Bulverism

Bulverism is a logical fallacy in which, rather than proving that an argument is wrong, a person instead assumes it is wrong, and then goes on to explain why the other person held that argument. It is essentially a circumstantial ad hominem argument. The term "Bulverism" was coined by C. S. Lewis. It is very similar to Antony Flew's "Subject/Motive Shift".

Source of the concept
Lewis wrote about this in a 1941 essay of the same name, later included in the anthology God in the Dock. He explains the origin of this term:

You must show that a man is wrong before you start explaining why he is wrong. The modern method is to assume without discussion that he is wrong and then distract his attention from this (the only real issue) by busily explaining how he became so silly.

In the course of the last fifteen years I have found this vice so common that I have had to invent a name for it. I call it "Bulverism". Some day I am going to write the biography of its imaginary inventor, Ezekiel Bulver, whose destiny was determined at the age of five when he heard his mother say to his father — who had been maintaining that two sides of a triangle were together greater than a third — "Oh you say that because you are a man." "At that moment", E. Bulver assures us, "there flashed across my opening mind the great truth that refutation is no necessary part of argument. Assume that your opponent is wrong, and the world will be at your feet. Attempt to prove that he is wrong or (worse still) try to find out whether he is wrong or right, and the national dynamism of our age will thrust you to the wall." That is how Bulver became one of the makers of the Twentieth Century.

Pattern
The form of the Bulverism fallacy can be expressed as follows: or
 * You claim that A is true.
 * Because of B, you personally desire that A should be true.
 * Therefore, A is false.
 * You claim that A is false.
 * Because of B, you personally desire that A should be false.
 * Therefore, A is true.

Examples
From "Bulverism" by C.S. Lewis:

"Suppose I think, after doing my accounts, that I have a large balance at the bank. And suppose you want to find out whether this belief of mine is 'wishful thinking.' You can never come to any conclusion by examining my psychological condition. Your only chance of finding out is to sit down and work through the sum yourself. When you have checked my figures, then, and then only, will you know whether I have that balance or not. If you find my arithmetic correct, then no amount of vapouring about my psychological condition can be anything but a waste of time. If you find my arithmetic wrong, then it may be relevant to explain psychologically how I came to be so bad at my arithmetic, and the doctrine of the concealed wish will become relevant — but only after you have yourself done the sum and discovered me to be wrong on purely arithmetical grounds. It is the same with all thinking and all systems of thought. If you try to find out which are tainted by speculating about the wishes of the thinkers, you are merely making a fool of yourself. You must first find out on purely logical grounds which of them do, in fact, break down as arguments. Afterwards, if you like, go on and discover the psychological causes of the error."

From A Reply to Professor Haldane by C. S. Lewis: The Professor has his own explanation ... he thinks that I am unconsciously motivated by the fact that I ‘stand to lose by social change’. And indeed it would be hard for me to welcome a change which might well consign me to a concentration camp. I might add that it would likewise be easy for the Professor to welcome a change which might place him in the highest rank of an omnicompetent oligarchy. That is why the motive game is so uninteresting. Each side can go on playing ad nauseam, but when all the mud has been flung every man’s views still remain to be considered on their merits. I decline the motive game and resume the discussion.