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Re: Comments on "SPARQL 1.1 Uniform HTTP Protocol for Managing RDF Graphs"

From: Nathan <nathan@webr3.org>
Date: Sun, 20 Mar 2011 15:30:02 +0000
Message-ID: <4D861D7A.9050102@webr3.org>
To: Pat Hayes <phayes@ihmc.us>
CC: Tim Berners-Lee <timbl@w3.org>, Kjetil Kjernsmo <kjekje@ifi.uio.no>, SW-forum Web <semantic-web@w3.org>, David Booth <david@dbooth.org>
Pat Hayes wrote:
> On Mar 19, 2011, at 9:31 PM, David Booth wrote:
> 
>> On Sat, 2011-03-19 at 07:49 +0530, Tim Berners-Lee wrote:
>>> On 2011-03 -19, at 02:21, Nathan wrote:
>> [ . . . ]
>>>> So perhaps the question being answered is, can we feasibly carry out
>>>> a conversation where we refer to both a birth certificate and a red
>>>> lightbulb by a single ambiguous name? using RDF?
>>>>
>>>> Possibly, but why even try?
>>> Seeing that without having caught up on the thread,
>>> No, I really do not want to go down that route, of trying that.
>> But you have no choice.  It is *impossible* to define something in a way
>> that is universally unambiguous (except perhaps in vanishingly few
>> cases).  Granted, the example of the birth certificate and lightbulb is
>> rather extreme.  But once you admit this fundamental limitation, it is
>> merely a question of where and when you run into this issue -- not
>> *whether* you will run into it.
>>
>> For example, suppose instead of birth certificates and lightbulbs we
>> were talking about LED lightbulbs versus filament lightbulbs.  Clearly
>> one could imagine an application in which "we refer to both [an LED
>> lightbulb] and [a filament lightbulb] by a single ambiguous name", and
>> the application may work just fine, because it has no *need* to
>> distinguish between LED and filament bulbs.  But to a different
>> application the distinction between LED bulbs and filament bulbs may be
>> as critical as the difference between birth certificates and lightbulbs
>> is to an application that computes power consumption.
> 
> David is exactly right. Here is my favorite example. Consider the claim that Everest was climbed by Edmund Hilary. This is probably stated in RDF somewhere by now, using a URi to identify Everest. But what exactly *is* Everest? Is it a geographical place? (What are its boundaries?) Is it a large piece of rock? (What does it weigh?) Maybe it is a 'mountain', but then you can find many definitions of 'mountain', all of them subtly incompatible. Now, of course, none of these questions matter a damn to the truth of, or even the meaning of, that simple sentence; it is not necessary to answer them in order to determine the truth of the sentence. But if we (wrongly) claim that the meaning or truth of sentences depends upon being able to uniquely and unambiguously identify the referents of all terms used in the sentences, then we are forced into focussing upon all such unanswerable and irrelevant details. 
> 
> Even if we could answer them, by the way, this state of grace would only last for a while, since ironically, the very ability to make new ontologies means that formerly unambiguous names can become ambiguous. Science bristles with such cases. Elements are revealed to be mixtures of isotopes. So "carbon" is now, for some purposes, ambiguous: did you mean carbon-12 or carbon-14? Even apparently robust things like personal names become ambiguous when we have rival ontologies of personhood. "Sir Tim Berners-Lee" might refer to the man in the present, or to a temporal continuant, or to a 4-dimensional entity extended in time. You might object that you don't care about such pettifogging obscurantism, and I agree; but this makes exactly our point, since there are those who do care, and in fact there are clashes between internationally accepted and widely used ontology standards which turn on such distinctions. So your proper name, even if it does uniquely "identify" you, is still
 *ontologically* ambiguous, because (I'm sorry to have to be the one to tell you this, but...) *you* are ontologically ambiguous. Ambiguity of reference is inherent in the very act of naming and describing. There is just no way to avoid this fact, no matter how unpalatable you may find it. 

I hear what you're both saying, and I see the problem in human terms, 
but don't think we have it on the semantic web.. to explain:

For each thing we have names and descriptions, descriptions are always 
ambiguous to an extent, but not the names. Do note that's plural, each 
name is a name for a thing, not the name, each description is a 
description of a thing, not the description of it.

Which leads to saying that a name refers to a thing as described by x, 
or as used in conversation by x, y and z. And this is built in to URIs, 
unlike human names, around the web we do not simply say "lightbulb" or 
"everest", we use full URIs, we can each use our own unique name to 
refer to a thing. Moreover we couple those names to our descriptions to 
describe what we are using the name to refer to, and further still we 
equate names together to say that <me:lb> refers to the same thing as 
<another-source:clb>, to say that when I say <me:lb> I do not refer to a 
filament lightbulb or an LED lightbulb, but to the general class of all 
lightbulbs, the same thing that <another-source:clb> refers to.

This is why we couple descriptions to names, to give an indication of 
what we are using a name to refer to, sure our descriptions may be 
ambiguous and open to refinement, but our names are not; because we are 
not using simple string token names "everest" or "lightbulb", we're 
using distinct URIs. Its not the case that you're saying "lightbulb" and 
I'm saying "lightbulb" and we each refer to two different things by the 
same name, rather we're using two or more distinct URIs as names, and we 
simply need to establish that when you say <x> and I say <y> that we are 
both refering to the same thing.

Jonathan (Rees) pointed to a fantastic example of the semantic web being 
played out in real life the other day 
http://blog.cleveland.com/metro/2011/03/identifying_photocopy_machine.html 
if you look closely you'll see that neither name was ambiguous, on the 
surface it may look like confusion over the term "photocopying machine", 
but in reality we had two terms "marburger:photocopier" 
"patterson:xerox", marburger pointed his term to his description, 
patterson pointed his term to his description, and it was established 
(via the descriptions) that both the terms were used to refer to the 
same thing. Note, the terms that were equated here were not photocopier 
and xerox, but rather "photocopier as described by marburger" and "xerox 
as described by patterson", "marburger:photocopier" and 
"patterson:xerox". It's the semantic web model through and through :)

So, to move back to the "everest" example, luckily we're not saying just 
"everest", you're saying "you:everest" was climbed by 
"people:EdmundHilary", and I can say "mountain:everest" was climbed by 
"pople:EdmundHilary", then we can establish or infer that when you say 
"you:everest" you are referring to the same thing as I am when I say 
"mountain:everest". Notice the names are both still distinct unambiguous 
identifiers, all we've done is refine the descriptions by equating them 
as describing the same thing.

So, I have to conclude that the names aren't ambiguous here, and that 
there isn't a problem - rather this ambiguity in reference is exactly 
what the naming with URIs and offering descriptions process is designed 
to handle.

I could be way off here though.

Best,

Nathan
Received on Sunday, 20 March 2011 15:30:52 UTC

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