Re: NULLs

On Jan 3, 12:49 am, David BL <davi..._at_iinet.net.au> wrote:
> On Dec 30 2007, 12:00 am, JOG <j..._at_cs.nott.ac.uk> wrote:
>
>
>
> > On Dec 28, 10:59 pm, Hugo Kornelis
> > > I doubt it. Given this information:
>
> > > * "Employee Jack is male"
> > > * "Employee Mary is female"
> > > * "Employee Jack is 43 years old"
> > > * "Employee JJ is 32 years old"
>
> > > how would you answer the below questions:
>
> > > * "List all employees aged 40 and above".
> > > * "What is the average age of our employees?"
> > > * "For each employee, how many years left until retirement (assume a
> > > country with laws for retirement age of 65)"
> > > * "List all employees that are female, under 35 years old, or both"
> > > * "Is JJ older than Mary?"
> > > * etc
>
> > Yup, I catch your drift. However I believe these questions should have
> > "in propositions that you know" concatenated to them, so we still have
> > 2VL, and leaving us humans in the real world to interpret the results.
> > The CWA is a nonsense imo. But I do agree that with CWA, in RM, 3VL is
> > very difficult to avoid, and nulls are incredibly tempting to use. My
> > personal conclusion therefore is that perhaps we are trying to
> > address /symptoms/ and not the root cause of the issue, and a step
> > back is required.
>
> It seems to me that it's the OWA that invites 3VL. I'm thinking that
> the CWA simply means that an extension is uniquely defined given the
> intension and when intensional definitions are stated properly 2VL
> applies.
>
> I think OWA means a recorded extension may only be a subset of the
> actual extension corresponding to the stated intension, because OWA
> asserts that a system's knowledge is incomplete. It is the OWA that
> forces the logical inference system to deal with partial information.
>
> Have I got it wrong?

Those seem pretty neat definitions as far as I can tell. However there is then an issue as to what fully completing an extension means. Does the database then assume:
(a) I possess communicated statements that represent the whole current truth about the world
(b) I possess the whole current set of communicated statements about the world.

Its a really subtle difference but its always bugged me. (b) is interesting in that it completely precludes a user from asking the database any questions about the world proper, restricting us instead to asking questions about propositions themselves (I said it was subtle...). i.e. instead of "how many employees are there", one must ask "how many propositions are there with employees in". This seems entirely sensible to myself and completely rules out 3VL, but leaves the user to interpret the results.

I wonder whether (a) describes the assumptions of an inference engine and (b) that of a database. Received on Fri Jan 04 2008 - 02:25:54 CET