- From: <Joachim.Peer@unisg.ch>
- Date: Mon, 14 Jan 2002 15:19:20 +0100
- To: www-rdf-logic@w3.org
hi All, until recently I really had difficulties to understand the benefits of the semantic web "layer cake"-model (which says that ontology languages like DAML+OIL should build on a generic RDF(S)-layer, which, in turn is written down in XML.) Well, now I think I may have grasped the idea, but I am not sure if I got it right. Therefore I post my interpretation to this list, in hope of responses. The trouble I had with the layer cake model was that I did not see any benefits of the use of RDF/RDFS: The main point was, that even if formalisms like DAML+OIL or other DL's build on top of RDF(S), an agent still needs to have *built-in* capabilities to interpret and reason on DAML+OIL ontologies. So, even if ontologies in DAML+OIL and other languages are written down in one common way (RDF), they still represent completely different worlds! Many papers tried to relativate this, stating that RDFS-based agents which are not capable of e.g. DAML+OIL could understand DAML+OIL ontologies at least partially, because some properties of DAML+OIL constructs are defined in RDF schema. (However this never convinced me. I do not trust systems which act upon partial (=uncertain) information, should i?) So, my main problem was, that most of the semantics of languages like DAML+OIL where declared *outside* their respective RDFS definition (e.g. DAML+OIL's semantic is defined using KIF axioms in [1]) But why not encode the *complete* semantic interpretation of a language like DAML into RDF(S)? This would enable agents to "load" and use different problem solving mechanisms "on the fly" - - and that would really justify the role of RDF as a generic KR layer. I am aware of two papers which approach this problem: In [2] W. Conen and R. Klapsing propose an extension to RDFS by introducing a property "isDefinedAs" which "hosts" arbitrare logical constructs, e.g. prolog clauses, which might be applied as follows: <Property rdf:ID="union"> <rdfs:isDefinedAs rdf:parseType="Literal"> in(X,S,P,O) :- P = union, instanceOfSet(X,S). in(X,S,P,O) :- P = union, instanceOfSet(X,O). </rdfs:isDefinedAs> </Property> Now, if for example the whole DAML+OIL specification would be encoded this way (much work already done in [3], which expresses some DAML+OIL-semantics as sequences of prolog clauses), any RDFS capable agent in combination with a prolog interpreter could make use of the semantics of DAML's epistomological constructs. One problem with this approach is of course, that the use of a specific logical notation (e.g. horn clauses) may not scale in Semantic Web context (again we deal with "different worlds") Another approach is presented by D. McDermott and D. Dou in [4] which proposes the encoding of logics as reified RDF statements. The logical formulas are "quoted" in RDF in a clean, RDF conform way. Thus, logical formulas written down in reified RDF statemens are parseable by any RDF parser. however, one problem still remains: is there a logical calculus (e.g. HOL) or notation (e.g. KIF) available which is able to express all other (relevant!) logical calculi? [which calculi may be considered relevant, which not?] If this question could be solved, then the SemWeb layer cake would finally make sense to me: a nearly selfdescribing multilayered semantical framework, building on top of a generic syntactical framework would be possible. perfect infrastructure for a semantic web ! :-) what is your opinion on this? thanks in advance, Joachim References -------------------- [1] Fikes, R; McGuinness, D.L.: "An Axiomatic Semantics for RDF, RDF-S, and DAML+OIL", 2001http://www.ksl.stanford.edu/people/dlm/daml-semantics/daml+oil-axioms-october2001.htm [2] Conen, W; Klapsing, R: "Utilizing Host Formalisms to Extend RDF Semantics", 2001 http://wonkituck.wi-inf.uni-essen.de/rdfs.html [3] Zou, Y. : "DAML XSB interpretation", lab notes, 2001http://www.cs.umbc.edu/~yzou1/daml/damlxsb.P.txt [4] McDermott, D; Dou, D. :"Embedding logic in DAML/RDF", 2001 ftp://ftp.cs.yale.edu/pub/mcdermott/papers/wwwconf.pdf Joachim Peer Research Assistant Institute for Media and Communications Management Blumenbergplatz 9 CH-9000 St. Gallen Tel.: ++41 (0)71 224 3441
Received on Monday, 14 January 2002 09:19:23 UTC