Attacking the character of the person with whom you are arguing rather than finding fault with his or her argument (eg, "I loathe that elder, so will ignore his counsel to me, even though it's straight from the bible". eg, 'That ex-JW is so bitter and just wants to bad-mouth the Organisation, so I will dismiss out of hand anything they have to say' . eg, 'the Organisation has failed to adequately protect children against child molesters, therefore I reject all their teachings'. eg 'Those Silentlambs people are just apostates, so I don't believe anything they say'. ). The move is traditionally known as arguing ad hominem (from the Latin for 'to the person'.) Getting personal is, in most parts, a technique of rhetoric, since discrediting the source of an argument usually leaves the argument itself intact.
For instance, if a politician argued that lowering the speed limit in built-up areas would reduce accidents involving children, and a journalist attacked this on the grounds that the politician had been fined for drunken driving and speeding on several occasions, this would be a case of getting personal. The question of whether or not the politician is a safe driver is irrelevant to the question of whether lowering the speed limit in built-up areas will reduce accidents. The politician's claim is best assessed by examining the evidence in support of the conclusion. The journalist deflected attention from the argument under consideration towards the alleged hypocrisy of the person who put the argument forward But it is clear that hypocrites can put forward excellent arguments: many do.
Take another example. If a member of an appointments committee makes a very strong case for a particular applicant being given the job, and it is subsequently learnt that this applicant had been having an affair with him at the time, then this fact might be taken to undermine the case made to the appointments committee. The member of the appointments committee had a vested interest in seeing that particular candidate succeed. However, the nature of the personal relationship in no way destroys the force of the case. If good reasons were given for employing this person above other candidates, then they remain good reasons. What would probably be unfair in such a situation is that the other candidates would not have had such a motivated advocate working on their behalf. If there was prejudice in favour of this particular candidate then getting personal about the appointer's involvement would be appropriate.
Obviously if an argument involves our taking factual premises on trust, then it would be appropriate to point out that the arguer is a compulsive liar, if this is so. In this sort of case getting personal is focused on a relevant aspect of the arguer's character and so is an acceptable move to make. However, in most cases getting personal focuses on irrelevant aspects of character, thereby deflecting attention from the arguments given.
(taken from 'Thinking from A to Z' by Nigel Warburton)