W3C home > Mailing lists > Public > www-tag@w3.org > July 2003

Re: New issue - Meaning of URIs in RDF documents

From: pat hayes <phayes@ihmc.us>
Date: Wed, 16 Jul 2003 11:56:15 -0500
Message-Id: <p0600121abb3b259fce33@[10.0.100.23]>
To: www-tag@w3.org
Cc: timbl@w3.org
There are several statements made in the following proposal which 
should not be made by any WG. To make authoritative assertions of 
propositions which are clearly or provably false does not make them 
true; it only destroys public trust in the source making the silly 
assertions.

1. " each URI
identify one thing ("Resource": concept, etc)."

Exactly what is meant by "identify" here is not exactly clear, but if 
this means something  close to what it usually means then it is 
simply untenable to claim that all names identify one thing. Existing 
W3C standards already provide counterexamples: what single thing is 
identified by the URI reference 
http://www.w3.org/2000/01/rdf-schema#Class? This is supposed to 
*denote* the class of all RDFS classes; but that is not a single 
well-defined notion, by the very nature of formal semantics: it 
varies from interpretation to interpretation.

Perhaps 'identify' doesn't mean 'denote' or 'refer to'. What does it 
mean, then? Note that if we were to say that 'identify' means MORE 
than simply 'denote' or 'refer to' - if, say, it also has a 
connotation that the URI can be somehow used to retrieve some 
information about the referent - then the claim would become even 
more false.

See 6 below also.

2."  An RDF statement "S P O" means that a given binary relation
identified by P holds between to things identified by S and O. (S, P
and O are URIs)"

This would be true if "identified" meant "denoted"; but then it 
should, if stated strictly, have the qualification "in a given 
interpretation", illustrating the reason why the first claim is false.

3. " The OWL specification is a vocabulary of properties allowing an RDF
document to say things about RDF Properties"

First, OWL is more than an RDF vocabulary: it is an RDF vocabulary 
with a particular semantics applied to it. It is the semantics which 
allows the document (strictly, the RDF graph) to make nontrivial 
assertions, most of which cannot be made in RDF; so it is the OWL 
document making the assertions, not the RDF document (true, an OWL 
document can be described as an RDF document with an OWL semantics, 
but it is misleading to use the syntactic criterion when we use 
phrases like "say things about".)

Second, it is misleading to claim that the assertions made in an OWL 
document are about RDF properties. They may be: but they may also be 
about classes or individuals (ie anything); in most cases it will be 
impossible to say what particular thing they are 'about'. (The use of 
this phraaseology, by the way, is odd: most texts in most languages, 
formal or informal, cannot be said to be 'about' any one thing.)

4.  "The combination of these architectural parts allows information to be
published so that the recipient of an RDF statement  "S P O" can, by
dereferencing P, get information about the relation being asserted. "

Wrong. First, there is no particular semantic importance attached to 
the P part of the triple. Properties have no special status in RDF. 
Second, the relation is not being asserted: the triple is. Third, 
there is no particular reason why dereferencing P will get you to the 
information you might require in order to draw the appropriate 
conclusions; and indeed most RDF applications would not work if this 
were an architectural requirement. Finally, this conclusion does not 
follow from the architectural points made previously.

5. "This information, directly or indirectly acquired, may be
human-readable and/or machine readable, the latter including for
example ontological statements in OWL, or rules, or other logical
expressions."

This is an extremely contentious and potentially confusing claim. It 
is *impossible* for software agents to respond to or utilize 
"information" which is only human-readable: it must be 
machine-readable. So to lump these categories under the single 
heading of 'information' is an architectural disaster, if 'recipient' 
in the previous sentence is supposed to refer to an architectural 
element (such as an agent of some kind). This point is not new, of 
course: it has been made already in many intense discussions and 
debates, many of them archived.

