Reductio Ad Absurdum
An especially powerful valid deductive argument form is reductio ad absurdum. If you are unsure how to directly argue P, you may use reductio ad absurdum. First, assume it is not the case that P; second, include known truths amongst your premises; lastly, draw a conclusion that is a contradiction. It is impossible for a valid argument with true premises to result in a contradiction. Ergo, the premise P must not be true.
Suppose your friend maintains that every opinion is equally correct. You disagree but do not know how to argue against her directly. So you try reductio ad absurdum. You say to her, “Let us assume that you are right, that is, that every opinion is equally correct. Now, my opinion that you are wrong is an opinion, so it is correct. That is, it is correct to say that you are wrong. So, you are wrong. Thus, you are right and you are wrong, which is a contradiction. Therefore by reductio, the initial assumption that you are right must be rejected. This, you are wrong. (McHenry & Yagisawa, p 22)