I recently posted a piece on what a good thing the Argumentum ad hominem is, as used by Blessed John Henry Newman and Dom Gregory Dix. What I need to make clear is that 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".
In other words, 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.) 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 through the Parks with a friend to watch the Varsity Cricket Match, 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 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, shared by Dr D, poor silly old thing. So, to him, 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 holds dear 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 I that I commend Dr Newman's and Dr Dix's attachment to the Argumentum ad hominem.
In a little while I shall post on this blog in full an interesting example (from Dix) of a (Lockean) Argumentum ad hominem. Dix argues, implicitly, that classical 'orthodox' Reformation Protestants cannot logically deny 'Popish additions to the Faith' on the grounds that they are 'unscriptural', while they themselves accept the dogma of the Divinity of the Holy Spirit, which is equally - or in fact, more - 'unscriptural'.