Thanks Bijan, John and all. May I suggest (that the next person
continuing this conversation) to please move this thread to the
[ontology-forum] list, which would be the more appropriate venue for
the conversation. Tx. =ppy
-- (01)
On 6/10/10, Bijan Parsia <bparsia@xxxxxxxxxxxx> wrote:
> On 10 Jun 2010, at 01:27, John F. Sowa wrote:
>
>> Leo, Jeff, and Elisa,
>>
>> I went to the TrOWL web site ( http://trowl.eu/ ) and found some
>> very strange comments:
>>
>>> TrOWL utilises a semantic approximation to transform OWL2-DL
>>> ontologies into OWL2-QL for conjunctive query answering and
>>> a syntactic approximation from OWL2-DL to OWL2-EL for TBox
>>> and ABox reasoning.
>>
>> Why does that word 'approximation' pop up twice in a single
>> sentence? If you have a well-defined semantic foundation,
>> all the permissible transformations are exact.
>
> In these contexts, approximation refers to a possible lossy
> transformation from a more expressive representation to a less
> expressive one. Hence "approximation".
>
>> Are you claiming that DL, QL, and EL have different semantics?
>
> He's definitely not. He's claiming that he can take an OWL2-DL
> ontology (with NEXPTIME complex satisfiability) and rewrite it as an
> OWL2-QL (with PTIME complex satisfiability) with, perhaps, some loss
> of information, but that that loss of information is controlled and
> suitable for various purposes.
>
> To take a simple example, I can approximate an arbitrary OWL2-DL
> ontology as an RDFS ontology by extracting the asserted, explicit
> subsumption hierarchy. This will be sound wrt atomic subsumptions, but
> (possibly) incomplete. I can do a better approximation if I first
> fully classify the ontology and then extract the subsumptions. Now, my
> approximation will be sound and complete wrt atomic subsumptions in
> the original ontology, but it will not be complete for arbitrary
> concept expression satisfiability, subsumption, etc. nor for instance
> retrieval. Plus, it requires the ontology to be classified.
>
> These are the sorts of trade off.
>
>> I can understand the idea of having different levels of
>> expressive power for different subsets. But then the
>> mappings between versions are just subset/superset.
>>
>> Why do these systems require approximations? What is the
>> point of multiple notations with a common name (OWL), but
>> incompatible semantics?
>
> Concept approximation is a typical non-standard service for
> description logics. It can be useful for modelling as well as
> performance. Unsound and incomplete approximations (more typically
> incomplete) are often used as optimization inside a sound and complete
> procedure (i.e., you fall back to the more complex procedure when your
> sound approximation fails to return an answer). Unsound and incomplete
> approximate reasoning is exposed to users when the approximations are
> good enough that the user can accept the tradeoff.
>
> (This is certain by no means restricted to DLs. Anytime reasoning
> procedures, if halted before completion, provide (typically) sound but
> incomplete reasoning. See
>
>http://folli.loria.fr/cds/2006/courses/Harmelen.Hitzler.Wache.ApproximateReasoningForTheSemanticWeb.pdf
> for more.)
>
> The most obvious common example is to use rule based implementations
> of OWL Full which are known to be wildly incomplete but "useful".
>
> Cheers,
> Bijan.
>
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