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I would also add one more reason to reject conspiracy theories in general: once you accept the premise of an all-powerful conspiracy, you lock yourself into a closed universe where supporting evidence proves the conspiracy, lack of evidence proves a coverup, and contrary evidence proves how pervasive the coverup really is. In other words, the only evidence that counts is that which supports the already-postulated conspiracy; everything that doesn't accomodate it has to be twisted or ignored to fit. Anyone with any comon sense can see that this is not a rational process, and therefore can't be trusted.