The history of the ad hominem contains an ambiguity that has led to some confusion in textbook treatments. In the textbooks, that argument is mostly interpreted as a personal attack, as outlined above, [sic] But there is also a meaning of ad hominem which takes it to mean an argument ex concessis that uses an arguer’s commitments (previous concessions) to try to maker [sic] her follow a certain line. This meaning of ad hominem is found in Locke, Galileo, and Johnstone, but it can be traced back to Aristotle.
(D. N. Walton, "Poisoning the Well," Argumentation 20 [2006]: 273-307, at 285 [endnote omitted])
Note from KBJ: Argumentation, to be effective, must be ad hominem. That is to say, it must be directed to a particular person with particular beliefs. If my interlocutor doesn't accept my premises, then my argument gets no traction; and if it gets no traction, then what is its point? The point of argumentation is to show people the implications of what they already believe, so that, if they don't like the implications, they can make adjustments in their beliefs. I cannot dictate which adjustments my interlocutor makes. If I show you that you cannot believe both of two propositions, it is up to you, not me, which proposition you give up. Put formally, if I argue from 1 and 2 to 3, I am claiming that the set consisting of 1, 2, and ~3 is inconsistent. You can avoid the inconsistency by either denying 1, denying 2, or affirming 3. Which you do is up to you.
Keith Burgess-Jackson, J.D., Ph.D. 