clarity through the clouds of communication

“brevis esse laboro, obscurus fio.
(when i labor to be brief, i become obscure.)
quintus horatius flaccus (from horace’s ars poetica c. 18 bc)

“homo sum, humani nihil a me alienum puto.”
(i am human, so nothing that is human is foreign to me.)
publius terentius afer "terence”

argumentum ad hominem “ad hominem”
is an attempt to negate the truth of a claim by pointing out a negative characteristic or belief of the person supporting it. often described as a “logical fallacy” it is not always fallacious; in some instances, questions of personal conduct, character, motives, etc., are legitimate and relevant to the issue.

the philosopher charles taylor has argued that ad hominem reasoning is essential to understanding certain moral issues, and contrasts this sort of reasoning with the apodictic reasoning of philosophical naturalism.

halo effect
ad hominem arguments work via the halo effect, a human cognitive bias in which the perception of one trait is influenced by the perception of an unrelated trait, e.g. treating an attractive person as more intelligent or more honest. people tend to see others as tending to all good or tending to all bad. thus, if you can attribute a bad trait to your opponent, others will tend to doubt the quality of their arguments, even if the bad trait is irrelevant to the arguments.

see
http://en.wikipedia.org/wiki/Ad_hominem

and
http://en.wikipedia.org/wiki/Halo_effect

(wikipedia and internet triangulated sources)