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".
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