You did it too!
Fallacies of Relevance > Ad Hominem Arguments
Explanation of the Tu Quoque
The Tu Quoque fallacy is a form of the ad hominem fallacy which does not attack a person for random, unrelated things; instead, it is an attack on someone for a perceived fault in how they have presented their case. This form of the ad hominem is called tu quoque, which means "you too" because it typically occurs when a person is attacked for doing what they are arguing against.
Examples and Discussion of the Tu Quoque
Usually, you will see the Tu Quoque fallacy used whenever an argument has gotten very heated and the possibility of civil, productive discussion may have already been lost:
1. So what if I used an ad hominem? You insulted me earlier.
2. How can you tell me not to experiment with drugs when you did the same thing as a teenager?
As you can see, the arguers in these examples are trying to make the case that what they have done is justified by insisting that the other person has also done the same. If the act or statement in question was so bad, why did they do it?
This fallacy is sometimes referred to as "two wrongs don't make a right" because the implication that a second wrong makes everything alright. Even if a person is being completely hypocritical, though, this does not mean that their advice is not sound and should not be followed.
Tu Quoque and Sincerity
This fallacy can also occur more subtly, for example, by attacking a person's sincerity or consistency:
3. Why should I take your arguments for vegetarianism seriously when you would accept a transfusion of blood that has been tested using animal products, or accept medication that has been tested using animals?
The reason this example qualifies as a tu quoque fallacy is because the argument reaches the conclusion "I don't have to accept your conclusion" from the premise "you don't really accept your conclusion either."
This looks like an argument against the consistency of an argument for vegetarianism, but it is actually a argument against a person arguing for vegetarianism. Just because a person fails to be consistent does not mean that the position they are arguing for is not sound.
You can be inconsistent in following a sound principle and consistent in following an unsound principle. This is why the consistency with which a person follows what they are arguing for is irrelevant when it comes to the validity of their position.
Of course, this doesn't mean that it is illegitimate to point out such glaring inconsistencies. After all, if a person does not follow their own advice, it may be that they don't really believe it themselves - and if that is the case, you can ask why they want you to follow it.
Or maybe they don't really understand what they are saying - and if they don't understand it, it is unlikely that they will be able to present an effective defense for it.
You Would Do It Too
A closely related tactic is to move from saying "you did it, too" to saying "you would do it too, if you had the chance." In this way, people can construct arguments like:
4. The leaders of that country are insane, and would attack us if they had the chance - so we should attack them first and thus protect ourselves.
5. Christians would persecute us again if they were given the opportunity, so what's wrong with persecuting them first?
This is fallacious for the same reason that the usual tu quoque is a fallacy - it doesn't really matter what someone else would do if they had the chance, because that alone doesn't make it right for you to do it yourself.