Bulverism is a logical fallacy in which, rather than proving that an argument in favour of an opinion is wrong, a person instead assumes that the opinion is wrong, and then goes on to explain why the other person held it. 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".
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