1.) Start by assuming that the theory is false
2.) Next, we go about our proof and eventually run into a contradiction, that is something that doesn't make sense.
3.) The contradiction from step 2 proves our assumption of the theory being false not to be the case, so the theory must be true.
An indirect proof is also called a proof by contradiction, because we are literally looking for a contradiction to a theory being false in order to prove that the theory is true.
