W3C home > Mailing lists > Public > public-semweb-lifesci@w3.org > March 2013

Re: owl:sameAs - Is it used in a right way?

From: Pat Hayes <phayes@ihmc.us>
Date: Tue, 19 Mar 2013 23:04:06 -0500
Cc: Alan Ruttenberg <alanruttenberg@gmail.com>, Jeremy J Carroll <jjc@syapse.com>, Umutcan ŞİMŞEK <s.umutcan@gmail.com>, Kingsley Idehen <kidehen@openlinksw.com>, "public-semweb-lifesci@w3.org" <public-semweb-lifesci@w3.org>
Message-Id: <6F1FE281-AEE9-4A80-8640-D2E6EDB8AB3F@ihmc.us>
To: David Booth <david@dbooth.org>

On Mar 18, 2013, at 4:04 PM, David Booth wrote:

> Hi Pat,
> On 03/17/2013 10:02 PM, Pat Hayes wrote:
>> Hi David
>> On Mar 16, 2013, at 11:26 PM, David Booth wrote:
>>> Hi Alan,
>>> On 03/16/2013 01:49 PM, Alan Ruttenberg wrote:
>>>> David's assertion that a uri can mean different things in
>>>> different graphs is an opinion
>>> An opinion?  It is direct consequence of standard RDF Semantics!
>> No, it isn't.
>>> Read the spec: http://www.w3.org/TR/rdf-mt/
>> Indeed. Section 1.2, first paragraph: "... the semantics simply
>> assumes that ... a single URI reference can be taken to have the same
>> meaning wherever it occurs. Similarly, the semantics has no special
>> provision for tracking temporal changes. It assumes, implicitly, that
>> URI references have the same meaning whenever they occur."
> Wow.  Well, maybe I have completely misunderstood the RDF Semantics spec all these years -- in which case we're *really* going to have to go back to basics to straighten this out.

Maybe :-)

>  But presumably that passage from Section 1.2 means ". . . whenever they occur _in the *given* graph, i.e., the graph whose semantics are being determined_",

No, it means wherever they occur, period. If they occur in several graphs, they all refer in the same way in all of them. 

> since the spec would be meaningless if it were predicated on the assumption that a URI always has the same meaning everywhere:

It had better not be, as it is predicated upon that assumption :-)

> 1. If a URI always denoted the same resource then there would be no need in the RDF Semantics for multiple interpretations that map URI references to different resources, because every interpretation would necessarily map a URI to the exact same resource.

No. Lets make sure we are speaking very carefully here. An interpretation is a mapping from names (URIrefs - now called IRIs - and literals) to things in a universe. Each such mapping is from the names to what they are being interpreted, in this interpretation, to name. (In the RDF 1.1 spec just being written, by the way, every interpretation mapping is a mapping on *all* names, not just a 'vocabulary V' of names.) So each interpretaton is a decision about what the names mean. 

But of course there can be many of these interpretations. One of them might map "ex:PatHayes" to me, considered as a continuant, and another might map it to the number 37.5, and yet another to my cat's right front paw. But each of the many (maybe uncountably many) interpretation mappings maps each *name* to some thing, and it applies to *all occurrences* of that name. There is no such thing as an RDF interpretation which maps an URI in one graph to one thing and the same URI in a different graph to something else. 

>  The RDF Semantics would need only one, unique  global mapping!  But as we know (and Section 1.3 states): "there is no such thing as 'the' unique interpretation of an RDF graph".  Presumably these non-unique interpretations can and do map the same URI to different resources, do they not?

DIfferent interpretations map the same URI to different resources, yes. But each interpretation fixes that resource for each URI, and it applies to every occurrence of the URI. 

> 2. As we know, a false assumption entails everything.  If I were a drug dealer, I could one week declare to my co-conspirators that this week, http://dbooth.org/x means heroin, and next week it means morphine.

True, but you and your evil henchmen would then be using RDF in a noncompliant way which does not fit with the normative specs. (People do do such things, of course; but specifications do not cease to be normative when people fail to conform to them.)

