Re: "entailment regime"?

On 4 Jul 2007, at 15:33, Sandro Hawke wrote:

> (This message is likely to be controversial among the RDF folks, but I
> think it's important.  It may also not be something we can agree on --
> other Semantic Web working groups have been challenged by this -- but
> let me at least try to make the case.)
>
> In http://www.w3.org/2005/rules/wg/wiki/Arch/RDF, Jos writes:
>> Furthermore, this data set has a particular entailment regime
>> associated with it (e.g. simple, RDF, RDFS).
>>
>> rif:rdfEntailmentRegime a rdf:Property ;
>>     rdfs:domain rif:RuleSet ;
>>     rdfs:range  rif:RDFEntailmentRegime .
>
> If I understand this approach correctly, I'm afraid I have to disagree
> with it.  RDF entailment regimes should *not* be specified.  It's a  
> key
> element of the architecture of the Semantic Web that semantics are
> implied by the vocabulary in use,

If it is, then it's violated by RDF and OWL and poor practice in a  
number of cases (e.g., why shouldn't I be able to query an OWL  
document under simple entailment only?)
[snip]

> The idea, though, is that you use all the entailment regimes which are
> defined for the vocabulary in use.

Since this is not true in theory or in practice, I think it cannot be  
a part of the semantic web *architecture*, esp. not a key part. You  
could propose it as part of it, but that's a bit different.

(Note: A consequence of this requirement is that I cannot correctly  
use an RDF entailment based reasoner on an OWL document...which seems  
to strongly contradict the "process what you can" model.

I guess you could argue that the *semantics* are implied, but the  
*processing* can vary, but I think that certainly isn't in most  
people's head :))

>   So for an RDF/XML document that uses
> no RDFS or OWL terms, RDF entailment applies.  If you use RDFS terms,
> RDFS entailment applies.  If you use OWL terms, OWL entailment  
> applies.
>
> At a high enough level, this is equivalent to just using all the
> entailment regimes you can.

? I don't see why "can" or "cannot" have to do with it. Once you make  
it a requirement, then arguably I should *punt* if I "can't"  
understand the RDFS terms. If I don't have to punt, one reason to  
*not* to use RDFS entailment even when I could is to interoperate  
with a system that doesn't understand the RDFS tools. (And what do we  
mean by "can"?) Since I already would have to specify what sort of  
entailment to use out of band, what's the issue?

Plus this is similar to content sniffing which in some lights is a no- 
no and in other lights a good thing :) Isn't mime-type a way to  
override? Should I always interpret <html></html> as html even if i  
retrieve it with mimetype text/plain?

(Or consider validating a document with different schemas....they  
could, for example, add different defautl

> I know for people concerned with
> theoretical properties of logics, this is painful,

It is?

> and it does present
> some challenges.  But if there's a case where this approach is a real
> problem to users, I'd like to hear about it.

It's not the worst default in a lot of cases, but I don't think it's  
a panacea.

> (Among the flaws in the current specs, I realise, is that there are
> OWL-Full entailments for a OWL-DL graph which are not OWL-DL
> entailments.  That's a bug, though, and will hopefully be fixed some
> day, by getting the semantics fully aligned.)

Even that wouldn't be *vocabulary* dispatch, but *syntax*  (i.e., use  
of vocabulary) dispatch.

It's worse than that, consider:

	:P rdfs:subPropertyOf :R.
	:R rdfs:range :C.

is it entailed that
	:P rdfs:range :C?

under RDFS semantics? (No! Yeek!) Yet is entailed under owl semantics  
(including owl full).

In general, I favor giving people the ability to directly specify the  
intended semantics and to being able to override that as they see  
fit. So, for example, I think it'd be great if I could publish an  
rdfs ontology at a sparql endpoint, have in the ontology that I  
intend the owl semantics, and to let the user request that the query  
be evaluated under the rdfs semantics. The endpoint might not be able  
to respect that request, but I see no harm in having endpoints that do.

Oh, if I have a statement "I am to be understood under owl semantics"  
isn't that vocabulary? In use? :)

Cheers,
Bijan.

Received on Wednesday, 4 July 2007 16:28:56 UTC