“You too!”

Lat. “tu quoque!”; tu “you”, quoque “too”

In Latin and in English, the “you too” argument scheme is named after the statement that typically realizes the argument.
In the general case the reply:

S1:   — I do A because X does so.

is a strategy of legitimation by imitation. The fact that X makes A creates a precedent@ legitimizing A, and if S1 considers X as a model, it gives A a second form of legitimacy. Such legitimations are part of the “You too!” argumentation; its scenario is as follows:

S1 performs such action A.
S2 blames him
S1 replies: “But you do it too! You do the same!

S2 criticizes S1 for an action that he presents as blameworthy. S1 can reply in a variety of ways:

(i) He can first answer to S2 that others do the same: since Landru (a popular French serial killer) murdered his mistresses, why couldn’t I? The degree of legitimation depends on the severity of the transgression and the number of transgressors. I run a red light in the open country, when there is no traffic and the visibility is perfect, and I feel justified in saying “well, this is forbidden, but everyone does it, the guy ahead went through, I just followed him”.

(ii) In the case where the wrongdoer is not another third party but S2, S1 has two possibilities:

— As in the previous case, S1 can quietly legitimize his or her action by the (bad) example given by S2.

— S1 can also reply using a counter-accusation, which seeks to put S2 in the face of the contradiction between what he preaches and what he does, S. Ad hominem. S1 acknowledges his or her misbehavior, but considers that, due to his or her own misbehavior, S2 is in no position to teach him or her a lesson. In terms of stasis, the defendant does not recognize the legitimacy of the judge, S. Stasis:

S1: — It suits you well to blame me! Please, not you! I have no moral lessons to receive from you.

Two wrongs don’t make a right

The phrase “two wrongs don’t make a right” can be understood in two different ways.

— First, as “one does not fight evil with evil”, that is, “evil must be fought by legal means”, a very important principle; many would be tempted to add the clause “as far as possible”. In other words, the good end — the struggle against evil — should not be pursued by evil means; such as torturing the former torturer to stop torture. This would amount to a case of autophagy.
This principle is invoked to reject the justification of a mistreatment made to somebody by arguing, in a sort of anticipatory law of retaliation, that, had he been in our place, this is what he would have done to us, S. Reciprocity (after FF, Two wrong).
— Secondly, it can express the rule that “bad behavior does not become legitimate because widespread”; many wrongs never make a right. The common transgression (argument from number) never creates an against-the-law legitimacy, S. Consensus.

In practical life, thanks to a minor miracle, an error sometimes compensates for another. This also seems to occur in science:

Kepler knows that Tycho Brahe [obtained] the best possible accuracy on the measurements of the positions of the planets (including the planet Mars), and this accuracy was of two minutes of degree. With the mathematical model of a circular orbit on the Mars planet that he (Kepler) used, Kepler noticed discrepancies of eight minutes of degree between the positions observed by Tycho Brahe and the calculated positions. Trusting the precision of Tycho Brahe’s measurements, Kepler renounces the circular orbit of Mars. He revises the orbit of the Earth and, thanks to two compensating errors, discovers his first law: “In the motion of a planet, the vector ray sweeps equal areas in equal times.”
Edgar Soulié, [Johannes Kepler (1571-1630), the Protestant astronomer who discovered the laws of motion of the planets], (no date).[1]

[1] Edgar Soulié, Johannes Kepler (1571-1630) L’astronome protestant qui a découvert la loi du mouvement des planètes. http://www.astrosurf.com/rtaa/rtaa2016/documents/kepler-edgar-soulie.pdf (01-09-2017).