Evidence and Support
Ultimately the question of the legitimacy of a conspiracy theory comes down to evidence. It comes down to the question of whether or not the claim is supported by facts external to the belief of the claimant. Conspiracy theories are the products of a desire. A person wants an explanation then sets out to find anything which supports it to the exclusion of any kind of contradictory evidence. This is problematic in that the desire will have the effect of pushing what would normally be a tenuous link to being conclusive evidence. "...all the devils are here." Let's take my personal favorite conspiracy theory: the claim that William Shakespeare was a fraud. The evidence for this theory is presented like a checklist of things that we all kind of knew but no one pointed out to us. For instance, why do we study this guy's work in PhD programs while he, himself, had no education other than elementary school? More familiarly it's posted like this: how could the son of ...