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Re: A shot back at #rdfms-resource-semantics

From: Frank Manola <fmanola@mitre.org>
Date: Tue, 08 May 2001 15:12:20 -0400
Message-ID: <3AF84514.C4470E59@mitre.org>
CC: w3c-rdfcore-wg@w3.org
Dan Connolly wrote:
> 
> Martyn Horner wrote:
> [...]
> > OK. The M&S says that RDF asserts facts about resources and that
> > `resources are always named by URIs plus optional anchor ids'. That
> > doesn't define resources but the glossary says that `resources represent
> > entities'.
> 
> Er... where? I don't see that in the glossary entry you excerpted.
> Quite the other way around: resources are represented by
> entities. Think of the case of a resource being a person;
> an entity representing the state of a person might be a photo
> of the person. Clearly the person doesn't represent the photo.
> 

Actually, I think Martyn only slightly misquoted the M&S glossary entry
for "resource", the crucial piece here being the first sentence:  

"Resource:  An abstract object that represents either a physical object
such as a person or a book or a conceptual object such as a color or the
class of things that have colors."  

If you take an "entity" as being a physical object, then you have
Martyn's reading that resources represent entities.  In fact, as Dan
points out, this seems backwards, assuming a (at least one) common
understanding of "represents":  the M&S glossary seems to say that a
resource (an abstract object) can "represent" a physical object, whereas
a common understanding of "represents" would say that abstractions don't
"represent" physical things;  instead, physical things represent
abstract things.  Complicating the definitional problem is that Section
2.1 of the M&S talks about resources "being" (not "representing")
things, e.g., "A resource may also be an object that is not directly
accessible via the Web;  e.g., a printed book."  Note that this doesn't
say a resource may "represent" such an object, but that it may "be" such
an object.  Those aren't the same thing. 

Further contributing to the confusion is the definition (or lack of
same) of "entity".  In an earlier message, Dan gave us the definitions:

>       "Resource
>          A resource can be anything that has identity.  Familiar
>          examples include an electronic document, an image, a service
>          (e.g., "today's weather report for Los Angeles"), and a
>          collection of other resources.  Not all resources are network
>          "retrievable"; e.g., human beings, corporations, and bound
>          books in a library can also be considered resources.
>          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."
> 
>         -- http://www.ietf.org/rfc/rfc2396.txt

and

> "resource 
>      A network data object or service that can be identified by a URI, as
>      defined in section 3.2. Resources may be available in multiple
>      representations (e.g. multiple languages, data formats, size, and
>      resolutions) or vary in other ways. 
> entity 
>      The information transferred as the payload of a request or
>      response. An entity consists of metainformation in the form of
>      entity-header fields and content in the form of an entity-body, as
>      described in section 7. "
>         -- http://www.ietf.org/rfc/rfc2616.txt

Unfortunately, I doubt if the definition of "entity" as used in rfc2616
is that same as that intended in rfc2396.  For example, where rfc2396
says "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", I can't see "entity" here referring
to "the information transferred as the payload of a request or
response", since there may be no such information to transfer (the
resource may not be network-retrievable).  Note also that TBL's "Generic
Resources" paper at http://www.w3.org/DesignIssues/Generic.html says "A
"resource" is a conceptual entity".  It seems to me that "entity" is
used in a rather generic sense in rfc2396 (and I think Brian's model is
getting along in the right direction where "entities" are concerned),
but this all needs some fleshing out.  

Bottom line:  I think we need to decide on a coherent picture of
resources, URIs, the things in the physical and network "worlds" that we
want to say things about, etc., and come up with good words** to
describe that picture, somewhat independently of the various specs we
have in hand (but trying to be as consistent with them as we can, in
order not to break things).  I suspect rfc2396 would be a reasonable
starting point, but there's stuff about "entities" and "conceptual
mappings" to be filled in, among other things.  I seem to recall the
Topic Map folks using some concepts making related distinctions (not
necessarily more clearly expressed, unfortuately).  

--Frank

** Actually, we need a mathematical-logic-style formal specification,
the "good words" being its natural language correspondent

-- 
Frank Manola                   The MITRE Corporation
202 Burlington Road, MS A345   Bedford, MA 01730-1420
mailto:fmanola@mitre.org       voice: 781-271-8147   FAX: 781-271-8752
Received on Tuesday, 8 May 2001 15:13:06 EDT

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