We continually see examples of people using the phrase Argumentum ad hominem with the following meaning: "Arguing against somebody by simply hurling personal abuse at him".
In all linguistic matters, I cannot deny that usage can change the meanings of words and phrases. But I think it is a tremendous shame that this immensely useful phrase is in danger of losing the totally different meaning it originally had. I want to make clear that, on this blog, I use the phrase in the original sense in which it was used by Locke: "to press a man with Consequences drawn from his own Principles and Concessions".
As you might expect, an example, nearly contemporary with Locke himself, is found in the 1738 chapter of Let Dons Delight.
So ... to be boringly clear about ths ... I have not used, do not and will not use, argumentum ad hominem in the sense of 'personal abuse'. Permit me to be even more explicit, and even nastier: I think this new meaning now very widely attached to the phrase originated in a common human failing. And that failing is: trying to sound erudite by using a piece of Latin ... often, as in this case, misunderstood Latin.
Allow me to explain what 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".
For example, if you are strolling into the Parks with a friend to watch the University playing Barsetshire, and he says "I don't believe Mary is the Mother of God", you can ask him if he believes that Jesus Christ is 'fully' God. If he replies "Yes I do", you can then say "So Mary, who is the Mother of Jesus Christ, is the Mother of God".
Let us suppose that your friend says "Ah yes! I do see! She is Mother of God in the the sense that she is the Mother of the one hypostasis of the Incarnate Second person of the Glorious Trinity. Fair enough". You and he can then warmly embrace; an ecumenical advance has been made. But Stay!!!: behind that substantial Rhododendron Bush was lurking Dr Dawkins of this University, beautifully coiffured as ever, eavesdropping upon your dialogue. He now leaps through the leaves and blossoms screaming maniacally "But there is no God at all and so Jesus Christ is not God and his Mother is not the Mother of God because there isn't a God for her to be Mother of because there isn't a God". Premises which were common ground between your interlocutor and yourself are not, you gather, intuitively shared by Dr D, poor silly old thing. So, to him, to Dr Dawkins, you have proved absolutely and totally nothing.
Indeed, your argument was not intended to convince someone of Dr Dawkins' limited understanding; you were only endeavouring to convince your friend, by appealing to his beliefs, to his 'Principles', to what he already deems true and has asserted. Your argument was designed to persuade him by presenting him with the dilemma: "Either withdraw your belief that Jesus Christ is God; or withdraw your statement that Mary is not the Mother of God".
It is in this old Lockean sense of the phrase that I commend Dr Newman's and Dr Dix's attachment ... and my own ... to the Argumentum ad hominem.
To be concluded.