The informal fallacy of supposing that a proposition should be denied because of some disqualifying feature of the person who affirms it. This fallacy is the mirror image of the appeal to authority. In its abusive form, ad hominem is a direct (and often inflammatory) attack on the appearance, character, or personality of the individual.

Example: "Jeremy claims that Susan was at the party, but since Jeremy is the kind of person who has to ride to work on the city bus, it must be false that she was there."

A circumstantial ad hominem accuses the person of having an alternative motive for defending the proposition or points out its inconsistency with the person's other views. Tu quoque (the "so do you" fallacy) uses a similar method in response to criticism of a position already held.

Recommended Reading: Douglas Walton, Ad Hominem Arguments (Alabama, 1998).

Also see FF and GLF.