What is "tu quoque"

Latin, literally "thou also" (or, in modern vernacular, "so are you!"); an argument which consists in retorting accusations.

n. An argument whereby an accusation or insult is turned back on the accuser.

Tu quoque (; Latin for, "you also") or the appeal to hypocrisy is an informal logical fallacy that intends to discredit the validity of the opponent's logical argument by asserting the opponent's failure to act consistently in accordance with its conclusion(s).

Tu quoque "argument" follows the pattern:

1. Person A makes claim X.
2. Person B asserts that A's actions or past claims are inconsistent with the truth of claim X.
3. Therefore X is false.

An example would be

Peter: "Based on the arguments I have presented, it is evident that it is morally wrong to use animals for food or clothing." Bill: "But you are wearing a leather jacket and you have a roast beef sandwich in your hand! How can you say that using animals for food and clothing is wrong!"."

It is a fallacy because the moral character or past actions of the opponent are generally irrelevant to the logic of the argument. It is often used as a red herring tactic and is a special case of the ad hominem fallacy, which is a category of fallacies in which a claim or argument is rejected on the basis of facts about the person presenting or supporting the claim or argument.

It is not a fallacy, however, to simply point out that the opponent is being hypocritical.