Tu Quoque is a form of ad hominem. It means “You too”. It's often called “The You Too Fallacy”.
The way Tu Quoque works is that a person's argument is dismissed because he is guilty of violating his own argument. He is hung on his inconsistency.
Dave: Marriage is a sacred institution Brian, you need to take it seriously and work hard to keep your marriage healthy. Cheating on your wife is the most destructive thing you can do to your family and your marriage.
Brian: What do you know about marriage and cheating? Your first two marriages ended when you were caught cheating.
Brian dismisses Dave because he was guilty of that which he is trying to persuade him to stop doing. Not only does this avoid the argument about how to preserve a marriage, it dismisses the fact that Dave is trying to stop Brian from making the same mistakes. Teenagers are famous for using Tu Quoque arguments against their parents.
In politics, Tu Quoque often shows up as “flip-flopping”. For example, John Kerry supported the war in Iraq before he opposed it. His original support for the war does not disqualify his latter opposition, however his reasons for that opposition may provide reason to dismiss his opinion. Likewise John McCain favored amnesty for illegal aliens in 2000, but opposed it in 2008. While his change of opinion could be chalked up to political expediency, dismissing him on the grounds of a change of mind would be logically fallacious.
Another Tu Quoque political argument is common among non-politicians.
Rick: “I can't support Democrats because I am disgusted by all of the corruption they have been involved in. The governor of Illinois tried to sell Obama's senate seat to the highest bidder. Tim Geitner and Charlie Rangel cheated on their taxes, and several Senators had their votes bought in the Health Care bill, like Nelson of Nebraska and Landrieu of Louisiana.”
Tom: Maybe they got caught, but all politicians do it.
One corrupt politician doesn't excuse another's corruption. Tom is avoiding the issue.
Arguments must be addressed on their own merit, and Tu Quoque is just another way of avoiding the merits to shift the discussion.
Next Lesson: The Genetic Fallacy
