Re: Dark triples are inherently nonmonotonic (from Pat Hayes & R.Guha)

If I'm understanding this correctly, it means that we should look to a 
syntactically-signalled darkening rather than a semantically-signalled 
mechanism (based on schema inferences)?  (The other approach in Jonathan's 
note, I recall.)


At 03:08 PM 5/30/02 -0700, R.V.Guha wrote:

>Dark triples are inherently nonmonotonic.
>The dark triples idea is to provide a way to indicate that some triples 
>have their RDF meaning cancelled when read as OWL. Since the only point of 
>doing this is to allow OWL to be a same-syntax extension of RDF, we will 
>assume that this indication of darkness is itself an RDF statement, though 
>of course its OWL *meaning* will go beyond the RDF MT itself.
>The most recent proposal was to 'darken' a uriref simply by asserting a 
>triple of the form
><ex:aaa> <rdf:type> <owl:Dark>
>(say that this triple 'darkens' the ex:aaa) and assume that this means 
>that any triple using the ex:aaa term is not required to have the same 
>meaning in OWL as it has in RDF, and to assume that
><owl:Dark> <rdf:type> <owl:Dark>
>but the exact details do not matter to the point being made here.
>Now, since this darkening is expressible in RDF, one can imagine a 
>situation where an RDF graph G contains some vocabulary and another RDF 
>graph GD darkens that vocabulary. The result of merging the two graphs 
>then would also darken the vocabulary, even in the first graph, so that 
>any conclusions made in OWL from the first graph that depended on the RDF 
>meaning of that vocabulary would become invisible, and hence no longer 
>OWL-entailed by, the merged graph. This is a classical non-monotonic 
>situation: something is entailed by A but is no longer entailed (A and B) 
>for some B.
>(There are many examples of nonmonotonic inference from AI using a 
>construction due to McCarthy called the ab predicate. Ab stands for 
>'abnormal' and the idea is that one is allowed to infer that things will 
>happen normally  (when planning actions) until the predictions go wrong, 
>and then one infers that something was abnormal. (This was one way to deal 
>with the notorious frame problem.) If the 'not-abnormal' assumptions are 
>made explicit the reasoning is monotonic, but the idea is to not make them 
>explicit, but to non-monotonically assume that everything is normal until 
>it goes wrong, then change one's mind. Obviously it would be easy to map 
>this kind of reasoning into the dark-triples framework by darkening all 
>the 'ab' vocabulary. Then the planning inference would be done in OWL but 
>one would have to revert to RDF when things went wrong, and the 
>conclusions would differ in each case. One can do similar tricks to 
>recreate the classical 'Tweety' examples, by darkening the 'unless it is a 
>penguin' exception case statements.
>One might argue that it should be impossible to infer any darkening triple 
>in OWL, since part of the convention is that <owl:Dark> is itself 
>darkened, so any antecendents which involve it will not be visible to OWL. 
>This response is only partly satisfactory, however. The darkened 
>vocabulary is not *invisible* to OWL;it simply no longer has its RDFS 
>meaning; and since owl:Dark does obviously have a meaning in OWL, albeit 
>an unusual one, there is no a priori reason why it should not take part in 
>OWL inferences. Moreover, if we ever make "imports" a first class language 
>construct, which can get inferred, then an OWL ontology might import an 
>ontology which darkens part of its own vocabulary.
>All this applies pretty much unchanged whether the darking is done in RDF 
>or in OWL; the only difference is which language gets to be nonmonotonic.
>The only way to ensure a modicum of global monotonicity would be to 
>strictly restrain the ways that the darkening machinery can be deployed. 
>For example, we might require that all OWL graphs must darken certain 
>parts of the RDFS/OWl vocabulary, and no other parts. This would produce 
>such a rigid convention that it hardly seems worth incorporating the 
>darkening process into the triples-store at all, however; it could be 
>simply built into the OWL model theory as a global OWL assumption.
>Pat Hayes
>R. V. Guha

Graham Klyne

Received on Friday, 31 May 2002 08:37:49 UTC