[HPforGrownups] Re: Script from JKR's reading

[Jordan Abel]

No: HPFGUIDX 156856

On 8/11/06, Sally Altass <sallyaltass at yahoo.co.uk> wrote:
> > Neri:
> > If A was a doctor then B didn't die. If it's also given that B
> > *did* die then the unavoidable conclusion is that A can't be a
> > doctor.
>
> Just a quick point that Neri doesn't seem to have grasped.  A could
> could still be a doctor, but not necessarily an MD.

Which still only means "If A was a doctor then B didn't die" is false,
NOT that "if B died A is not a doctor" doesn't follow from it.

Proof that the latter follows from the former:
(A is a doctor) IMPLIES NOT (B died) [original statement]
NOT (A is a doctor) OR NOT (B died) [definition of implication]
NOT (B died) OR NOT (A is a doctor) [disjunction is commutative]
(B died) IMPLIES NOT (A is a doctor) [definition of implication]