Reductio Arguments

A "Reductio" proof (aka a “proof by contradiction” or an “indirect proof”) is an argument with a very particular form. A reductio proof begins by assuming the opposite of what it is trying to prove, and then showing that this assumption leads to a contradiction.

To construct a reductio proof, you need to do two things:

First, assume the opposite of what you are trying to prove.
Then, show that that assumption leads to a contradiction.

The first example of a reductio proof we saw in this class was in Step 2 of Anselm's Ontological Argument.