38 How to Win Every Argument
Never believe what people tell you about patent medicine; they are
always liars. It is because you know that I am truthful about this that you
will know I am also telling the truth when I tell you that my snake-oil
here is the most remarkable...
(Remarkable.)Contradictory premises
No matter how good the logic may be, you cannot rely on an
argument which has certain falsity built into it. For a sound
argument true premises are needed, as well as valid logic. The
problem with contradictory premises is that they cannot both be
true. If one is true, the other must be false, and vice versa. In
other words, we can be certain that at least one of them must be
false, and cannot therefore generate a sound argument.
Everything is mortal, and Cod is not mortal, so God is not everything.
(This might look like an argument against pantheism, but it is in fact
an argument against common sense. Since the premises contradict
each other, one must be false. This makes any conclusion unreliable.)The fallacy is an interesting one because it permits the logic to be
valid. It usually amazes non-logicians to hear that with incon-
sistent premises any conclusion, no matter how irrelevant, can be
validly inferred. Logicians, however, do not use the word 'valid'
to mean 'sound', If there is known falsity built in, as there must
be with contradictory premises, then it matters not how good
the logic is: the argument is not sound.
This is the fallacy which enables us to prove that the moon is
made of green cheese. The proof is quite complicated, but quite
fun: