to quoque

\TOO-KWOH-kwee\

noun

: a retort charging an adversary with being or doing what he or she criticizes in others.

...i.e., the accusation upon discovering hypocrisy.

I think of such other phrases as:

It takes one to know one!

He can dish it out, but he can't take it!

Why don't you look in the mirror, buddy!

I added that last one; I don't know if it's an actual idiom.

The TU QUOQUE entry in The Free Dictionary is complex and lists both illegitimate and legitimate uses of the Latin phrase:

Illegitimate uses—

In many cases tu quoque arguments are used in a logically fallacious way, to draw a conclusion which is not supported by the premises of the argument.

You-too version

This form of the argument is as follows:

A makes criticism P.

A is also guilty of P.

Therefore, P is dismissed.

This is an instance of the two wrongs make a right fallacy.

Example:

"He cannot accuse me of libel because he was just successfully sued for libel."

Inconsistency version

This form of the argument is as follows:

A makes claim P.

A has also made claims which are inconsistent with P.

Therefore, P is false.

This is a logical fallacy because the conclusion that P is false does not follow from the premises; even if A has made past claims which are inconsistent with P, it does not necessarily prove that P is either true or false.

Example:

"You say airplanes are able to fly because of the laws of physics, but this is false because twenty years ago you also said airplanes fly because of magic."

Legitimate Uses—

Not all uses of tu quoque arguments involve logical fallacy. They can be properly used to bring about awareness of inconsistency, to indirectly repeal a criticism by narrowing its scope or challenging its criteria, or to call into question the credibility of a source of knowledge.

You-too version

A legitimate use of the you-too version might be:

A makes criticism P.

A is also guilty of P.

Therefore, the criticism is confusing because it does not reflect A's actual values or beliefs.

Another legitimate use of this version asserts:

A makes criticism of P for Q.

A is also guilty of Q.

Therefore, the criticism is confusing because it does not reflect A's beliefs.

Example:

Version 1: "You say that taking a human life is wrong under all circumstances, but support killing in self-defense; you are either being inconsistent, or you believe that under some circumstances taking a human life is justified."

Version 2: "You claim to believe that taking a human life is always wrong, and you criticize John for it. Contradictory to this, you also support killing in self-defense. You are a hypocrite and are inconsistent in your criticisms."

Note the difference between this legitimate usage and the fallacious one: in the latter, we attempt to use A's hypocrisy to prove that criticism P is false. This is illogical, since the truth value of a claim does not depend on the speaker. In the former, we are showing that A does not make a good critic, therefore arguing for greater skepticism toward his/her claims.

Inconsistency version

A legitimate use of the inconsistency version might be:

A makes claim P.

A has also made claims which are inconsistent with P.

Therefore, A is an inconsistent source of information.

Inconsistent sources of information are untrustworthy.

Therefore, A is an untrustworthy source of information.

Example:

"John Smith told the police he was at home alone on Friday night, but later said he was with friends at a bar; we can't take what he says about the crime at face value since he lied about his alibi."

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Wow. There's your reading for the day. Take a break. Stretch. Open a new window and check your Facebook.

I'm kidding.

It's important to understand the algebraic essence of linguistics.