On 03/27/2013 11:02 AM, Oliver Ruebenacker wrote:
Hello David,
So if I understand your view correctly, then it could be expressed
in a language close to yours as:
"Some people believe that if a URI occurs twice within a graph or
statement, it refers to the same thing. But this is a myth! RDF never
guarantees that two occurrences of the same URI mean the same thing."
No, that's not correct. If you are talking about what the URI means
**within that graph**, then:
- For any interpretation, every occurrence of that URI in that graph
refers to the same thing; BUT . . .
- Different interpretations can be applied to that graph, such that
the URI means one thing in one interpretation, but means something
different in another interpretation.
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It all comes down to the fact that, in essence, the RDF Semantics spec
defines a function -- call it RS -- of *two* arguments: one
interpretation and one graph (or a piece of a graph, but that's an
unimportant detail). You can apply this function to determine the truth
value of any <interpretation, graph> pair. If Interpretations is the
set of all RDF interpretations, and Graphs is the set of all RDF graphs,
then RS can be defined as:
RS: (Interpretations x Graphs) -> {true, false}
For any I in Interpretations and G in Graphs, RS(I, G) is true
iff the RDF Semantics specification says that G is true under
interpretation I.
Given RS, we can then define another function -- call it SI -- that
determines the set of satisfying interpretations for a graph, i.e., the
set of interpretations under which that graph would be true:
SI: Graphs -> PowerSet(Interpretations)
For any G in Graphs, SI(G) = {i in Interpretations | RS(i, G) }
Since the set of satisfying interpretations differs depending on the
graph, to do anything useful with the RDF Semantics, the key question
that you must first ask is: **What graph do you wish to consider?**
Once you have answered that question, you can do useful things, like
determining the set of satisfying interpretations, or inputting that
graph to an app that applies its chosen interpretation to it.
David
