Re: NULLs

"David BL" <davidbl_at_iinet.net.au> wrote in message news:17031909-5d14-42ac-a7fd-4a81b3cf709f_at_p69g2000hsa.googlegroups.com...
> On Jan 8, 7:50 am, Hugo Kornelis
> <h..._at_perFact.REMOVETHIS.info.INVALID> wrote:
>
> > And finally, I think that the closed world assumption will be a hard
> > sell for businesses. I found this definition "Closed world assumption:
> > if you cannot prove P or ~P from a knowledge base KB, add ~P to the
> > knowledge base KB." athttp://cs.wwc.edu/~aabyan/Logic/CWA.html. If I
> > understand this correctly, this implies that, if a knowledge base holds
> > information about a person with ID XP55303, but no information
> > whatsoever about the birthdate of this person XP55303, this person does
> > not have a birthdate at all and hence can not exist.
> >
> > Most businesses will (unless legally prohibited) gladly do business with
> > persons who refuse to state their date of birth - as long as they can
> > wave a valid credit card. Telling a customer she doesn't exist is bad
> > for business. Ergo, most businesses choose the Open World assumption.
>
> The above formal definition of the CWA above is compatible with
> missing information when the intensional definition is stated
> appropriately. Eg
>
> P = it is known to HR department that Alice is 25 years old.
>
> If P cannot be proven from the DB, then we deduce ~P.
>

I'm with you right up to this point. The problem is that, in practical terms,
people confuse ~P with the following:

Q=it is known to the HR department that Alice is not 25 yars old.

Note that Q is not the same as ~P. Received on Tue Jan 08 2008 - 05:56:04 CET