Erasmo points out, in effect, that I am using a priori reasoning to justify ignoring empirical evidence. This is itself a very unscientific thing to do. So I am the one who is being unscientific and not (necessarily) the conspiracy "theorist."
He is quite right that by implication I am saying that scientists and scholars are right to ignore the latest conspiracy theory, without even looking at the evidence for that theory. But I would add that a priori reasoning, of this sort, is not alien to science at all.
Suppose I am an astronomer, specializing in the study of planets. I am deluged with new findings about extrasolar planets, more than I can keep up with. One fine day I get a self-published monograph in the mail, detailing "new evidence" that planet earth is flat. You know what I am going to do. I'm going to toss the book, and without looking at the new evidence. Is this unscientific? Is this irrational?
I think the answer to both questions is "no." We have many different methods for determining the truth, and they vary widely in quality. The highest-quality procedures -- such as (in science) a controlled experiment or a (in the legal realm) a trial with elaborate rules of evidence -- are difficult and expensive, and consequently are reserved for resolving a special category of issues. There are, however, many questions we answer every day of our lives, and as rational beings we generally apply lower-quality truth-determining procedures to them. An astronomer who tosses a flat-earth treatise is acting, with barely a moment's conscious thought, on a lifetime of experience that tells him or her that the earth is not flat at all. This almost instantaneous reaction is a very very low-grade truth-determining procedure, admittedly. It is barely a procedure at all! But I would point out that it is not anti-empirical. It is a priori only in the sense that it does not collect any new evidence.
Now you've probably guessed where I am going with this analogy. I probably will not go out and investigate all the web sites that Erasmo recommends to me. (I also, though this will take a lot of will power on my part, am planning to never read another book on the Kennedy assassination.) There are a number of reasons for this, but one, and a sufficient one, is just this: these theories basically posit the existence of super-brilliant human beings with amazing superpowers, who can bring off wildly complicated, all-but-perfect deceptions. If I did believe that such beings are possible, I certainly would not think that they work for the federal government. And if I did believe that, I certainly, certainly would not identify them with the Bush administration. I admit that having such a thought and acting on it constitutes a very low-grade truth-determining procedure. Like I say, it barely qualifies as a procedure at all. But it does not consist in ignoring empirical evidence. To the contrary! If I were to waste a lot of time reading 9/11 conspiracist websites I would be ignoring empirical evidence: six decades of experience with my fellow-humans, which convinces me that this is just not the way the world works.
That is one pretty good reason for rejecting conspiracism, lock, stock and kaboodle, without looking at the details of each theory. Here is another: Conspiracism, like religion, is an obvious case of motivated belief. It's obvious why Christians believe they have immortal souls, and it has nothing to do with evidence. If the belief is true, then the world is the sort of world they want to live in. Therefore, they believe it. In a similar way, it is obvious that 9/11 conspiracists have reasons, political or religious or both, to believe that 9/11 was not brought off by nineteen hate-crazed religious wack-jobs from the Middle East. A world in which this is true is more their kind of world. Ergo, they believe in it. This, in fact, is painfully obvious to everyone but the conspiracists themselves. Conspiracism is a kind of thinking that, in general and as a kind of thinking, does not deserve to be taken seriously.
Mark Twain's defense of lying -- "The truth is the most valuable thing we have: let us economize it!" -- can also be applied to empiricism, in the sense of examining new empirical evidence. We have to economize it, and that means investing it on hypotheses that deserve that high-quality sort of attention. Conspiracism, I am afraid, just does not make the grade.