As far as problems of logic in conspiracy theory are concerned, I see the following two (which are related):
1. Argument from Ignorance/Appeal to Ignorance (argumentum ad ignorantiam) -- a fallacy
E.g., "Since it has not been proven that the U.S. government did not itself commit the 911 attacks or was not complicit in them, then the U.S. government must have."
Note: The converse of the above ("Since it has not been proven that the U.S. government did commit the 911 attacks or was complicit in them, then the U.S. government must not have") is also a case of argument from ignorance.
2. Shifting the Burden of Proof
In general, it is up to the person making an argument (or an accusation for that matter) to prove it. In the case of the 911 attacks, it is up to the person making an argument (or rather an accusation) that "the U.S. government itself committed the 911 attacks or was complicit in them" to prove it. The burden of proof does not fall upon the U.S. government (in other words, it is not its obligation to provide proof that it didn't commit the criminal acts); nor does it fall on the skeptical audience. However, a conspiracy theorist illicitly shifts the burden of proof, making the accused (in this case the U.S. government) -- and the skeptical audience -- responsible for proving that the accusation is untrue. "Let them deny it if they can" is a slogan of witch-hunters (e.g., Senator Joseph McCarthy: "I do not have much information on this except the general statement of the agency that there is nothing in the files to disprove his Communist connections") & doesn't belong in political discourse of the Left. -- Yoshie
* Calendar of Anti-War Events in Columbus: <http://www.osu.edu/students/sif/calendar.html> * Anti-War Activist Resources: <http://www.osu.edu/students/sif/activist.html> * Anti-War Organizing in Columbus Covered by the Media: <http://www.osu.edu/students/sif/media.html>