Saturday, February 20, 2010

Contradictory Premises



XKCD brings up an interesting point.

If you start with any set of contradictory axioms (ie p and not p are both true for some p), you can "prove" the truth of just any statement you wish.

For example, given 4 = 3, can you prove that you are the Pope?

Alfred Whitehead, logician of Russell and Whitehead fame, was once asked this very question. His answer:

4=3 (by axiom)

2=1 (Subtract two from both sides)

It is well known that the Pope and I are two people.

Therefore, the Pope and I are One.


There are, of course, many ways of obtaining credit card numbers through false premises, though none of the famous ones involve logical deduction.

2 comments:

Maverick's Musings said...

but wch axiom says 4=3 and why do we assume that..

The Mad Hatter said...

@Maverick's Musings

The problem was posed as "given 4=3, prove that you're the pope". See above.