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“”If the facts are against you, argue the law. If the law is against you, argue the facts. If both the facts and the law are against you, abuse the other side's attorney.
The phrase ad hominem argument (often called an ad hominem attack) comes from the Latin "to the person." It also sometimes applies to any argument that centres on emotive (specifically irrelevant emotions) rather than rational or logical appeal.
As most people use the phrase in recent times, an ad hominem argument occurs when one attacks the person making an argument rather than the argument itself. It is therefore a special case of the broader category of formal logical fallacies, the non sequitur, in which the conclusion urged, e.g. that the disputant is incorrect, does not follow from the premise asserted, e.g. that the disputant is a dick. Even if the ad hominem attack is true, that fact has no bearing on whether the disputant's argument is logically sound.
 How ad hominem works
Ad hominem arguments work via the halo effect, a 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 your opponent's arguments.
 Strict usage
An ad hominem argument has the basic form:
- A makes claim X
- There is something objectionable about A
- Therefore X is false
 Subtle uses
Often, ad hominem attacks are used subtly in order to influence the views of spectators. There are many forms of this, such as pointing out bad things they (the opponent) have done in the past in arguments about morality (the argument that the opponent does not practice what they preach is a special case called ad hominem tu quoque). While ad hominem attacks are used mostly in arguments about philosophy or morals, it can also be used in other debates, such as when people supporting evolution are said to be "justifying genocide and mass suicide," though this could also be referred to as a straw man fallacy.
Another case is the circumstantial ad hominem or appeal to motive where an opponent's argument is discarded on the basis that they have some motivation for making it; for example, that it is in a banker's best interests to say he has not stolen from his company's accounts, so obviously he has. All "well they would say that, wouldn't they?" arguments are based on this form of ad hominem.
 Blatant uses
Ad hominem attacks are hardly ever used plainly, and people who do are generally trolls who want to provoke people to fight.
 False positives
Ad hominem attacks are strictly fallacious when the attack has little or no bearing on the argument at hand. For example, dismissing a female scientist's opinion on a subject because she is a woman would be a fallacious ad hominem argument. Dismissing it for being incorrect and making a sexist comment at the same time would not be, though the fact that something is not a logical fallacy does not automatically make it an effective or desirable debating tactic.
Calling someone an idiot when you have explained the evidence five times and they still refuse to address it, or provide counterexamples, is not an ad hominem attack, but rather a valid logical conclusion based on their actions. Similarly, tacking an insult onto the end of any argument might be bad form, but it doesn't automatically make it an ad hominem. It's only an ad hominem if you say the other person must be wrong because they are an idiot - not the other way round. Creationists sometimes make the mistake of calling a personal insult an ad hominem attack when it is not intended to address the truth or falsity of the creationist's claim, but merely to denigrate the creationist. Likewise, creationists are known to mischaracterize logically sound arguments as ad hominem attacks in an attempt to obscure the soundness of the argument or their burden to respond by making an emotional appeal.
A criticism is also not an ad hominem argument if a person's merits are actually the topic of the argument. A habitual liar is not physically incapable of telling the truth, and therefore dismissing their claims entirely is not valid, but it is certainly not incorrect to weigh their testimony as less trustworthy than that of someone with a reputation for studious honesty if comparing contradictory claims by the two. In cases of testimony the goal is to establish which claim is more likely to be true, so the character of witnesses is a valid subject of discussion.
 Alternative meaning
The traditional meaning of the phrase is that an "argumentum ad hominem" is one tailored to appeal to the person with whom one is arguing rather than to impartial reason. This definition may be seen in Modern English Usage by H. H. Fowler, a book whose explanations of usage are often taken as highly authoritative, if not definitive. For example, if you wanted to convince someone that Costa Rica was a superior place to go on vacation and you knew that the person was a keen birdwatcher, you might point out that Costa Rica is full of interesting bird life.
Fowler also mentions the following types of argumenta:
... ad crumenam (purse), one touching the hearer's pocket
... ad baculum (stick) or baculimum, threat of force instead of argument;
... ad ignorantium, one depending for its effect on the hearer's not knowing something essential;
... ad populum, one pandering to popular passion;
... ad vericundiam (modesty), one to meet which requires the opponent to offend against decorum.
 See also
- Ad hoc
- Argumentum ad cellarium
- Argumentum ex culo
- Emotional appeal
- Non sequitur
- Red herring
- Shill gambit
- Mike Bara