Here is a nice little paradox:
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.
(In reply to
another solution by Logan)
1.S<=>(S=>G)
2.S
3.(S=>G)
4.S
5.(S=>G)
6.(S<=>(S=>G)) from steps 3 and 5
I'm confused as to how you reached step 6. You can't form a proof by
contradiction by supposing first an antecedent is true and then that it
is false and then using both as if they were both true at the same
time. This can be shown if we include premise dependencies alongside
the statement dependencies:
1. P
2. P
3. From 2, from premises 1,2.
4. P
5. From 4, from premises 1,2.
6. From 3 and 5, from premises 1, 2 and 4.
Now instead of just saying "from
steps 3 and 5", we see that it is from premises 2 and 4, respectively.
Therefore, the conclusion is based on the premise "S and S", which is
a contradiction in the first place, and from a contradiction anything
can be proven, including step 6.
But note that this isn't the same contradiction you claim to have shown
 it's not a contradiction stemming from premise 1 (which you want to
reject), but from premises 1, 2 and 4, which means you can't conclude
simply by rejecting 1.

Posted by Sam
on 20050309 10:30:43 