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Re: Information resources?

From: Roy T. Fielding <fielding@gbiv.com>
Date: Wed, 8 Sep 2004 17:18:28 -0700
Message-Id: <C6161F6F-01F5-11D9-83F4-000393753936@gbiv.com>
Cc: www-tag@w3.org, Mark Baker <distobj@acm.org>
To: Sandro Hawke <sandro@w3.org>

On Sep 8, 2004, at 3:11 PM, Sandro Hawke wrote:
>> Hmm, I think this paragraph goes too far ...
>> On Wed, Sep 08, 2004 at 04:30:13PM -0400, Sandro Hawke wrote:
>>>    Information Resources are the only kind of resources which can 
>>> have
>>>    representations.  The number 1, which is not an Information
>>>    Resource, might be said to be represented by the two-octet 
>>> sequence
>>>    0x0001, but not in the sense of "representation" used in this
>>>    document.
>> No?  Why not?  It seems to me that it does.
> What information is contained in the number 1?

All that is inherent in its identity as a concept.  If that were not
information, the concept could not be taught to math students, let
alone small children who just want to know how many helpings of dessert
they are allowed to eat.  Since you can communicate the concept to me
with just two words (and a lot of context), the concept can and is
represented as such even if that is not sufficient (alone) to
encompass its "meaning".

>>>  A web-accessible control dial, set to "1", might respond
>>>    to HTTP GET requests with a representation of its state: 0x0001.
>>>    In this example, 0x0001 acts an identifier for the number 1 within
>>>    the data format being used.

No, it acts as one representation of its state as mapped to the format
described by the media type.  It doesn't say anything about how the dial
is implemented, nor the meaning of 0x0001, though that may be discovered
elsewhere or elaborated by the media type.

>> I think it's representing, not identifying.  If it were an
>> identifier, it should be a URI (or an EPR, I suppose 8-).
> It could indeed be a URI, but people (eg Patrick Stickler in RDF Core
> :-) argued that more compact text/bits were needed for things like
> numbers.

Umm, like "n:1"?  [I don't believe that all identifiers need to be 
Let's say that, instead, the response comes back as

    HTTP/1.1 200 OK
    content-type: application/uri


(and someone actually bothered to register those things) where the
media type is defined to mean "that which is identified by the enclosed
URI".  What then?  Is that not capable of representing *everything* we
know and mean by the abstract concept of 1?

Furthermore, any degraded form of representation is also sufficient
to represent 1 provided that the recipient can still understand its
meaning, in the same way that various lossy graphic formats can be
used to represent the same resource as a 24bit TIFF.  That is what
representations do: they do not define the nature of the resource --
they merely have to be consistent with the resource.

>>>  An HTTP GET of a URI for the number 1
>>>    itself could meaningfully be met with an error or redirect, but 
>>> not
>>>    with a representation.
>> Gotta disagree with you there (even before pointing out that errors
>> and redirects are representations 8-).
> The MIME Entity (representation) carried in an error or redirect HTTP
> response (in the cases where there is one, like 404) is not stated in
> the spec as being a representation of the named resource, as far as I
> can tell.

It is a representation of the error as defined by the server, not of
the requested resource.  Mark is right -- the assertion you give does
not correspond to how representations are used on the Web.

Personally, I think the distinction between resource and information
resource is unnecessary.  It is an attempt to fix broken assertions by
claiming that there is an artificial divide among classes of resources,
thereby defining systems that bridge that divide to be "bad" just to
satisfy a philosophical argument over the limitations of RDF.

I think it would be better to simply accept that some assertions
are on resources and other assertions are on representations of
resources, and any attempt to make an assertion on a resource that
isn't true of all of its representations is just a bad assertion
(like any other bad assertion).  It can be fixed by defining new
property URIs that are more specific about what is being asserted,
or by doing what TopicMaps does in distinguishing the targets.
It cannot be fixed by creating an artificial divide, because the
same problems occur when making assertions about a content
negotiated (information) resource.

And the only reason this argument has persisted for so long is
because people insist on framing it as something "new" that only
affects the semantic web, and using obscure examples of dogs and cars.
If you simply take the same example and apply it to "my home page",
wherein the page actually consists of multiple representations
that are independently authored and may or may not be provided
depending on the requesting user agent, then you can use the same
invalid dc properties to make contradictory assertions about
"my home page" that you would for a "car" or "picture of the car",
or for anything else for that matter.  This is not the fault of
the person who had the nerve to place "my home page" on the Web:
it is the fault of using ambiguously targeted assertions within
a language that assumes properties are unambiguous.

Received on Thursday, 9 September 2004 00:18:38 UTC

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