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. 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_", since the spec
would be meaningless if it were predicated on the assumption that a URI
always has the same meaning everywhere:
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. 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?
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. 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.
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. Have I completely
misunderstood something fundamental here?
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 that is *parameterized* by a
given interpretation (called "I"), and a given RDF graph or piece
thereof (called "E"). 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? 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*.
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. AFAICT, it says nothing about any other RDF
graph that is not under consideration.
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. 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?]
Of course, but again, merging is defined on a *given* set S of RDF
graphs. 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, 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! And in this case, those
interpretations will necessarily map http://example/h to different
resources. 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. 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.
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. And when URI
definitions are shared, the RDF Semantics (and its semantic extensions)
provide a very neat way to bound that ambiguity.
David
