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Re: [oor-forum] Ontologies vs Theories / Axioms vs Rules

To: Pat Hayes <phayes@xxxxxxx>
Cc: OpenOntologyRepository-discussion <oor-forum@xxxxxxxxxxxxxxxx>, Ali SH <asaegyn+out@xxxxxxxxx>
From: Bijan Parsia <bparsia@xxxxxxxxxxxx>
Date: Wed, 19 Oct 2011 01:12:00 +0100
Message-id: <E39534A1-4FEE-4CE9-89E8-D03A8DD16661@xxxxxxxxxxxx>
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)

Bijan.    (021)

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