One of the most fundamental principles of reasoning and investigation is what has come to be known as Occam's Razor. Named after the 14th century logician William of Occam, it is the principle which favors the least complicated of two or more possible explanations for an observation. Needless to say, most conspiracy theories don't satisfy this rule.

In practice, Occam's Razor is used to cut away elements of theories which cannot be observed. For example, Einstein described space-time in the special theory of relativity. Lorentz had theorized that space-time fluctuations are caused by motion through the "ether". However, Lorentz's ether cannot be observed even though his equations produce the same results as Einstein's, so it represents an unnecessarily complicated model. It doesn't prove Einstein right and Lorentz wrong, but because there's a whole lot less baggage to Einstein's model, it's more likely to be correct given the current set of observations.

Conspiracy theories generally entail the opposite of Occam's Razor. That is, when explaining observations, the conspirators often propose more complicated explanations than the commonly believed story. Their conclusions often require us to believe in additional postulated events or factors for which there is seldom any direct proof. Occam's Razor clearly requires us to eliminate candidate explanations which imply the existence of unobserved phenomenon.

Both NASA and the conspiracists offer explanations which fit the observable phenomena. But some Apollo conspiracy theories require us to believe in things like NASA death squads and top-secret soundstages in remote locations. There is no direct evidence for either of those. The possibility that these things -- if they existed -- might explain the conspiracists' observations is not proof that those things exist.

On a grander scale, conspiracists often have an elaborate explanation for one photograph or statement and another completely different but equally elaborate explanation for the next photo and so on. Soon these piecemeal propositions start contradicting each other. And you get different explanations depending on which conspiracist you ask.

It's not suspicious that different conspiracists have different ideas. That's how investigation works. But it is a big deal when one conspiracist's theory, taken as a whole, propounds into a looming mass of unfounded speculation. Instead of the typical process of looking at all the possibilities and deciding which of them best makes sense, conspiracists generally follow a line of reasoning which first demands that the conspiracy exist. They then follow whatever tortured path of conjecture is necessary to arrive at that conclusion.

The resulting line of reasoning may appear airtight. The reader can follow the argument from first principles to conclusion. But the reader often fails to ask whether that line of reasoning really is the only possible one, and whether the conspiracist's argument requires the reader to believe in extraneous propositions for which there is no evidence.