>  In which case, that URI clearly would *not* have the same meaning whenever it occurs.  So if the RDF Semantics were predicated on the false assumption that it does, the RDF Semantics would be meaningless.

No, it would have the consequence that if someone were to follow the RDF specs in this case, they would get into trouble. That often happens to people who believe the specs and try to act on them, when other people don't follow the specs. Still, that distasteful fact about the dirty real world does not make normative specifications meaningless. 

> In short, I can only see how the RDF Semantics could possibly make sense if the idea that "URI references have the same meaning whenever they occur" is talking about a *single* interpretation.

Yes, it is talking about a single **interpretation**. See above. Whatever each URI means - we might not know what it means, hence the need to consider alternative interpretations, but whatever it actually means - every occurrence of it means that. It means what it means globally.
>  Have I completely misunderstood something fundamental here?

Possibly. I'm not sure. Do my answers make sense to you?

>>> The RDF semantics is only defined for a *given* RDF graph.  It does
>>> not constrain a URI's resource identity across *different* graphs.
>> No, it does so constrain what URIs mean.  It presumes that a given URI
>> denotes some single thing, wherever it occurs. The interpretation
>> mappings are defined as mappings upon URIs. not upon occurences of a
>> URI token in a particular graph; and the semantics does not mention
>> contexts or any other mechanism which would allow a URI in one place
>> to denote differently from the same URI in another place (or at
>> another time, cf the above quote.)
> The RDF Semantics spec defines a procedure

Well, not actually a *procedure*. It is not required to be computable. But OK, leave that aside for now...

> that is *parameterized* by a given interpretation (called "I"), and a given RDF graph or piece thereof (called "E").

No, it defines an interpretation mapping and explains how to *apply* that mapping to a graph E. E is an argument, not a parameter. 

>  The spec is riddled with references to those parameters, and semantic rules are written in terms of them.  A typical example from Section 1.4 reads:
>  "if E is a ground RDF graph then I(E) = false if I(E') = false
>  for some triple E' in E, otherwise I(E) =true."
> How could this procedure possibly define the semantics of a graph that it has not been given -- i.e., a graph that is not E?

E can be *any* ground graph. The symbol "E" in this text is not the name of a particular graph, it is a universally quantified varaible ranging over all ground graphs. ALL of them. 

>  Of course one may choose any graph to consider, so any graph *may* be considered E, in which case the procedure tells you the semantics *of* *that* *graph*.

Yes, exactly. 

> But AFAICT, there is no way around *first* deciding what RDF graph and interpretation you wish to consider, and *then* the procedure defined in the RDF Semantics will tell you the truth value of *that* graph relative to *that* interpretation.

This is mathematics, not an algorithm. There is no first/then involved. 

>  AFAICT, it says nothing about any other RDF graph that is not under consideration.

It says something about all (ground) graphs, and it does not mention being "under consideration", whatever that means. 

> I see no requirement in the RDF Semantics that interpretation "I" be the *same* interpretation for every graph "E" to which this procedure is applied.

There is none, of course. The semantic definitions you are citing here apply to all interpretations and all graphs. (I really don't understand what point you are making here, to be honest.) The semantic rules simply specify when a graph (any graph) is true in an interpretation (any interpretation). But interpretations are not defined "on" graphs: they are mappings from *names* to things. That does not mention graphs at all. So to then start talking about one graph in one interpretation and another graph in another interpretation simply misses the point. The fact that there are many possible interpretations is a reflection of the fact that we typically are in a state of doubt about what the names (URIs) actually refer to. But we are not in doubt that they do actually, or are intended to actually, refer to something, and that **what they refer to is not dependent on where they occur**: a given URI in one graph refers to the same thing as it does in another graph, which is why we can glean RDF from many sources and mash it together to draw inferences. The RDF semantics is predicated on this assumption. And a context version of RDF would not assume this: it would allow a single URI to mean one thing in one graph and a different thing in another graph, consistently, in a *single* interpretation. 

>  Am I right, or have I completely misunderstood something fundamental?
>> Entailment, for example, is defined upon *sets* of RDF graphs, and
>> the definition of merging makes sense only if the URIs in these
>> various graphs all denote in the same way.
> [Side note: As discussed above, surely you mean that "merging makes sense only if the URIs in these various graphs all denote in the same way" *in* a given interpretation.  Right?

