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.