In reality, ad hominem is unrelated to sarcasm or personal abuse. Argumentum
ad hominem is the logical fallacy of attempting to undermine a speaker's
argument by attacking the speaker instead of addressing the argument. The mere
presence of a personal attack does not indicate ad hominem: the attack must be
used for the purpose of undermining the argument, or otherwise the logical fallacy
isn't there.
[...]
A: "All rodents are mammals, but a weasel isn't a rodent, so it can't be
a mammal."
B: "You evidently know nothing about logic. This does not logically follow."
B's argument is still not ad hominem. B does not imply that A's
sentence does not logically follow because A knows nothing about logic. B
is still addressing the substance of A's argument...
This is too beautiful, thorough, and precise to not post. HT to sfk on HN.
Incidentally, I think one of Bond's "real life examples" exposes an important ambiguity:
A: "I can even handle misplaced apostrophes every now and then. Not excessive amounts of them, [...]"
B: "Perhaps double-check your grammar before you write a grammar rant that refers to 'amounts of apostrophes'."
C: " ...the ad hominem nature of [B's reply] takes the sanctimonious angle that any who criticize must be without stain."
Bond writes, "B's reply was not ad hominem. It was not a counter-argument to A, but an attempt to point out what B saw as A's hypocrisy."
But actually it is ad hominem, i.e. directed 'to the person', though not (of course) an ad hominem fallacy in the usual sense. See: Ad hominem tu quoque.
Stephen Bond writes the definitive word on ad hominem in "the ad hominem fallacy fallacy":
This is too beautiful, thorough, and precise to not post. HT to sfk on HN.