"Ad hominem" is a term used in debate and law that refers to the practice of trying to invalidate a person's argument by attacking his or her personal failings or characteristics. It is a logical fallacy, or incorrect reasoning, that is often cited as a poor way to debate. Ad hominem arguments are quite common in law and politics, however, and they are an emotional plea rather than a logical one. This type of argument can hurt the arguer's credibility while simultaneously damaging his opponent's reliability. In politics, this type of argument is often known as negative campaigning.
Personal attacks of one's character or practices can be considered ad hominem attacks. The personal attacks very often have little or nothing to do with the issue being debated, and the attack is meant to discredit the person as a reliable source of information in general rather than as a source pertaining to the relevant issue at hand. An example of such an ad hominem attack follows:
"Bill shouldn't be able to have a say in planning gym activities because he used to be obese."
The insinuation that Bill used to be obese has little or nothing to do with his ability or inability to plan activities at the gym, but the ad hominem attack is meant to discredit Bill as a source in general and attack him on a personal level.
A circumstantial ad hominem attack assumes that a person will make a decision or behave a certain way based on that person's characteristics or one's perceptions of that person's characteristics. If, for example, a person says, "Jim would of course choose to go to the playground instead of to school" based on the fact that Jim prefers play over school is an ad hominem attack because it assumes Jim is incapable of making any other decisions. This type of argument is valid only if Jim has some sort of conflict of interest; for example, if Jim owned the playground and wanted people to come there instead of to school, it may be safely assumed that Jim would choose the playground because he has an interest in being there.
Other types of ad hominem arguments include guilt by association arguments and "tu quoque" arguments. Guilt by association assumes a person will behave a certain way because someone closely associated with that person has acted a certain way. Tu quoque arguments contend that a certain behavior one has been accused of is okay because another person has done it as well. Tu quoque arguments are sometimes known as "You Too" arguments.