What is an ad hominem fallacy?
by Edward Feser
As students of logic know, not every appeal to authority is a fallacious appeal to authority. A fallacy is committed only when the purported authority appealed to either does not in fact possess expertise on the subject at hand, or can reasonably be supposed to be less than objective. Hence if you believed that PCs are better than Macs entirely on the say-so of either your technophobic orthodontist or the local PC dealer who has some overstock to get rid of, you would be committing a fallacy of appeal to authority — in the first case because your orthodontist, smart guy though he is, presumably hasn’t much knowledge of computers, in the second case because while the salesman might have such knowledge, there is reasonable doubt about whether he is giving you an unbiased opinion. But if you believed that PCs are better than Macs because your computer science professor told you so, there would be no fallacy, because he presumably both has expertise on the matter and lacks any special reason to push PCs on you. (That doesn’t necessarily mean he’d be correct, of course; an argument can be mistaken even if it is non-fallacious.)
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Similarly, not every ad hominem attack — an attack “against the man” or person — involves a fallacious ad hominem. “Attacking the man” can be entirely legitimate and sometimes even called for, even in an argumentative context, when it is precisely the man himself who is the problem.
Attacking a person involves a fallacy when what is at issue is whether some claim the person is making is true or some argument he is giving is cogent, and where the attacker either (a) essentially ignores the question of whether the claim is true or the argument cogent, and instead just attacks the person giving it (in which case we have a kind of red herring fallacy) or (b) suggests either explicitly or implicitly that the claim can be rejected false or the argument rejected as not cogent on the basis of some irrelevant purported fault of the person giving it (in which case we have a poisoning the well fallacy, or perhaps a tu quoque).
Hence, suppose you put forward an argument against “same-sex marriage” and someone responds either by calling you a bigot, or by suggesting that the only reason you are putting forward such an argument is to rationalize some religiously motivated prejudice. Here we have classic examples of ad hominem fallacies…
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