x = x
>
> No, that is not the only reasonably straightforward understanding of
> identity.  That is the specific mathematical axiom of identity and not
> identity itself.  We already had that discussion on www-tag.
>
> Everything does not have identity.  "Everything" includes both 
> things that
> have been identified and those that have not yet been identified.  
> Something
> only becomes a resource after it has been identified, but not 
> necessarily
> by a URI.
>
> The definition is correct as it stands.

Maybe as an add-on to this:

WebDAV working group decided to call a URL without a resource
(e.g. 404 on GET) an "unmapped URL".

In that context, one could call a resource without URI a
"unmapped resource".

So instead of going metaphysical with names and identity, it
maybe makes more sense of talking about *the* mapping
between the set of resources R and the set of names N
and define properties of this mapping M.

M: R -> N

M is neither injective, nor surjective, that is
   there are (many ;) x in R without n in N so that M(x) = n
   there are n in N without x in R, so that M(x) = n

and as stated before: |R| > |N|

Of interest to RFC 2396 is the subset of names which are uniform
and follow a specific syntax. Let's call the set of such names
U. So what RFC 2396 describes is which elements of N are
also elements of U. Elements of U are called URI.

"Having identity" means than that there is a mapping between
U and R. Such a mapping can be established in several defined
ways and, as of the uniformity, in endless, future, not-yet
defined ways.

For the subset of URIs "below" http://www.greenbytes.de, my
web server is the authority over the mapping between those
URIs and the set of resources on my server (which might no all
have a URI as it might refuse to offer any
file://greenbytes.de mappings or might not even have a file system).

(If I take my server down, the mapping with URI is lost, although
the resources still exist and probably have names given to them
by the operating system.)

So, if we restrict RFC 2396 to only talk about the mapping of
URI to resource and defining that "having identity" in RFC 2396
means that such a mapping exists (established by whomever),
do we then get rid of the metaphysical issues?

//Stefan
Received on Tuesday, 10 September 2002 08:09:28 UTC

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