Poe's law:
Poe's Law points out that it is hard to tell parodies of fundamentalism (or, more generally, any crackpot theory) from the real thing, since they both seem equally insane. Conversely, real fundamentalism can easily be mistaken for a parody of fundamentalism.
Originally the law only made the claim that someone will mistake a parody of fundamentalism for the real thing - that if someone made a sarcastic comment stating that evolution was a hoax because "birds don't give birth to monkeys" then there would be a high probability that someone, later in the discussion, would miss the joke and explain (in all seriousness) how the poster was an idiot. However, the usage of the law has grown to include three similar but different concepts:
1. The original idea that at least one person will mistake parody postings for sincere beliefs.
2. That nobody will be able to distinguish many instances of parody posts from the real thing.
3. That anybody, not already in the grip of fundamentalist ideas, will mistake sincere expressions of fundamentalism for parody.
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Poe's Paradox:
The "Poe paradox" is a corollary to Poe's Law. It states that:
“ In any fundamentalist group where Poe's Law applies, a paradox exists where any new person (or idea) sufficiently fundamentalist to be accepted by the group, is likely to be so ridiculous that they risk being rejected as a parodist (or parody). ”
History: The term was first used by RationalWiki editor The Lay Scientist to describe an apparent paradox in the management of editing rights at Conservapedia:
"Any new member of the CP project who's not as Conservative as them is liable to be chucked out. However, any new member who is as Conservative as them is in serious danger of being called a parodist, and chucked out. Is this the first living example of a Poe Paradox?"
More:
http://rationalwiki.org/wiki/Poe%27s_Law