On Nov 17, 2012, at 3:59 AM, Michael Brunnbauer wrote: (01)
>
> Hello Pat,
>
> On Fri, Nov 16, 2012 at 02:20:50PM -0800, Pat Hayes wrote:
>> I wish this were true. Unfortunately, there are many users who either ask
>> for, or worse simply *presume* decideability. So that if you give them a
>> system which sometimes says "no answer", they will simply take this as the
>> answer, "no". Which is one of Ian's arguments: whatever you give users,
>> they will treat it as an oracle.
>
> Sounds like those users would also have a problem with "not deriveable", which
> should also be a possible answer of decideable systems ? (02)
Yes indeed. I am only reporting the argument. not endorsing it. Ian's argument
here is really an argument for *efficient* decideability, and (to give him
credit) this is exactly what he is a world leader in providing. (03)
> I am confused by the Wikipedia article about OWL:
>
> https://en.wikipedia.org/wiki/Web_Ontology_Language#OWL_DL
>
> It says that "OWL DL was designed to provide the maximum expressiveness
> possible while retaining computational completeness (either phi or not phi
> belong), decidability (there is an effective procedure to determine whether
> phi is derivable or not)".
>
> The statement about computational completeness of OWL DL is wrong, isn't it ? (04)
I had a *huge* fight with the DL people (mostly Ian Horrocks) about this. Ian's
view is that logics that aren't decideable are simply faulty, broken,
inadequate, not of interest, etc.. Any "reasonable" logic should be decideable.
So, he simply *assumes* decideability as a basic property of a logic. OK, that
settled, we turn to completeness. If the logic is decideable, then one can
phrase completeness as there being an effective procedure to determine
entailment. The fact that this definition does not apply to non-decidable
logics is not relevant, in Ian's view. (By the time we had this argument, the
WG business was almost finished and everyone was very, very tired, and the WG
just wanted to get it over with. And as Ian had proposed the wording and I was
objecting to it on what most people thought were arcane
technical/logical/academic details, I lost the vote.) (05)
>
> OWL Full is undecidable (06)
Actually I don't know that OWL Full is decideable. I suspect it is not, but I
don't really care :-) (07)
> but less expressive than FOL for reasons of
> better computational complexity of reasoning ? (08)
Again, I don't know what the complexity class of OWL Full is. Those who care
about such things are usually working on description logics in any case. (09)
Pat (010)
>
> Regards,
>
> Michael Brunnbauer
>
>
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