In *all* interpretations, but yes, OK. 

> ]
> Of course, but again, merging is defined on a *given* set S of RDF graphs.

It is defined on sets of graphs. Being "given" is not a meaningful mathematical term, AFAIK.

>  Once the user of the RDF Semantics spec has *chosen* S, the spec defines entailment upon *that* S -- not on some other, unspecified set.  Is that correct

No. Entailment is a relation between sets of graphs, and graphs. No choosing involved.

> , or have I completely misunderstood the spec?
>>> And here is a trivial existence proof that demonstrates that a URI
>>> does *not* necessarily denote the same resource in different
>>> graphs.
>>> Graph 1 (assuming standard owl: prefix):
>>> <http://example/h> a <http://example/WhiteHorse> .
>>> <http://example/WhiteHorse> owl:disjointWith
>>> <http://example/BlackHorse> .
>>> Graph 2:
>>> <http://example/h> a <http://example/BlackHorse> .
>>> <http://example/WhiteHorse> owl:disjointWith
>>> <http://example/BlackHorse> .
>>> Each graph (by itself) has satisfying interpretations per standard
>>> RDF (and OWL) semantics.  And <http://example/h> denotes a resource
>>> in each graph.  But clearly it denotes a *different* resource in
>>> each graph.
>> The conclusion you should draw here is that these graphs cannot both
>> be true. And indeed, if you merge those graphs into one, then that
>> merged graph is owl-inconsistent. But that does not mean that the
>> URIs denote differently in the two graphs. What it does reflect is
>> the fact that no interpretation of names will make a contradiction
>> true.
> Of course both graph 1 and graph 2 can both be true!  They are just true under different interpretations!

There is no interpretation that makes them both true. That is the precise way to say that they cannot both be true. 

>  And in this case, those interpretations will necessarily map http://example/h to different resources.

Well, they might interpret http://example/WhiteHorse and http://example/BlackHorse differently. But I expect this is nit-picking. 

>  On the other hand, the merge of graphs 1 and 2 is necessarily false under any interpretation.


> Unless I am terribly mistaken, the RDF Semantics spec does not mandate the use of a single, a unique interpretation that must be applied to every RDF graph.

It does not mandate a single interpretation, of course, But it does presume that each interpretation determines the truthvalue of every graph. 

>  Indeed, to my mind the main value of the RDF Semantics is that, given an RDF graph, it allows one to determine the range of possible interpretations under which that graph is true.

Yes. And it does that for *every* graph :-)

>>>> that does not concur with either the web specifications
>>> Correct.  As I pointed out, the AWWW's statement that "a URI
>>> identifies one resource" is a good goal, but it does not concur
>>> with standard RDF semantics.
>> It is quite consistent with, indeed presupposed by, RDF semantics. As
>> to whether it is really the case, that is a whole more complicated
>> question. But note that when AWWW says "identify", it apparently does
>> not always mean what RDF semantics means by "denote". There is no
>> assumption in RDF theory or practice that a URI must denote what it
>> 'dereferences' to using http.
> Right, that's a different question.
> It might well be the case that
>> awww-identification is unique even while denotation is not.
>>> nor the goals they were built to satisfy. Caveat emptor.
>>> Not true!  As I said before, I *agree* with the goal stated in the
>>> AWWW, that a URI should denote one resource!  But that does not
>>> change the reality: that a URI does *not* necessarily denote only
>>> one resource.
>> You are probably right, but to the extent that they do not, then
>> meaningful communication fails. So communication usually presumes
>> that they do, until circumstances force this assumption to be
>> revisited.
> I think that is overly pessimistic.  I think useful communication can still be achieved, provided that the ambiguity is bounded.

Ah, we in fact agree here, but your way of phrasing it led me astray. OK. 


>  And when URI definitions are shared, the RDF Semantics (and its semantic extensions) provide a very neat way to bound that ambiguity.
> David

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Received on Wednesday, 20 March 2013 04:04:34 UTC

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