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Re: homonym URIs (Re: What if an URI also is a URL)

From: Pat Hayes <phayes@ihmc.us>
Date: Tue, 12 Jun 2007 11:21:47 -0500
Message-Id: <p06230903c29471c79c4e@[]>
To: "Sandro Hawke" <sandro@w3.org>
Cc: semantic-web@w3.org

>Pat Hayes <phayes@ihmc.us> writes:
>>  Tim, as this discussion gets to the heart of what
>>  Ive been trying to argue for several years,
>>  please take the comments below as intended in a
>>  spirit of analysis rather than just pins and
>>  angels.
>Pat, I'm going to jump in here, if you don't mind.

Of course not. I copied your rather short CC line, though.

>  I think my position
>on these issues is pretty much the same as Tim's but I could be wrong.
>I don't argue that John's "dance" isn't required, just that part of the
>Semantic Web version of the dance is: don't make your URIs unnecessarily
>ambiguous.  One might even say: don't pun.

Well, one man's punning is another man's efficient use of language. 
If this same attitude is going to say that the same URI can't be used 
to denote both an individual and a class and a property, then to hell 
with it. Its just going to be a sheet-anchor stopping technical 

I'd like to see some evidence that punning me and my email address is 
ever going to cause an actual problem, for that matter. Now, punning, 
say, me and my wife, or me and my eldest son, *is* likely to cause a 
problem. But the appropriate thing to say is not to denigrate 
punning, but to explain what is wrong with doing it badly.

>>  And what about a URI
>  > that I own and wish it to denote, say, the planet
>>  Venus, or my pet cat? What do I do, to attach the
>>  URI to my intended referent for it?
>You publish a document (an ontology) so it's available through that URI.
>If it's a hash URI, you publish the ontology at the non-hash version.
>If it's a slash URI, you publish the ontology at the far end of a 303
>redirect.  And you content-negotiate HTML and RDF.
>So when users paste that URI into their browser, they get the official
>documentation about it.

None of that attaches a URI to my cat (though see below)

>And when RDF software dereferences that URI, it gets some logical
>formulas which should be understood (like the HTML) to be asserted by the
>URI's owner/host/publisher.  Those formulas constrain the possible
>meanings of that URI, relative to other URIs.

Neither does any of that (and in this case, I can *prove* it, using 
Herbrand's theorem.)

>  They can't nail a URI to

Quite. In fact, none of this can nail a URI to ANYTHING other than 
something accessible using a transfer protocol.

>, but they can use other ontologies to provide useful (and possibly
>very constraining) information, like that it's an astronomical body with
>a mass of about 5e+24kg.

You are begging the question. Suppose an ontology asserts

ex:Venus rdf:type ex:AstronomicalBody .

Now, what ties that object URI to the actual concept of being an 
astronomical body? And so on for all the other URIs in all the other 
OWL/RDF ontologies. The best you can do is to appeal to the power of 
model theory to sufficiently constrain the interpretations of the 
entire global Web of formalized information. But that argument from 
Herbrand's theorem (basically, if it has a model at all then it has 
one made entirely of symbols) applies just as well no matter how 
large the ontology is.

The only way out of this is to somewhere appeal to a use of the 
symbolic names - in this case, the IRIs or URIrefs - outside the 
formalism itself, a use that somehow 'anchors' or 'grounds'  them to 
the real world they are supposed to refer to. If we all assume that 
English words are so grounded (not a bad assumption) then this can be 
done in principle by using the URI in English sentences or to other 
kinds of representation which are widely accepted as real-world 
identifiers, like SS numbers or facial images. I did all three in


If the TAG said this somewhere, and recommended how to do it, that 
would be great.

>My advice here is, I confess, not widely followed.  But I hear more and
>more people converging on the idea that this is both practical and
>likely to be sufficiently effective.

I agree. Still, its important to describe it properly. It doesn't 
mean that URIs have a unique denotation.

>>  The point surely is that URIs used to refer (not
>>  as in HTTP, but as in OWL) do *not* have a
>>  standardized meaning. Standards are certainly a
>>  chore to create, but they only go so far. OWL
>>  defines the meanings of the OWL namespace, but it
>>  does not define the meanings of the FOAF
>>  vocabulary,
>No, that's up to the owner(s) of the FOAF terms.
>>  or the URIrefs used in, say,
>>  ontologies published by the NIH or by JPL.
>And that's up to the NIH and JPL, respectively.

I understand that. I was reacting to Tim's comments, which seemed to 
suggest that all this should be determined by standards-setting 

>>  The
>>  only way those meanings can be specified is by
>>  writing ontologies: and finite ontologies do not
>>  - cannot possibly - nail down referents
>>  *uniquely*.
>Ah -- there we go.  There must be a long history of this subject in
>philosophy.  Can things ever be nailed down uniquely?  I haven't a clue.
>But that's the wrong question.

Surely this is exactly the question. I didn't raise the issue, Tim 
did. There is a claim, often repeated and sometimes cited as 
doctrine, that a URI *must* identify a *single* referent. To do this 
requires that things are nailed down uniquely (isn't that EXACTLY 
what it says?) but they can't be.

>  In this thread, I don't think we're
>talking about whether we can really be sure what we mean when we say
>such a URI denotes Venus.

Well then don't SAY that is what you are concerned with, for 
goodness's sake. That is what is implied by "the URI for Venus has a 
unique denotation".

>  Instead, we're talking about whether it's a
>good practice to use a single URI to denote clearly distinct things

Aaaaargh. What do you think is 'clearly' distinct?

The second rock from the sun might be a continuant or an occurrent. 
Those are as clearly distinct as a rock and a Roman goddess. I know 
people are a lot more familiar with the second kind of clearly 
distinct, but ontologies aren't people. And the first kind of 
difference is more important, if anything, than the second, for an 
ontology. The second kind of muddle is easily resolved. The first 
kind can be fatal.

>such as:
>    (1) the second rock from the sun
>    (2) the Roman goddess of love
>    (3) a star tennis player
>    (4) ... etc
>The term "ambiguity" covers both these issues, but we don't need to
>combine them.

Well, you tell me how to distinguish them, then.

>  The first is a kind of imprecision, a fuzziness

No, its worse than that. Its like the distinction between an object 
and a process. Fuzziness/imprecision is what gives you the 'Everest' 
kind of examples.

>, while
>the second is the re-use of a word for a second meaning, a homonym.
>(Homonyms seem to be called "overloading" in computer programming.)
>I think we know how to work with homonyms, but since we're engineering a
>new system, it seems like a good design decision to forbid them, doesn't

Well, actually, no. Overloading is widely used for good engineering 
reasons. And on an open system like the Web, we arent going to be 
able to prevent it happening, so we will need to have methods of 
dealing with it. Once those are deployed, one might as well take 
advantage of them. Making grand statements about what should be done 
seems to me like trying to tell evolution what it ought to be doing.

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Received on Tuesday, 12 June 2007 16:22:03 UTC

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