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Re: rdfs:Graph ? comment on http://www.w3.org/TR/rdf11-concepts/#section-dataset and issue 35

From: Pat Hayes <phayes@ihmc.us>
Date: Thu, 12 Sep 2013 00:18:17 -0700
Cc: Sandro Hawke <sandro@w3.org>, www-archive <www-archive@w3.org>
Message-Id: <7B471F1A-A4DE-42F6-877C-6977920EFAC9@ihmc.us>
To: David Booth <david@dbooth.org>

On Sep 11, 2013, at 9:33 PM, David Booth wrote:

> [Let's move this discussion to www-archive@w3.org

Sure. I thought we were doing that, in fact. Sorry about the slip there. 

> please, as it isn't relevant to Jeremy's comment.  All follow-ups there please.]
> On 09/11/2013 10:32 PM, Pat Hayes wrote:
>> On Sep 11, 2013, at 5:38 PM, David Booth wrote:
>>> On 09/09/2013 02:51 AM, Pat Hayes wrote:
>>>> The question though is, whether
>>>> I(<http://my.graph.name.example.org/>) = the graph you want it
>>>> to mean. The problem is that there are people who want to use an
>>>> IRI to simultaneously denote a person (say) but also be the name
>>>> of a graph (eg of information about that person). And they have
>>>> deployed systems and much money vested in being able to do this.
>>> Uh . . . this may be opening up a can of worms, but what you're
>>> saying sounds a lot like the IRI resource identity ambiguity issue
>>> that has been discussed quite a lot in the past.  In short, there
>>> is no conflict if either: (a) the class of persons has not been
>>> asserted to be disjoint with the class of graphs
>> Indeed. I am assuming throughout this discussion that graphs and
>> persons are disjoint classes, and that this is known by all parties
>> involved.
> Okay, but not all software needs to make that distinction.  So unless it has been explicitly stated in the graph (or implied as a valid entailment)
>>> ; or (b) the IRI denotes a person in one RDF interpretation (e.g.
>>> in one system) but denotes a graph in a different RDF
>>> interpretation (e.g. in a different system).
>> That is nonsense, as I have explained to you many times in the past.
> Baloney!  *Each* interpretation maps an IRI to one resource, but **RDF ALLOWS MULTIPLE INTERPRETATIONS**!

Of course. This is a little like getting excited about the fact that 2+2=4. 

>  And different interpretations can perfectly well map the same IRI to different resources.

Of course. That is what interpretations are for.

>  Please stop trying to look at RDF in terms of only one interpretation!  That is *not* the only way -- or the only correct way -- to think about RDF.

I am not looking at it that way. And this discussion is not about RDF, by the way, it is about the basic ideas of model-theoretic semantics, which I regret to have to tell you, you apparently do not fully understand. 

>> Interpretations are not systems: they are alternative ways to
>> construe what IRIs denote.
> Yes, and different systems (or people) can and do construe them differently.

>> But each IRI denotes one thing, in all
>> possible interpretations.
> No, in *each* possible interpretation, not in *all* possible interpretations.

Yes. In every interpretation, it is the case that each IRI denotes one thing. Anything true in all possible interpretations is a logically necessary truth. Therefore, that each IRI denotes one thing, is a logically necessary truth.

Here's a terse way to show it, using a more conventional logic than RDF. Suppose we have equality in our logic. Then 


is logically true, true in all interpretations, necessarily true. Why? Because, in every interpretation, *both* occurrences of 'a' denote the *same thing*. The fact that in one interpretation, the first 'a' might denote me, and in another interpretation,. the second 'a' might denote you, does not make the equality false in any interpretation. 

>  I.e.,
>  For any interpretation I and URIs U1 and U2,
>  (U1=U2) => (I(U1) = I(U1))
> NOT:
>  For any interpretations II and I2, and URIs U1 and U2,
>  (U1=U2) => (I1(U1) = I2(U2))
> I.e., the uniqueness does not hold *across* interpretations.  It only holds within *each* interpretation individually.

Of course. But since it holds in all of them, it is always true: in fact, necessarily true.

>> (The current RDF 1.1 semantics socument
>> makes thie very explicit, by the way.)
> Yes, I noticed that, and the current wording is *incorrect*.

No, it is exactly correct.

>  It needs to be fixed, as it wrongly implies that RDF may only be viewed from the perspective of a single RDF interpretation, and that is simply *wrong*.

I have no idea what this 'perspective' language is siupposed to mean. Each interpretation is a possible way that the universe  that RDF describes might be configured. That is what "interpretation" means. It is sometimes called a "possible world", though that term comes with lots of extra baggage which is best avoided, IMO. 

>  I have not yet raised that issue, but I will.  I wanted to talk it over with you first, before causing a long email thread.

I suspect it will be quite short, in fact. 

>> If we want to allow different
>> occurences of an IRI to denote different things, then we would need
>> some kind of context mechanism in RDF, which it currently does not
>> have, and providing which would have been beyond this WGs charter.
> You are talking about something entirely different than what I am talking about.  I am not and never have been talking about that kind of notion of context.

Actually you are, although you apparently do not realize it. That kind of notion of context is the only way to have different occurrences of the same name denoting differently. That, in fact, is often taken to be the *definition* of what makes a logic contextual.

>  I am talking about the *existing* RDF Semantics, BUT from the perspective of looking at the set of satisfying interpretations for an RDF graph -- not from the perspective of a single interpretation.

This entire idea of 'perspectives' arises from a confusion that you have about how model theory works. The whole point of interpretations is to provide an exact statement of entailment, which as I am sure you know, refers to *sets* of interpretations. 

Sorry, David, but this debate is pointless. Go read a book about semantics, or something. You are just plain confused. 


>>> I don't know if this observation would help resolve the problem
>>> that you're mentioning though.
>> Neither of them do, I'm afraid.
> Okay.  I don't know enough about the graph naming debate that you mentioned to know if it was relevant, so I'll take your word for it.
> David

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Received on Thursday, 12 September 2013 07:18:45 UTC

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