Argumentum Ad Hominem

Logical Penguins

I always knew argumentum ad hominem as a personal attack on a person (he stinks, his teeth are yellow, he beats his dog) as a means of distracting you from the argument he is making. Father John Hunwicke tells us of another meaning of the term that is probably beyond the ken of those who make these slanderous types of attacks.

Allow me to explain what [John] Locke and I mean by Argumentum ad hominem.

If a man says "Matilda never lies", and two paragraphs later he says "Matilda lied when she said X", then you are entitled to 'press' him with this. Logically, he must withdraw one of these two statements, or he is involved in a contradiction and cannot expect us to follow him. (He can, of course, also withdraw both.) Whether or not Matilda is truthful is, at least for the time being, irrelevant. It is the speaker's inconsistency which requires analysis.

If a man says "I believe A" and later says "I do not believe B because it is not explicitly in Scripture", he has handed you a hostage to fortune: if you can successfully demonstrate that A also is 'not explicitly in Scripture', then either he must preserve the 'principle' in his second statement "I do not believe B" by also discarding his belief in A; or, continuing to believe in A, he must cease to deny B on the grounds that it is not explicitly in Scripture (he might, of course, if he is quick on his feet, be able to think up a different reason for denying B and argue that; what he cannot maintain is his original and stated reason for denying it.)

This is called an argumentum ad hominem because, strictly speaking it does not prove anything at all; it only demonstrates that this particular man cannot maintain two contradictory or inconsistent statements. So it is "an Argument against [that particular] Man".

Via Monday Evening