[SCL] The semantics of "semantics"

Thu Jan 16 17:33:06 CST 2003

Chris-
  one of the problems is that when one says "use RDF for ?x" we can 
actually mean a lot of different things.  Here a couple of things we 
might need different terminological terms to use amongst ourselves to 
distinguish:

(BEFORE YOU ALL JUMP ON ME - let me be clear that I'm being informal 
in the below, only trying to convey some basic memes, not trying to 
explain the details and subtle distinctions that require the genius 
of a Pierce, Frege or Wichtenstein)

1) RDF is a document language that has a meaning of its own.  Thus, if I say
http://www.cs.umd.edu/users/hendler rdf:about "james Hendler" it 
carries some sort of assertional meaning (and the model theory for 
that meaning is a document Pat has edited [1])

2) One can define langauges with their own more specific semantics 
that sit on top of RDF.  For example, OWL, the product of my Working 
Group, has its own model theory [2] which give special semantics to 
certain terms such as owl:class or owl:datatypeProperty.

3) One can use the RDF or OWL languages and their own MTs to develop 
languages that use, rather than extend, this semantics.  For example, 
I could state:

<owl:Class rdf:ID="Rule">
<rdfs:subClassOf>
     <owl:Restriction>
         <owl:onProperty rdf:resource="#antecedent"/>
         <owl:cardinality
	   rdf:datatype="&xsd;NonNegativeInteger">1</owl:cardinality>
     </owl:Restriction>
   </rdfs:subClassOf>
   <rdfs:subClassOf>
     <owl:Restriction>
       <owl:onProperty rdf:resource="#consequent"/>
       <owl:cardinality
	  rdf:datatype="&xsd;NonNegativeInteger">1</owl:minCardinality>
     </owl:Restriction>
   </rdfs:subClassOf>
</owl:Class>

which, loosely interpreting the OWL and RDF model theories,  means 
there is a thing called a rule which must have each of two 
properties, one called an antecedent and one called a consequent. 
(but, of course, there is no explicit semantic relation of this thing 
to what we humans call a "rule" - i.e. there's obvious nothing in 
either model theory that says anything about the entailment 
relationship between the antecedent and consequent properties).  The 
use of this, of course, is that now I can have files that have many 
instances of this thing called rule that can be "semantically" 
checked for some sort of well-formedness, even if the properties are 
defined in different files and linked together (i.e. if I discover a 
rule with two consequents or no antecedents, etc. I know it is not a 
"rule" as defined here, whereas if I find a antecedent in one file 
and the consequent in some other one, I can determine it may be a 
rule since it meets the requirements expressed above).

I could imagine all or some of SCL being described with relation to 
any or all of these uses of RDF.  Note also that RDF is quite 
different from XML in all of the above - it has no MT and thus one 
first has to invent one for one's own domain to make it work.  This 
has some advantages (i.e. SCL could be defined from scratch without 
having to sit on top of another MT) but also some disadvantages, 
particularly with respect to the third use above, as creating an XML 
"langauge" only allows for syntactic verification, and allows no 
automated semantic checks with respect to a "model" being defined, 
and also requires a locality of reference that RDF doesn't.

so, when we talk of mapping SCL to RDF, there's a big difference 
which of the above we mean.

  -JH  (let the jumping on begin...)

[1] http://www.w3.org/TR/2002/WD-rdf-mt-20021112/
[2] http://www-db.research.bell-labs.com/user/pfps/owl/semantics/

At 16:47 -0600 1/16/03, Chris Menzel wrote:
>On Thu, Jan 16, 2003 at 03:53:09PM -0500, John Sowa wrote:
>>  I believe that somee recent lapses in communication have resulted
>>  from undefined notions of what "semantics" means.  Those of us who
>>  have been working with logic for some time naturally equate that
>>  term with "model-theoretic semantics" as defined by Ockham, Peirce,
>>  Tarski, Kripke, Montague, etc.
>  >
>>  An "impedence mismatch" has come to the fore in the exchange
>>  of notes between Chris and Tanel ...
>
>All of your points are very well taken, John; I agree strongly with
>Jim's assessment.  But I do want to point out that I don't *think* there
>was as much of a mismatch between me and Tanel on this point as you
>suggest.  He was clearly using the term "semantics" in a way that is
>common in the theorem proving, ontology, and Sem Web communities -- as
>something more syntaxy in orientation -- and, although it is not the way
>I would use the term talking with you or Pat, I was trying to follow
>suit.  In (one variation of) this usage, one provides a "semantics" for
>a term by fleshing it out in terms of sentences (axioms, if you will)
>involving that term -- residing ideally in a robust knowledge base --
>that can then be reasoned upon.  That is roughly the sense I was using.
>My question to Tanel was simply how RDF was to be used for any sort of
>"semantic translation" of an SCL-conformant language when it is not
>capable of providing the "semantics" (i.e., axioms) for a term that an
>SCL language is capable of providing.  What he was calling semantics
>just looked like quotation to me.
>
>Again, I invite reproof and correction!  I'm *happy* to be set straight
>on this and related points, to be shown that I just don't have the right
>picture of RDF's role here, or that I just don't really grok the notion
>of translation that is at issue.  I have no axe to grind.  A lot of the
>Sem Web landscape is still rather foreign to me and I don't for a minute
>want to suggest that I know what is best, or who is confused, etc.  I am
>only striving to understand the issues and avoid the sort of mismatch
>you caution so clearly against -- mismatches that can arise very easily
>when representatives of different communities come together to solve a
>problem.
>
>Regards,
>
>-chris
>
>--
>
>  /\ ASCII ribbon | Chris Menzel -- http://philebus.tamu.edu/~cmenzel
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-- 
Professor James Hendler				  hendler at cs.umd.edu
Director, Semantic Web and Agent Technologies	  301-405-2696
Maryland Information and Network Dynamics Lab.	  301-405-6707 (Fax)
Univ of Maryland, College Park, MD 20742	  240-731-3822 (Cell)
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