|From:||Ali Hashemi <ali@xxxxxxxxx>|
|Date:||Thu, 12 Aug 2010 11:08:44 -0400|
To add one more perspective to this discussion.|
From a very practical standpoint, the proliferation of "guaranteed logics" has at least two unintended consequences.
1) Company A uses decidable logic L1 (say an internally developed temporal logic), while Company B uses decidable logic L2 (say OWL-DL). Later A and B realize they want to exchange information and/or combine systems. The model and proof theory of their logics are somewhat idiosyncratic. Without a more encompassing framework (i.e. Common Logic?) or a common semantic foundation, the translation between these logics (syntax, semantics and all), let alone their axioms is at best not trivial.
Perhaps this situation is mitigated if someone somewhere specifies mappings from L1 and L2 to some common foundation logic (see http://www.informatik.uni-bremen.de/agbkb/forschung/formal_methods/CoFI/hets/index_e.htm ). But note, that for each dialect of a logic (not just the ontology), a further mapping is needed. Unless of course we can somehow make sure that everyone uses the same specialized logic...
2) As a corollary to what John describes below, by sacrificing expressiveness for decidability, we also further hinder interoperability. Particularly, if we only represent the fragment of the problem under consideration that is expressible in said language, we are leaving a lot of the intended semantics of the problem domain external to the system of representation!
Any other person or machine coming along can at best only guess at what the actual intent of these squiggles are, since so much is consequently left unsaid. A lot of the problems that arise when trying to conduct semantic / ontology mappings become much more directly accessible. One need only look at the current state of the art in semantic mapping based on DL's to see the problems posed by this approach. We're trying to match lexical terms, guessing at instance data and using probabilities to guess at the intended semantics. If more were expressed, we could actually use model theory to specify much more robust and reliable mappings.
Note: one can still decide to infer / implement only particular fragments of the representation, but the representation is more complete and closer to the intended semantics of the ontologist / knowledge engineer.
None of this is meant to take anything away from the work done on OWL or DL's, but I submit that the framing or promoting of these languages as "more appropriate" ontology languages, because they guarantee decidability is missing the mark, and probably counter productive and needlessly divisive / creating silos in a community that really can't afford to fragment.
Fragments of logic which exhibit certain desirable properties are an important / invaluable toolkit in any ontologist / knowledge engineer's repertoire. Being able to mark a fragment of an ontology as exhibiting XYZ properties and guaranteeing ABC performance is important, and can make the difference in certain domains. But it is not the same as ontology, and it would be disingenuous to construe such languages as being somehow inherently more suited to computational ontology.
To my limited mind, they represent one of many useful applications. If I'm designing an autonomous robot, I'll probably be using a dedicated temporal reasoner, a dedicated spatial reasoner, etc. etc. OWL-2 is probably too expressive for these, and not nearly expressive enough to support narrative. No single optimized logic or formalism on its own is likely to suffice, and I'd rather represent as much as I can formally, and then worry about how best to compute these explicit assumptions - probably (no - in fact, exactly) based on the invaluable work that people in the DL community are doing determining the properties of various fragments of FOL. But to start with DL's as the end all seems sort of like lobotomizing logic...
My two cents.
On Thu, Aug 12, 2010 at 2:37 AM, John F. Sowa <sowa@xxxxxxxxxxx> wrote:
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