At 10:26 PM -0400 5/10/08, John F. Sowa wrote:
>Adrian,
>
> > Unfortunately, classical logic assigns a meaning to negation
> > that differs from the way databases are actually used.
>
>That is true. There are four widely used options:
>
> 1. The closed-world assumption (CWA) in which any ground clause
> (n-tuple of constants) not asserted to be true is assumed
> to be false. This is typical of airline reservations: if
> your reservation is not in the DB, you don't have one.
>
> 2. The open-world assumption (OWA) in which any ground clause
> not asserted to be true has an unknown truth value. This
> is typical of most databases generated from observations.
> Anything not observed is not known to be true or false.
>
> 3. Three-valued logic (TVL) in which some ground clauses are
> asserted to be true, others are asserted to be false, and
> any others have an unknown truth value.
>
> 4. The whatever assumption (WA) in which anything not asserted
> to be true may be false, unknown, or whatever. This is the
> widely used method of code-and-debug until you get tired
> or the deadline has passed.
>
>
> > Unfortunately, classical logic assigns a meaning to negation
> > that differs from the way databases are actually used.
>
>Classical negation corresponds to the CWA. (01)
I think you mean the OWA. (02)
Pat
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