Tu quoque means ‘you also’, and is committed when a case is undermined by the claim that its proponent is himself guilty of what he talks of. It is a change of subject from a claim made by a proponent to one made against him. (‘You accuse me of abusing my position, but you’re the one whose company car is seen propping up the rails at the local race-course!’)
With a little more subtlety, the tu quoque can be used to undermine an accusation by discrediting the accuser.
And now I turn to Mrs Green's charge that I deliberately misled this society over my personal interest in the firm concerned. May I remind you that this charge comes from the same Mrs Green who kept very quiet when her son-in-law benefited from our decision over the surplus land. Hardly a source entitled to make such charges, you must agree.
(I reckon he did it.)
The fallacy of the tu quoque occurs because it makes no attempt to deal with the subject under discussion. A new subject is introduced, namely the record of someone involved. The truth or falsehood of an assertion has nothing to do with the back-ground of the person who makes it. Evidence for or against that assertion is not altered by details of the previous actions of the one who is putting it forward.
Another version of the tu quoque seeks to undermine what is being said by showing it to be inconsistent with the previous views of its proposer.
Why should we listen to Brown's support for the new carpark when only last year he opposed the whole idea?
(For one thing, if the arguments changed his mind they might be worth listening to. For another, there might be more cars around.)
Because someone once opposed an idea it does not preclude their arguments in favour from being good ones. Despite this, the fallacy is supported by a strong tendency in us to appear consistent whenever we can. The new mayor finds it difficult to argue with sincerity this year in favour of the same official limousine which he opposed so vociferously for his predecessor.
The UK’s parliamentary question-time is the home of the tu quoque. Indeed, skill at handling questions is often measured exclusively in terms of the performer’s dexterity with this particular fallacy. This is why answers to questions about the present or the future invariably begin with the phrase:
May I remind my honourable colleagues…
(He is, of course, reminding those opposite that they did it sooner, longer, deeper, louder and worse. This is why their specious charges can be rejected.)
A parliamentary question is always known in the House as a ‘PQ’. There is a good case to be made for having the reply to one of them called a ‘TQ’.
The tu quoque is easy to use because everyone is inconsistent some of the time, and few people have a blameless past. You can argue that anyone who has changed their mind has thereby proved that they must be wrong at least some of the time, and that this occasion could well be one of those times. If you can find nothing at all to your opponent’s discredit, even this fact can be used in an attempt to undermine what he is saying. The rest of us have weaknesses, why doesn’t he?
As for the charges that I may just occasionally have helped myself out of difficulty to a small extent, all I can say is look at Mr High-and-mighty Holier-than-thou.
(And he is probably quite a lot holier than thou.)