People chant 'agumentum ad hominem' here as though it were some magical phrase of power which puts its utterer in the right on any occasion. It does not, and it is almost never used correctly.

The fallacy of argumentum ad hominem refers to a response attacking the person making the argument, by saying that he or he is unworthy to speak, or to be taken seriously, for reasons unrelated to the argument made. An example might be to say that since a person has gone bankrupt recently, his opinion that the United States should provide military assistance to Iraq cannot be taken seriously. The flaw cited, true or not, has no relevance to the subject. But if a person has expressed the opinion that stock in the Beefsteak Mine is an excellent buy at ten cents a share, pointing out that he filed for bankruptcy last month is not irrelevant to the point at hand, but speaks to the quality of the person's judgement on financial matters. Similarly, if a person states it as a fact that, say, the U.S. government is buying up great quantities of ammunition in order to drive up the price and so disarm the people, pointing out that earlier this person stated as a fact that the government has a machine that creates and directs tornadoes and uses it to destroy resistance to tyranny in the heartland is wholly appropriate, because it speaks to both judgement and pre-disposition, establishing the person is delusional and is in actual fact not worthy of being taken seriously. If someone predicts that, say, President Obama is going to resign in disgrace before the year is out to avoid impeachment, it is wholly legitimate to point out that earlier, this person predicted President Obama would be defeated in a landslide by Romney, thus establishing that this person's predictions are worth less than those of the average 'psychic friends' operator.

A leader or spokesman for Hamas being called to account for having circulated the classic blood libel, and having stating his aim is expulsion of all Jews from Israel whose ancestry traces to immigrants arriving there in the twentieth century, is wholly appropriate, and in no sense an argumentum ad hominem. It speaks directly to the worth of any statement he may subsequently make avowing an interest in any form of peaceful accommodation, or that he and his fellows are unfairly described as driven by hate. Viewed in the coldest light, it even calls into question whether the man is so gripped by delusion that it might be an error to consider him wholly sane. And of course, it would be quite easy to settle the matter when called to account for having made such statements: the man would need only to state that he does not stand by them, that he knows the blood libel is a despicable slander he regrets having circulated as truth, and that he knows he cannot expel nearly six million Jews from Israel, and does not desire to do so. That is all it would take, and if the man will not say that and move on, one is entitled to wonder why this is....