Ali,
<snip>
What do others think?
Best,
Ali
I think you’re right about one thing: the challenge of finding the right level of expressivity will elicit lots of comments.
We’ve been working with a set of OWL ontologies in which there are many restrictions of the form:
property exactly 1 owl:Thing
If you ask the Pellet reasoner to reason about these ontologies, you wait – well, Pellet doesn’t terminate overnight. If you replace these assertions with:
property some owl:Thing
which of course does not express quite the same semantics, Pellet terminates – in our case, requiring about 6 minutes. Not exactly real time, but acceptable for certain applications. If you want real-time or near-real time performance, you have to cut most of the restrictions. So far we have been working only with T-box assertions and we anticipate very large numbers of A-box assertions. So I expect that, for the applications we have in mind, cutting is what we’ll do.
(I want to make it clear I’m not criticizing Pellet. Other reasoners I’ve tried have their own problems. I mention Pellet because it worked, ultimately, and I measured the time.)
But the problem with figuring out what semantics to include or cut is that you don’t necessarily know the intended applications when you design an ontology. In an ideal world, an ontology is reusable. That means two things. (More actually, but never mind that now.) First, you include enough semantics to let other ontologists know how an element relates to their needs – whether a class is equivalent to, a superclass or subclass of, overlaps, or is disjoint with a concept they’re considering expressing as a class. Second, an ontologist doesn’t have to figure out semantics on his own – he benefits from your effort.
If you’re deliberately writing application-specific ontologies, you’re probably reducing the prospect of reuse. That would be a shame. At the very least, it has negative consequences for a semantic web. I hope our experience is atypical. It doesn’t bode well for reusing domain-specific ontologies, at least in the near term.
Regards,
Steve Wartik