So we need to lay down some terms for these conspiracy theories, that enable people to get them right (lets say we want to start a few bogus ones and see how they run, for example:-)
Rule 1. Lack of evidence proves that there is a cover up.
This is an essential rule of the conspiracy theorist - its part and parcel of the next rule:
Rule 2. A conspiracy theorist should strive to make their story unfalsifiable.
Falsifiable theories are for Popperians seeking objective knowledge, not for the hunter for objectionable ideas.
Rule 3. Ideally, a conspiracy theory should have great (in the sense of broad) explanatory power or applicability - it ought, at one fell swoop, to encompass several problematical domains (missing aircraft, beleaguered news readers, confused celebs). Conspiracy theories make the inexplicably complicated, suddenly comprehensible to a complete dolt.
Rule 4. Any decent conspiracy theory resonates with some fundamental cultural meme, especially one that engenders fear, uncertainty and doublets. Hence myths from eldritch times, alien technology, hidden rules of numbers and the odd way some people look at you when you talk about this in the context of UKIP's chances at ruling Scotland, are essential.
Rule 5. There is a cover up, which means that there are coverer uppers. There is some group (Illuminati, Opus Dei, the French, the Arms/Drugs/Car manufacturers, UKIP) who know something, and are not telling us.
Rule 6. A good conspiracy theory exploits the principle of maximum astonishment (see also Rule 3). On the other hand, a conspiracy theory might sound occasionally plausible for a moment - for example, the fact that mice, cats, dogs and horses are just different stages in the growth of the same creature, or that the names of passengers on flight MH370 were the same as the travellers on the Marie Celeste, or that 3D printers contain secret scanners to make sure you don't ever copy people, especially not members of UKIP.
