From: Stefan Decker (stefan@db.stanford.edu)
Date: 05/30/01
Pat,
>>>Why by 5? It seems only to add one extra triple per entry, which is a
>>>multiple of 2 at worst.
>>4 for representing the reification, one for the isTrueInModel property.
>>That means 5 triples instead of one.
>
>I thought you meant reification of the context, not the assertion. Why is
>there any need to reify the assertion?
otherwise it is not possible to refer to the assertion.
>>>>>>One example where it is important to distinguish between
>>>>>>different kind of sets of RDF data is when it comes from different
>>>>>>sources, and that
>>>>>>it originates from different sources is a property of the data.
>>>>>>There are other applications areas, e.g. the computation of different
>>>>>>semantics (see my RDF Schema
>>>>>>example).
>>>>>
>>>>>We really must speak different languages.
>>>>>(1) To speak of "computing" semantics doesnt make sense to me; and (2)
>>>>>why would we want to be using different semantics in any case? Isnt
>>>>>the whole idea to have a single semantics?
>>>>f(1) E.g. Dix and Brewka in [1] define semantics as follows:
>>>>
>>>>"A semantics SEM is a mapping from the class of all programs into the
>>>>powerset of the
>>>>set of all 3 valued structured. SEM assigns to every program P a set
>>>>of 3-valued models of P:
>>>>SEM(P) \subseteq MOD^LP_{3-val}(P)"
>>>>
>>>>My remark above is (admittedly loosely) based on this notion of
>>>>semantics as the deductive closure.
>>>
>>>Oh, come on. This is a misreading of an idiosyncratic usage in a minor
>>>subarea. I don't have the book in front of me, but from the quote they
>>>seem to be making a distinction between the model and the deductive
>>>closure in any case.
>>
>>Taking only herbrand models into account (also common in the minor
>>subarea) there is a direct correspondence
>>between the deductive closure of a logic program and models of that program.
>
>Well, you never mentioned Herbrand models. But in any case, while there is
>a correspondence, but its hardly an isomorphism. A logic program
>terminates when it runs out of Herbrand models, not when it constructs one.
When a inference engine terminates depends on the proof mechanism used
(e.g. top-down (prolog) vs. top-down or mixed (XSB)).
Usually it is also query dependent, so the inference procedure does not
generate the complete minimal herbrand model (which might be infinite), but
only a subset.
>>>>(2) A single semantics for one kind of language. We will have multiple
>>>>languages represented in RDF, e.g.
>>>>UML (see http://www-db.stanford.edu/~melnik/rdf/uml/ ) and it would be
>>>>nice if the rule language we
>>>>design is able to deal with multiple languages at the same time.
>>>
>>>Again, I have no idea what you mean. How can *a* (note singular) rule
>>>language deal with multiple languages? ("Deal with" in what sense? )
>>
>>Be able to compute the deductive closure of a single set of RDF
>>statements with respect to a set of rules.
>
>Stefan, according to you there is no such thing as the deductive closure
>of a set of RDF statements. You have said earlier in this thread that RDF
>triples have no logical interpretation, and in this message you say that
>RDF is a datastructuring language. Concepts like 'deductive closure' only
>apply to languages with a proof theory. What are the inference rules of RDF?
I meant the deductive closure of the Facts + Rules. Sorry for the sloppiness.
RDF does not have inference rules, in the same way that a set of tuples has
no inference rules. Together with a rule language (e.g. SQL) I might have
inference
rules.
>>>And in any case what does it mean to say that there will be multiple
>>>languages "represented" in RDF? In the example in
>>>http://www-db.stanford.edu/~melnik/rdf/uml/ , the RDF graph is being
>>>used simply as a graph to encode a state diagram,
>>>in a way that completely ignores the RDF semantics (such as it is). If
>>>that is 'representation', then RDF is just being used as a
>>>datastructuring language.
>>
>>If I store a statemachine in a database, do I ignore the logical
>>semantics of a database?
>>Probably yes, but I can live with it and I guess other people can also.
>
>I guess at this point we should just declare that we are working in
>different worlds. I feel like someone who is trying to design better cars,
>arguing with someone who insists that an internal combustion engine can be
>used to cook hamburgers. Of course you are free to you store a
>statemachine in a database, but it seems to me that the designers of the
>database language are then under no obligation to support you or
>accomodate to what you are doing. If you do things like that, you are on
>your own.
If a database vendor wants to restrict in me what kind of data I'm allowed
to store in my database,
I don't buy the database.
An example: look at http://www.bpmi.org
These people are building an Ontology for representing business processes.
This incorporates states, events, dynamics etc.
Are you saying they are not allowed to express these concepts in an
ontology language and
express it in RDF? (Actually, they use XML Schema).
>>And yes, RDF is a datastructuring language.
>
>Well, Stefan, that is not what the rest of the RDF community seem to be
>saying. I wish y'all would get your act together and come out with a
>single consistent story. If RDF is just a datastructuring language, then
>what has it got to do with the Semantic Web? And what advantages does it
>have over LISP (say), or XML ?
It as consistent as every community where different people with different
opinions and different
background try to work together.
I already gave my arguments regarding interoperation several times.
We have different opinions in how to value them.
All the best,
Stefan
>Pat
>
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