Bulverism is a type of ad hominem rhetorical fallacy that combines circular reasoning and the genetic fallacy with presumption or condescension. The method of Bulverism is to "assume that your opponent is wrong, and explain his error." The Bulverist assumes a speaker's argument is invalid or false and then explains why the speaker came to make that mistake or to be so silly (even if the opponent's claim is actually right) by attacking the speaker or the speaker's motive.

The term Bulverism was humorously coined by C. S. Lewis after an imaginary character to poke fun at a very serious error in thinking that, he alleged, frequently occurred in a variety of religious, political, and philosophical debates.

Similar to Antony Flew's "subject/motive shift", Bulverism is a fallacy of irrelevance. One accuses an argument of being wrong on the basis of the arguer's identity or motive, but these are strictly speaking irrelevant to the argument's validity or truth.