Essentially, the ad hominem fallacy is attacking an argument by making some assertion about its source. Trollish commenters on this blog often make the claim that an argument that supports some liberal position is not to be credited because the person making it is a liberal. Really -- I just got not one but two comments making precisely that claim about my quotation from a spokesperson from the Union of Concerned Scientists on the previous post. Since I have a policy against publishing completely idiotic comments, that one did not get through. I know perfectly well that UCS is an advocacy organization dedicated to the accurate application of science to solve social problems. This often puts UCS in opposition to corporate interests and for sure, right now, to conservatives who deny inconvenient scientific truths such as anthropogenic climate change and the health consequences of air pollution. While not every supporter is a scientist, the individual I quoted is indeed one. I am also a real scientist, and I have personally been involved in studies of the health effects of air pollution.
The commenter in this case makes what is called the circumstantial fallacy. Quoting Wikipedia, the circumstantial fallacy "points out that someone is in circumstances such that they are disposed to take a particular position. It constitutes an attack on the bias of a source. This is fallacious because a disposition to make a certain argument does not make the argument invalid; this overlaps with the genetic fallacy (an argument that a claim is incorrect due to its source)." Virtually everyone who makes an argument has some sort of predisposition. If the circumstantial fallacy were permitted, argumentation would be impossible.
Bulverism is basically another way of stating this. The term was coined by C.S. Lewis who wrote:
You must show that a man is wrong before you start explaining why he is wrong. The modern method is to assume without discussion that he is wrong and then distract his attention from this (the only real issue) by busily explaining how he became so silly.
In the course of the last fifteen years I have found this vice so common that I have had to invent a name for it. I call it "Bulverism". Some day I am going to write the biography of its imaginary inventor, Ezekiel Bulver, whose destiny was determined at the age of five when he heard his mother say to his father—who had been maintaining that two sides of a triangle were together greater than a third—"Oh you say that because you are a man." "At that moment", E. Bulver assures us, "there flashed across my opening mind the great truth that refutation is no necessary part of argument. Assume that your opponent is wrong, and explain his error, and the world will be at your feet. Attempt to prove that he is wrong or (worse still) try to find out whether he is wrong or right, and the national dynamism of our age will thrust you to the wall." That is how Bulver became one of the makers of the Twentieth Century.
Update: There is a degree of doofosity (dufusitude?) that I find absolutely astonishing. I do not dismiss and argument based on who made it, but neither do I accept it on that basis if I am able to evaluate it based on its merits. In the case of the science-denying efforts of the Dump administration, I have the necessary personal expertise to know that they will kill people, and I quote approvingly someone who puts this succinctly and well.
As a generalization, we all have limited expertise. I have to take the word of physicists as authoritative about the age of the universe, the field theories of particle interaction and the standard model, and so forth. I am not a climate scientist but I understand the basic concepts very well and I am also able to see that there is not just one authority here but a multitude, who overwhelmingly agree about the basic issues. Therefore deniers must be motivated, and they have no good arguments as far as I can see. I do not take the Union of Concerned Scientists as an authority of factual scientific issues because that is not what they purport to be. They are advocates for the appropriate acceptance and use of science in public policy. They rely on the appropriate scientific authorities, who for the most part are not their members, to determine what is scientifically factual.
Our problem is that we are giving high school and college diplomas to people who have not learned critical thinking skills.