Poe's Law is an Internet axiom which states that it is difficult to distinguish extremism from satire of extremism on the Internet unless the author clearly indicates his/her intent. This notion is most frequently observed with highly polarized discussion topics, such as gender equality, religious or political fundamentalism and other social justice-related issues.
The earliest notion of such grey area was first made on December 2nd, 1983 by Jerry Schwarz, who posted a message on Usenet expressing his concerns about confusion during online interactions and how it may affect Usenet. He offered several suggestions for how to address these problems with online communication:
8. Avoid sarcasm and facetious remarks.
Without the voice inflection and body language of personal
communication these are easily misinterpreted. A sideways
smile, :-), has become widely accepted on the net as an
indication that "I'm only kidding". If you submit a satiric
item without this symbol, no matter how obvious the satire is to
you, do not be surprised if people take it seriously.
This observation was further elaborated more than two decades later on August 10th, 2005, in a Christian forum discussion about possible flaws in the theory of evolution by natural selection. In the thread, the original poster (OP) Heidi Carico, a well-known Creationist member on atheist / scientific discussion forums, attempted to dismiss evolution as a valid theory. After another user commented on the utility of using a winking smiley to indicate sarcasm, Nathan Poe reaffirmed this notion by saying:
POE'S LAW: Without a winking smiley or other blatant display of humor, it is uttrerly impossible to parody a Creationist in such a way that someone won't mistake for the genuine article.
The thread gathered 176 replies on 18 pages, and a total of more than 41,000 views as of October 3rd, 2011.
Similar to Murphy's Law, Poe's Law concerns internet debates, particularly regarding religion or politics. … In other words, No matter how bizzare, outrageous, or just plain idiotic a parody of a Fundamentalist may seem, there will always be someone who cannot tell that it is a parody, having seen similar REAL ideas from real religious/political Fundamentalists.
Another encyclopedic article was submitted to RationalWiki on September 1st, 2007. The article provided a list of examples in which Poe's Law could be observed, many of them relating to political, religious or philosophical discussions. Soon, similar entries defining the axiom followed on a number of reference websites including Wikipedia, TVTropes, Encyclopedia Dramatica and Conservapedia.
The adage has been often discussed in religion-oriented blogs In 2009, Poe's Law was used as a prime example of "[…] Group Polarization, and the Epistemology of Online Religious Discourse" in a university research paper by Scott F. Aikin and the concept was illustrated in a diagram titled Poe’s Law Of Discernmentalism on Don Jobson's wordpress blog. Poe's Law was also included in The Telegraph list of online laws and theories, behind Godwin's law but ahead of Rule 34. The single-topic blog Poeslaw.com was created in 2010.
The main corollary of Poe's Law refers to an opposite phenomenon where a fundamentalist sounds so unbelievable that rational people will assume it is a joke. According to Urban Dictionary:
It is impossible for an act of Fundamentalism to be made that someone won't mistake for a parody.
Another theory emerged after a Rational Wiki editor, The Lay Scientist, commented on Conservapedia's Modus Operandi concerning news editors in the Conservapedia talk page:
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?
Afterwards, an Urban Dictionary definition was submitted for Poe's Paradox on February 15th, 2009.
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).
 Poe's Law, Group Polarization, and the Epistemology of Online Religious Discourse – Western Kentucky University / 1-23-2009
 Internet rules and laws: the top 10, from Godwin to Poe – The Telegraph / 10-23-2009