I really like teaching this section. The text I use is excellent on, e.g., my favorite all-time informal logical fallacy: the genetic fallacy. What a cool fallacy! What a great example of ill-logic (yes, two 'l's).
Ad hominem fallacies abound on the internet. Facebook is a breeding ground for them; log in and watch them multiply like bunnies. Ad hominem arguments have irrelevant premises. As I was teaching this tonight I thought of an ad hominem argument, which went like this:
3. Therefore, (Belief X) is stupid. (More bwa-ha-ha-ha-ing...)
My class seemed to enjoy this form of irrationality. So I present it now to you. Premises 1 and 2 are irrelevant to the truth of Belief X (whatever X signifies). Hopefully my students understood this part!