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

Sorry, long response, but this is a big topic. 

On Mar 20, 2011, at 10:30 AM, Nathan wrote:

> 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..

There is absolutely no difference between the semantic web and 'human terms' in this discussion. The Web does not add anything to the basic issues.

> to explain:
> 
> For each thing we have names and descriptions, descriptions are always ambiguous to an extent, but not the names.

Yes, the names. If we are talking about what names refer to (It is not at all clear that is what TIm is talking about when he uses the term 'identifies', by the way; but IF...) then the names are ambiguous in the sense that they might have many referents. 

> 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.

That isn't what TimBL is saying, though. He is saying that URIs 'identify' unambiguously, with no qualifications like this.

> 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.

Humans use names to refer also: so what? URIs refer just like all other names or indeed naming expressions refer. (Or at any rate, if this is not the case, if they refer in some new way, then nobody has ever explained or even hinted at an exposition of how this new kind of reference differs from the old kind.) 

> 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.

All of this applies to all other languages used by humans to refer to things. Nothing here is new enough to escape the general issues of reference which have been explored in philosophy of language.

> 
> 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.

So, are you saying it is the *syntax* of URIs which gives them this magical quality? So one gets unambiguous reference by putting a colon in the name somewhere?  OK, forgive my sarcasm: but if this is not what you are saying, just what ARE you saying, that gives URIs this amazing ability to reach out into the world and seize upon their single unique referent?

> 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.

But 'using <x> as a name" is exactly the process which is inherently [1] ambiguous, as David and I have been saying. So it makes no difference if you are using one x and I am using another x. If anything, this can only make things worse, in fact. If I use a name which only I use, then there is absolutely no way you can determine what I might intent to refer to by that usage. It is exactly the *social* use of names which gives then their referential load. 

> 
> 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 :)

Actually I don't think it was, because everyone involved actually understood quite well what was going on. The verbal contorting was artificial in this case, to avoid making a commitment under oath to something blatantly obvious. 

> 
> 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".

BUt that is exactly what we cannot do unambiguously, which is the point. 

> 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.

No, as referring to things which are sufficiently 'similar' (misleading word) that whatever you say about one I can understand as a statement about the other without it mattering to our communication. In the case of Everest, the key point of this assertion has to to do with the identity of the summit of the mountain, which we can assume is independent of the mass or the geographical edges of the mountain. But this summit is not actually mentioned in the sentence, and may not have a URI. So a clear, unambiguous assertion is made using ambiguous URIs. This case is typical, not exceptional. I defy you to cite a single factual RDF assertion that I cannot find an ambiguous URI in it somewhere. 

> 
> So, I have to conclude that the names aren't ambiguous here

What would lead you to that conclusion? I don't see that you have argued for it anywhere. Like TimBL's claim, it seems to be a matter of W3C Dogma rather than an actual observation or even a rationally defended position. And as it is radically false, and indeed in many cases *provably* false, it seems rather obtuse to be defending it with so slender an excuse or argument. 

> , 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.

Handle as is 'ignore' , or handle as in 'make unambiguous" ?

Pat

[1] I should add, there are cases of genuine unambiguous reference, I think, but they all involve direct apprehension of the actual referent itself, so-called ostensive reference. I you ask, what is a spanner? And I pick an actual spanner and wave it at you and say "THIS is a spanner" then the referent of the word 'this' is determined by extra-linguistic means, by ostention. Or if a dog wanders into the room and I say "Ah, here comes Sally." One can do this linguistically, eg by quoting: " "Man" is a three-letter word." uses a quoted string to refer by ostention to an English word. But explicit ostention is a very restricted device. 

> 
> I could be way off here though.
> 
> Best,
> 
> Nathan
> 

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Received on Monday, 21 March 2011 19:33:34 UTC