So. Come to the common room to hear Dr C.S. Lewis, author of Mere Christianity and the Screwtape Letters prove logically that God created the Universe and then explain how He did so. Nothing too challenging: he’s got nearly an hour in which to cover it.
But he doesn’t start with God. He starts with two elderly Victorians having an argument.
What are they arguing about? What else but geometry?
Some day (says Lewis) I am going to write the biography of Bulverism’s 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 the 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 your opponent is wrong, and then explain his error, 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.
Very droll. But what point was Lewis making?
We all know what an ad hominem argument is. You can’t prove your client is innocent on the basis of the evidence; so you say that the prosecution lawyer is a rogue, a fool, and what’s more, that he’s wearing a silly tie. You can’t explain why a 45% income tax on earnings above £150,000 would be impractical and unfair; so you go on and on about that time the leader of the opposition didn’t sing the words to God Save the Queen. Obviously this is a particularly egregious error when you are talking about geometry.
We also know what it means to beg the question: to take your conclusion as your starting point; to take as proved the very thing which needs to be proved. My learned friend says that my client is a murderer: but how can you believe someone who would falsely accuse an innocent man? You have heard the witness say that he was on the other side of town when the crime was committed: but how can you trust the word of a murderer?
Mrs Bulver is arguably guilty of a third offence against logic: she has introduced something irrelevant into the discussion. If there was a widespread belief that gender affected perception of Euclidean geometry, there might have been an excuse for saying that her husband believed in the inequality of triangles because he was a man. If he had said “I believe that this tie goes well with this shirt” she might perfectly well have replied “You say that because you are colour blind.”
In the course of the essay, Lewis proposes eight further examples of Bulverism; four more turn up in his book on Miracles. Each of them is arguably circular, since they assume that the speaker is in the wrong. All involve an ad hominem argument: three attribute the opponent's beliefs to psychological causes; the rest allege an ulterior motive or vested interest. None of them commit Mrs Bulver’s fallacy of irrelevance: in each case the motive or cause has some plausible connection to the belief. The American Internet generally uses Bulverism as a synonym for personal attacks: but strictly speaking an argument needs to be both ad hominem and circular to qualify.
Lewis examples fall broadly into four categories: axiomatic; facts; subjective beliefs and religious and political beliefs. Let's look at them one at a time...