What is a tu quoque fallacy?

[Answered on Quora.com by Alex Johnston]

Ah, tu quoque. So common. So easy to do. And, like the Monty Hall Problem, the reason why it’s a fallacy is counter-intuitive.

Here is a simple tu quoque:

A: Hey, are you eating a third bacon & cheeseburger? You know, you shouldn’t, they’re really fattening.

B: But you eat eight bacon & cheeseburgers per day, and as a result, you’re morbidly obese and have had three heart attacks. So you can’t talk to me about that. Bingo! Ya burned!

Here’s why B has committed a fallacy: the fact that A eats eight bacon & cheeseburgers per day has absolutely nothing to do with the fact that three bacon & cheeseburgers really are fattening.

A’s observation that B is eating unhealthily is absolutely correct. The fact that A also eats unhealthily doesn’t make A wrong.

But, you will say, hypocrisy!

Yes, we reply, hypocritical, sure, but still 100% true. B should not eat three bacon & cheeseburgers, any more than A should eat eight.

The hypocrisy could be said to lie in the act of A pointing out that B is eating unhealthily. It’s irritating to B that A, of all people, should say this, but that still doesn’t make it untrue.

It is of course obvious that the tu quoque fallacy is one of the central weapons in political discourse today, and people don’t call it out because the fact that tu quoque is a logical fallacy has no purchase on people who are thinking not logically, but in terms of their tribal loyalties.