Following up on my previous post, I thought I'd mention another tactic irrational people use to avoid accepting evidence: asking for one piece of evidence that is impossible to obtain, and then claiming that the failure to present that evidence proves that they are right.
In regards to the theory of evolution, creationists and ID proponents use this tactic by asking scientists to "Show me a cat evolving into a dog." They ask this in the full knowledge that, for one thing, evolution happens at rates too slow in multicellular animals to show it happening in real time, and secondly, that the theory of evolution never claims that cats evolve into dogs in the first place. They avoid dealing with the mountains of evidence for the theory of evolution this way, sidestepping it all to claim victory.
Holocaust deniers do exactly the same thing. A classic Holocaust denial demand is: "Show me one order signed by Hitler ordering the Holocaust." When that order isn't produced, they claim victory: "Hitler didn't order it, then." In this way, they attempt to ignore all other evidence that indicates Hitler did, in fact, order the Holocaust, and the evidence of why no such order exists.
No such order exists because Hitler was protecting himself from association with the Holocaust. He never ordered it in writing because he didn't want a written record. Now, a critic would say, "Well, that's easy enough to say. You could say that about anything." Which is true. But this isn't just a theory: there is a vast amount of other evidence that indicates Hitler did protect himself this way, such as numerous letters and diaries in which those close to Hitler wrote down some of the orders Hitler gave them verbally in regard to the Holocaust, the many orders signed by Hitler that authorized things now known to be part of the Holocaust, and by the fact that Hitler used the same "code words" to discuss the Holocaust as those who carried it out: there is vast evidence that Hitler knew that "Final Solution" meant exterminating the Jews, that "police actions" meant shooting Jews, and so on. He never said, "Kill the Jews," but he did say, "Implement the Final Solution," which was the term they used for the Holocaust.
Taken separately, perhaps each of these various lines of evidence have weaknesses: though the evidence does indicate that Hitler knew what all those euphemisms meant, it's possible he didn't. But it's not possible that he didn't know what all those euphemisms meant, that he didn't know what any of the orders he signed to help implement the Holocaust meant, that all the people who separately recorded Hitler speaking about the Holocaust and getting orders about the Holocaust not only lied, but did so in a coordinated fashion, so that their recollections would all fit together in a consistent pattern. The evidence, as a whole, is too great to deny.
Unless you're a Holocaust denier. Then, you ignore all of that, in favor of repeating your demand for "one piece of paper" that "proves" Hitler ordered the Holocaust. Since you know there isn't one, you know no one will ever be able to produce it, and you can ignore all that other evidence and continue to believe what you wanted to believe in the first place.
Evidence doesn't matter to these people, and, as I noted before, when they request it, they are acting in bad faith, because they wouldn't care even if you gave it to them.