6. "-  the architecture is that a single meaning is given to each URI "
and
"- the architecture does not permit the meaning of a URI to be changes
by consistent misuse by others"

These are IMPOSSIBLE architectural requirements. There are no precise 
theories of meaning which make such statements other than fatuous 
(except when we are talking about programming languages: but the 
universe as whole does not satisfy the second recursion theorem.) 
There isn't a 'single meaning' for the addition sign, or the 
multiplication sign; and the 'not changed by others' condition is 
either clearly false (in some views of 'social meaning') or 
completely irrelevant (on referential theories of meaning); either 
way, it isnt much use insisting on it as an architectural condition. 
(FWIW, this seems to me to confuse meaning with intended meaning. 
Intended meanings, however, are not the kind of thing that one can 
impose *architectural* conditions on; they are more a matter for 
courts and priests to decide.)

Having a definitive ontology does not provide a unique meaning, by the way.

7. "The community needs
1) A concise statement of the above architectural elements from
different specs in one place, written in terms which the ontology
community will understand, with pointers to the relevant specifications."

Maybe, if I could make the suggestion without seeming to commit 
lese-majesty, it would be a good strategy for the W3C, rather than 
trying to render nonsense "in terms that the ontology community will 
understand", to ask if it might possibly learn something from 
actually *listening* to the ontology community; or at any rate, to 
anyone with a grasp of basic 20th-century results in linguistic 
semantics.

Pat Hayes

--------------

Resent-From: tag@w3.org From: Tim Berners-Lee < timbl@w3.org >
Date: Mon Jun 30, 2003  15:05:02 US/Eastern
To: tag@w3.org Subject: New issue - Meaning of URIs in RDF documents


The Semantic Web Coordination group at its meeting of 2003-06-30  and
passed on to the Tag the issue which had been loosely described in RDF
circles as "social meaning".  As background,

- The URI specification defines URI syntax and explains that each URI
identify one thing ("Resource": concept, etc).
- RDF documents use URIs as identifiers for things including for
relations. An RDF statement "S P O" means that a given binary relation
identified by P holds between to things identified by S and O. (S, P
and O are URIs)
- The HTTP specification provides for a set of URIs which have (a)
delegated ownership (b) publication and retrieval of information
resources.
- The OWL specification is a vocabulary of properties allowing an RDF
document to say things about RDF Properties
- The TAG has written on the desirability of using dereferencable URIs,
and of actually publishing relevant and useful information.

The combination of these architectural parts allows information to be
published so that the recipient of an RDF statement  "S P O" can, by
dereferencing P, get information about the relation being asserted.
This information, directly or indirectly acquired, may be
human-readable and/or machine readable, the latter including for
example ontological statements in OWL, or rules, or other logical
expressions.

The community needs

1) A concise statement of the above architectural elements from
different specs in one place, written in terms which the ontology
community will understand, with pointers to the relevant specifications.


2) Some outline guidance on specific questions brought up in email
questions
- Is a given inference engine expected to take into account a given
document under given circumstances?
- how does one avoid having to commit to things one does not trust?
- etc etc etc

3) There may be some need to clarify frequent misunderstandings by
making some things clear.

-  the architecture is that a single meaning is given to each URI (such
as P), that the URI ownership system makes statements by owners
authoritative weight, despite what other documents may say.
- the architecture does not permit the meaning of a URI to be changes
by consistent misuse by others.
- that use of a URI in RDF implies a commitment to its ontology, and if
there is doubt as to what ontology that is, the web may be used to
resolve it.
- that the web is not the final arbiter of meaning, because URI
ownership is primary, and the lookup system of HTTP is though important
secondary. (That is, if you hack a web server's ontology files, you do
not change hat the URI means, you just break a machine for a while)
- etc etc.

The proposal is that a draft finding be written which pulls this
together, with elaborations pointing into the various specs. Members of
the SWCG have volunteered and some members of members of the SWCG have
been volunteered to read early versions.

tim bl
-- 
---------------------------------------------------------------------
IHMC	(850)434 8903 or (650)494 3973   home
40 South Alcaniz St.	(850)202 4416   office
Pensacola			(850)202 4440   fax
FL 32501			(850)291 0667    cell
phayes@ihmc.us       http://www.ihmc.us/users/phayes
Received on Wednesday, 16 July 2003 13:52:38 GMT

This archive was generated by hypermail 2.2.0+W3C-0.50 : Thursday, 26 April 2012 12:47:19 GMT