I'd like to present this as a classic case study to investigate what lies behind the question in the OP. Here is a poster who believes that something about 9-11 hasn't been proved beyond reasonable doubt. When asked to say what it is, he replies by assuming his original conclusion, sarcastically stating a prejudice about the other side in the argument, and advancing a classic, if unusually unfounded, ad hominem fallacy. Clearly he isn't interested in learning anything, because he assumes he already knows everything he needs to, so anyone who disagrees with him is ignorant and wrong. This is a position that can only be arrived at from a combination of poor education and arrogance. Whatever the specific piece of misinformation that triggered his particular set of beliefs, the roots are much deeper.
Dave