W3C home > Mailing lists > Public > uri@w3.org > April 2003

resources having identity

From: pat hayes <phayes@ai.uwf.edu>
Date: Mon, 21 Apr 2003 13:16:58 -0500
Message-Id: <p05111b05bac9e58ccc0a@[10.0.100.12]>
To: fielding@apache.org, uri@w3c.org
Greetings.

I am writing to you as editor of the forthcoming document intended to 
replace RFC 2396, and more broadly to the URI WG, in order to comment 
on the issue

http://www.apache.org/~fielding/uri/rev-2002/issues.html#024-identity

and to request that you and/or the WG clarify the intended meanings 
of some of the introductory definitions.

1.  I appeal to the WG to please explain in more detail what the word 
"resource" is intended to refer to, if only in broad outline. In 
particular, If there is an intent to limit the meaning of "resource" 
to some subset of the universe of logically possible entities, it 
would be most valuable if this could be spelled out as clearly as 
possible. This issue appears to be central to many aspects of the 
semantic web, and probably to the web more generally. The language of 
the introductory sections of RFC 2396, reproduced in the current 
version of your document draft, is not sufficient to achieve a clear 
communication of this intent as it stands.

As some examples, are any of the following NOT resources in the sense 
used in your document?

a. A document which has not yet been written, eg a book in progress, 
which has not (yet) been assigned a title or ISBN number.
b. A particular elephant, eg one in a zoo.
c. A particular elephant which is now dead, eg the original Jumbo.
d. A particular elephant which it is hoped will be the product of a 
future mating between two elephants.
e. Santa Clause (in any sense, eg as a fictional character, or as a 
concept in folk mythology, or whatever. Or use Sherlock Holmes or 
Superman or any other fictional character, if you prefer.)
f. The planet Mars.
g. The number one thousand seven hundred and twenty-nine.
h. An abstract class or category, such as the class of all types of 
French red wine.

----

2.  Miles Sabin, in an archived email comment, points out that the 
phrase 'that has an identity' is redundant as a qualifier, since 
everything necessarily has an identity. Your response says that 'The 
goal of the sentence ("A resource can be anything that has an 
identity.") is to describe the essence of what it means to be a 
resource' and that 'sameness of identity is the ... defining 
characteristic of a resource'. The only way I can interpret this is 
as saying that a resource can be anything, since the defining 
characteristic is apparently a tautology.  Is that what you intended? 
If not, can you clarify your intended meaning? In particular, how do 
the following sentences differ in meaning, in your view?

A.  Anything with identity can be a resource but not necessarily is a resource.
B.  Anything can be a resource but not necessarily is a resource.

It might help if you could indicate what you consider the phrase 'has 
an identity' to mean, particularly when used as a qualifier, perhaps 
by giving an example of something that does not have an identity, in 
your sense.

----

3.  I would like to ask for some explication of the use of the words 
"can be" in the definition, to which you draw attention in your reply 
to Sabin. I take it that this is intended to convey that there is a 
distinction between entities which could possibly be resources, and 
those that actually are resources. If this is right, can you explain 
the criteria for distinguishing actual from merely possible 
resources? That is, suppose X is something which *could be* a 
resource; what would make X *actually be* a resource?

Can something become an actual resource at a time, or cease to be a 
resource at a time? Can something be intermittently an actual 
resource, or must each actual resource have an uninterrupted period 
during which it is being the resource that it in fact is? Questions 
like this will be central if we try to make formal theories of 
resource-hood for use by reasoners.

  ----

4. RFC 2396 includes a particular note which is very hard to interpret:
"The resource is the conceptual mapping to an entity or set of 
entities, not necessarily the entity which corresponds to that 
mapping at any particular instance in time.  Thus, a resource can 
remain constant even when its content---the entities to which it 
currently corresponds---changes over time, provided  that the 
conceptual mapping is not changed in the process."

There are several problems with this.

First, it does not specify what it means by "conceptual mapping", nor 
how such a mapping can remain constant while its range changes.

Second, it does not say what is meant by the phrase "entity which 
corresponds to [a] mapping at [an] instant of time".  What does it 
mean for something to 'correspond to' a mapping?

Third, the use of the word "content" seems to suggest that resources 
are something like representations or descriptions, rather than the 
entities which are represented or described; but this seems to be at 
odds with what the document says in the immediately preceding 
paragraphs. For example, we are told explicitly that a person or a 
book can be a resource, but neither people nor books are the kinds of 
entity which would normally be described as having "content".

Fourth, the reference to time and change seems to imply that 
resources are inherently temporal or dynamic in their nature; but 
this does not seem to be reflected in any other part of the document, 
or in URI syntax, or in the examples given explicitly in the 
immediately previous paragraphs. For example, what kind of mappings 
can have different things 'corresponding' to them at different times?

Fifth, is this paragraph supposed to apply to all resources, or only 
to indicate that some resources may be dynamic in the way indicated?

(My purpose, let me emphasize, is not to urge that any particular 
interpretation be put on these words, only that their intended 
meaning be spelled out more clearly. )

----

5. The RFC 2396 text explicitly asserts that "not all resources are 
network "retrievable" ",  but almost immediately then says: "having 
identified a resource, a system may perform a variety of operations 
on the resource, as might be characterized by such words as 'access', 
'update', 'replace' or 'find attributes' "

These assertions seem to be at odds with each other, and to reflect 
different notions of 'resource', since the second sentence seems to 
refer only to entities which are "network-retrievable". Clearly, a 
resource which is not retrievable is not available to have operations 
performed on it, even if it is in some sense identified. As an 
example, the SS number of a  dead US citizen is sufficient to 
'identify' that person in a sense, but does not provide any way to 
perform operations on the deceased.

Again, it would be helpful if the apparent contradiction could be explained.

----

Thank you for your attention.

Pat Hayes




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Received on Monday, 21 April 2003 14:17:02 GMT

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