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Tu quoque ("You too!" — pronounced "too kwoh-kway"), also known as appeal to hypocrisy, is a form of ad hominem fallacy that occurs when it is assumed that an argument is wrong if the source making the claim has itself spoken or acted in a way inconsistent with it. The fallacy focuses on the perceived hypocrisy of the opponent rather than the merits of their argument.
The usual structure of the fallacy is:
- Person A makes claim X about Person B.
- Person B points out that claim X is also true of Person A.
- Therefore, X is is irrelevant/false and A is a hypocrite.
This kind of reasoning is fallacious because criticism or objection to the person making the claim does not apply equally, if at all, to the argument itself. Certainly, if the premises are indeed true then source A is likely a hypocrite and should also be included in the guilty party, but this bears no relevance or relationship to the validity or factual-ness of the claim X. In essence, the claim X is being dismissed on grounds of a criticism of A, which is a non sequitur.
However, while this response does not prove that the person's position is false, it can sometimes be a valid counterargument, especially if A makes a universal claim. For instance, if someone says "Engaging in homosexuality is wrong. Homosexuality is not an inborn characteristic, and anyone who feels homosexual urges can overcome those urges and live a happy, fulfilled life without engaging in homosexuality," it would be valid to say "Well, seeing as how you have a gay lover, it appears that it isn't as easy as you make it out to be."
A conversation between rival politicians:
Politician 1: Is it true that you were at a brothel last night?
Politician 2: You should know better than I do, I was with you last night!
The fact that Politician 1 was also present at the brothel in no way absolves any of the moral blame for Politician 2 (unless P1 dragged P2 there against P2's will). If P2 is lying, he is making a pathetic attempt to switch blame. If P2 is telling the truth, P1 is being a massive hypocrite who should also go down with P2 — but he's a politician, so no surprises there.
 Tu quoque in response to criticism
The first common variant of the fallacy is the "you criticize X, but you use something related to X" argument:
Alice: "But you yourself smoke and drink a lot! You're wrong!"
The fact that Bob is a smoker and drinker doesn't mean that he is wrong about the effects of those habits. Still confused? A better rebuttal would be to accept the premise that alcohol and smoking really are cancer risks, but then ask why Bob continues to do them. Perhaps Bob knows full well about the dangers of such addictions, but he may or may not be a hedonist with no sense of self-preservation, or it's just because he cannot or is yet to break from his very own addiction, hence why he continues to do it. Or because he is consciously or unconsciously suicidal, which makes his self-harm a logical consequence.
Another example is how (former) drug addicts who have suffered for their drug usage can also be very well qualified in admonishing others to not get started in harmful, expensive, illegal and addictive substances — sometimes due to personal experience and physical proof of the damage incurred by the drug addiction.
 Tu quoque in response to support
The second common variant is "you support X, but you do not totally embrace X" argument, which is often doubly wrong:
Bob: If you like them so much, why don't you go live there?
This argument fails for two reasons: firstly, whether Alice moves to Canada does not affect the validity of the statement in any way; and secondly, health care policies are not the only factor in choosing one's place of residence. For instance, Alice might be unwilling to leave her family behind regardless of the chance for better health care. Therefore not only is this argument a tu quoque, but also Alice's actions are not even inconsistent with her claims.
 Guilt by association
This fallacy is very frequently paired with the guilt by association fallacy, so that the person engaging in the fallacy need not even show that the other person's actions are inconsistent with their position; rather, it is merely shown that the actions of some other person somehow associated with that person did something inconsistent with that person's position. For instance, if someone criticizes Bill Clinton for his dishonesty, the common rejoinder is to bring up Bush's dishonesty.