On 19 Oct 2011, at 00:59, Pat Hayes wrote: (01)
> Bijans quibbles are all quite correct, (02)
Indeed! :) (03)
> but I plead that I was trying to get across a basic almost sociological
>divide here (and was talking to logicians) and there wasnt time to go into too
>much detail. (04)
Sure. Just amending where I thought some more detail would help. Tastes vary. (05)
And you are definitely on the side of angels at the moment (ducks). (06)
> He is also quite right that OWL2 refers to "axioms", a fact I had simply
>forgotten. Its unusual, though, in this respect. Calling them axioms is by no
>means, um, axiomatic. (07)
Indeed. But I think it's winning :) In rdf land, "triples" is by far preferred
for many of the same reasons. (08)
> One counter-quibble: (09)
For what do we live, but to make quibble fodder for our neighbours, and quibble
at them in our turn? (010)
> On Oct 18, 2011, at 6:34 PM, Bijan Parsia wrote:
>
>> On 18 Oct 2011, at 23:52, Pat Hayes wrote:
>>
>> .....
>> [snip]
>>> Still, there has been widespread interest in extending the expressive power
>of a DL logic by adding some of the functionality of a rule language to it.
>This has the great appeal of keeping the DL fragment intact while allowing
>inference engines to step outside the DL world where needed,
>>
>> Er...why isn't this just "They are more expressive logics".
>
> Well, that is a very natural way to see it from a logically trained POV, I
>agree. But the sense I often get is that this not in fact how implementers see
>it, but more like using a rule engine to do quick patch-around hacks to
>overcome a local lack of expressivity (eg doing a very quick check for
>transitivity) (011)
This isn't my sense, qua implementor and friend of implementors, *except* maybe
for OWLRL and languages/implementations like that. AL-log explicitly used a
hybrid method, but while Pellet's rule support used a Rete it was *inside* the
tableau (and worked *on* the completion graph). KAON2 reduced *everything* (DL
axioms and DL Safe rules alike) to disjunctive datalog KBs. HermiT's
hypertableau naturally incorporates horn kbs. (012)
There is no "quick" check for transitivity in *class expressions*, of course.
But over the named individuals and their relations to each other it is faster
to do datalogish reasoning, for obvious reasons. I don't know of any OWL engine
that does that, though. (013)
> but not even claiming any kind of completeness or attempting to relate the
>rules to the logical semantics in other than a superficial way. I dont accuse
>you or your colleagues of such sloppiness, of course, nor do I mean to say
>that more careful or theoretically sound work is not done; but there are
>certainly more, um, shall we say, scruffy points of view which just want to
>get things working as quickly as possible. (014)
Right, but my suggestion, sociologically, is that is more "from below" than
"from above", i.e., going from a rule based implementation of RDFS to "some
useful bit of OWL". These implementations are, of course, not hybrid either,
but purely rule based. (015)
> And my point in the email was only that Ali might well have come across some
>discussion from within that more engineering-oriented kind of tradition, is
>all. (016)
No worries ;) (017)
>> I'm not sure I see, from the generic inference engine POV, the difference
>between, e.g., adding transitivity to ALC and adding DL Safe rules.
>(Obviously, from an implementation perspective, they are quite different. Some
>are sometimes amenable some of the time to hybrid proof procedures, but those
>aren't even always preferred these days, at least, in the sense of bolting
>together separatedly developed engines).
>
> Quite. I was talking about the bolting-together approach. (018)
Yes, this is the point of clarification. The bolting together is really quite
rare nowadays (though, interesting, current Pellet + Stardog the rdf store have
some of this): If you have a DL engine with rules, it's as likely to be
designed for rules (due to Boris Motik, mostly). If you don't, you likely just
have a rules engine with some random axiomitzation of some fragment of OWL. (019)
(Or you're in the polynomial DL space, where the two approachs coincide,
really.) (020)
Cheers,
Bijan. (021)
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