Logical Fallacy #1 

Argumentum ad hominem-Argument Against the Person

Probably one of the most common logical errors in casual conversation, the ad hominem argument is one of the most intellectually lazy tactics imaginable, because it completely avoids the substance of the argument and instead attacks the opponent personally. In the simplest form of ad hominem argument, the speaker simply calls his opponent names ("stupid" is a popular choice).

Example:

Q: Why do you dislike Al Gore's ideas?

A: Because he's a dumbass.

The example is childish but surprisingly common. It does not answer the question in a substantive way. Note that even if the respondent later gives material reasons for disliking Al Gore or his ideas, the gratuitous use of the ad hominem attack detracts from the argument.

A more sophisticated  but nonetheless inappropriate use of argumentum ad hominem is to weave it into a substantive argument:

"At this point in the history of science, even a cursory examination of the fossil record, combined with an understanding of recent advances in virology and genetic sequencing should lend sufficient credence to the theory of evolution that only a self-deluded rube like my opponent could ignore it."

This statement was actually doing quite well until it got to the name-calling. It demeans the dignity of the argument as much as if the speaker had called his opponent a "mean old poopy-pants."

Ad hominem takes an insidious but no less fallacious turn when the disparaging comments about the opponent are true. It hardly strengthens one's position to point out that one's opponent is in fact a habitual jaywalker, chronic alcoholic or convicted felon; such people can still make cogent arguments. An irresponsible person, a weak person, or a criminal is still potentially capable of rational discourse. To avoid the ad hominem fallacy, attack the argument, not the opponent.