If we understand a fallacy as a mistake in reasoning that occurs with some frequency in real arguments and is characteristically deceptive, there is no argumentum ad hominem fallacy. Arguing ad hominem in its original sense is a perfectly legitimate strategy of using an interlocutor’s concessions or commitments to show that the interlocutor is committed to a certain conclusion. The tu quoque, which emerged from this sense as an appeal to commitments implicit in the behaviour of one’s critic, legitimately challenges the critic to explain away an apparent inconsistency. The purely abusive ad hominem is either a legitimate attack on an opponent’s ethos or an objectionable diversionary tactic that is not a kind of reasoning. The circumstantial ad hominem, which attributes the position of one’s opponent to self-interest or a dogmatic bias, raises legitimate suspicion about the credibility of the opponent’s statements and arguments.