Is "tu quoque" really a logical fallacy?

by George J. Dance

"Tu quoque" is a Latin term that translates as "you too." It is the name of a line of argument that is considered a logical fallacy. Here is a standard explanation, from Wikipedia:

Tu quoque ... or the appeal to hypocrisy, is a fallacy that intends to discredit the opponent's argument by asserting the opponent's failure to act consistently in accordance with its conclusion(s)... An example would be:

Peter: "Bill is guilty of defrauding the government out of tax dollars."
Bill: "How can you say that when you yourself have 20 outstanding parking tickets?"
https://en.wikipedia.org/wiki/Tu_quoque

Wikipedia adds that tu quoque "is a fallacy because the moral character or actions of the opponent are generally irrelevant to the logic of the argument." It also calls it a "red herring"(a diversion) and an "ad hominem" (an attack not on the argument but on the person making the argument), but those two objections to it reduce to the first one: a statement can be a red herring or an ad hominem only if it is irrelevant to an argument.

So, then, a tu quoque is a fallacy only if it is irrelevant to an argument. A moral argument, though, is an argument about right and wrong (about what people ought to do and not do); and a person's actions (what he does and does not do) reflect his moral beliefs (what he believes people ought to do and not do). So: a tu quoque is not a fallacy in a moral argument.

Consider the above example. Peter's parking tickets are indeed irrelevant to whether Bill is withholding money from the government; but it is not irrelevant to whether Bill is doing something wrong. Peter is not just giving us the information that Bill is withholding money from the government. By his use of the terms "guilty" and "defrauding," he is relying on (or smuggling in) tacit moral premises to imply a few equally tacit conclusions:

1. It is morally wrong to commit fraud.
2. Withholding money from the government is fraud.
3. Bill is withholding money from the government.
4. Therefore, Bill is guilty of fraud.
5. Therefore, Bill should stop withholding money from the government; and
6. Therefore, the rest of us should think badly of Bill for withholding that money.

That is the argument that Bill is addressing. He is in fact challenging Peter's premises, with a similarly tacit argument.

1. By not paying his parking tickets, Peter is also withholding money from the government.
2. Therefore, if Peter believed his third premise were true, he would pay his parking tickets (by his first premise).
3. Therefore, Peter does not believe one of either his first or fourth premises. Peter's argument contains a false premise, by his own expressed belief.
4. Therefore, Peter's argument, by his own expressed belief, is invalid.

That puts the onus back on Peter. Moral rules are "universalizable," which is just a fancy term meaning that they are rules not about what Bill should do or not do, but about what people (ie, everyone) should do or not do. The principle of universalizability states that the moral rules that apply to one person apply to every other person, except where there is a good moral reason for treating them differently; equal treatment is the default.

In order to save his argument, Peter has to justify such a difference in treatment. He can argue either that not paying parking tickets is different from not paying taxes, or that different rules should apply to Peter from those that apply to Bill. On the other hand, by simply claiming a fallacy on Bill's part, Peter has to do nothing at all: he wins his point without having to make it; Bill loses to a bad argument, that Peter did not prove (or even properly make).

To repeat: tu quoque is not a fallacy in a moral argument; falsely labelling it as such is itself fallacios.