Statement S: If S is true then God exists
Logically, statement S must be either true or false.

1. Suppose S is false.

2. If S if false, then any statement that starts with "If S is true..." is true *(see note)

3. Specifically, the statement "If S is true then God exists" would be true

4. This is exactly what S says, so S would have to be true

5. This is in contradiction with 1., so S cannot be false.

6. Therefore S is true.

7. So the statement "If S is true then God exists" is true.

8. By modus ponens, since S is indeed true, then the second half of that statement is true.

9. God exists.

Note of course that you can make the same argument to prove that God doesn't exist, or anything else.
What, if anything, is wrong with this proof?

*Note: This is the part that I expect most people will comment on. It is one of the standard logical rules that if something, A, is true, you can say "If (~A) then..." and that will always be true. For instance, I could say "If George Washington is alive then the moon is made of cheese" and that would be considered true in natural logic.

are you sure? that isn't intuitive and seems wrong. i can see how the antecedent being false does not prove the statement false, but i don't see how it validates the statement. there's a difference between not proving something false, and proving something true. it would seem to me that both the antecedent and consequent need to be true for the statement to be true.

otherwise you could make the two statements:
if it is above 80 degrees tomorrow it will be sunny
if it is above 80 degrees tomorrow it will rain

but if it is below 80 degrees tomorrow then both statements would be true.

Posted by donnmike on 2003-12-15 23:02:37