The ad hominem fallacy is a specific form of the genetic fallacy, and the opposite of an appeal to authority. It is typically committed by those who have been presented with an argument but are unwilling to accept its conclusion. Its name, ”ad hominem”, is Latin meaning ”against the man”. The fallacy is committed by rejecting an argument not because of a flaw in its logic or because of the falsehood of its premises, but because of the person, the “man”, who presented it.

The logical form of the ad hominem fallacy is as follows:

(1) Argument a was presented by person S.
(2) Person S has property p.
Therefore:
(3) Argument a is unsound.

Property p here could be any property that doesn’t bear on the soundness of the argument. This pattern of argument is fallacious because the soundness or unsoundness of an argument is independent of the person offering it. A sound argument does not become unsound simply because it is uttered by an unreliable source.

Consider this example:

(1) George Bush argued that we ought not to act to curb carbon dioxide emissions because the evidence that climate change is man-made is inconclusive.
(2) George Bush was an oil man.
Therefore:
(3) It is not the case that we ought not to act to curb carbon dioxide emissions because the evidence that climate change is man-made is inconclusive.

There are a number of criticisms that could be levelled against Bush’s argument that we ought not to act to curb carbon dioxide emissions. We could attack its logic, suggesting that the conclusion does not follow from the premise: given the dire consequences of climate change, perhaps we should err on the side of caution and act to curb carbon dioxide emissions just in case they are the cause, even if the scientific evidence is inconclusive. Alternatively, we could attack its premise, arguing that the evidence that climate change is man-made is now conclusive, and that we therefore ought to act to curb our emissions.

The fallacious argument above, however, takes neither of these routes. Instead, it ignores both the logic of Bush’s argument and the question of whether its premises are true, and instead says that the argument should be rejected simply because it is Bush’s argument, and Bush was an oil man. It thus attacks the man behind the argument, rather than the argument itself, and so commits the ad hominem fallacy.