Sturgeon's revelation, commonly referred to as Sturgeon's law, is an adage commonly cited as "ninety percent of everything is crap." It is derived from quotations by Theodore Sturgeon, an American science fiction author: while Sturgeon coined another adage that he termed "Sturgeon's law", it is his "revelation" that is usually referred to by that term.
The phrase was derived from Sturgeon's observation that while science fiction was often derided for its low quality by critics, it could be noted that the majority of examples of works in other fields could equally be seen to be of low quality and that science fiction was thus no different in that regard to other art.
The first written reference to the adage appears in the March 1958 issue of Venture, where Sturgeon wrote:
I repeat Sturgeon’s Revelation, which was wrung out of me after twenty years of wearying defense of science fiction against attacks of people who used the worst examples of the field for ammunition, and whose conclusion was that ninety percent of SF is crud. Using the same standards that categorize 90% of science fiction as trash, crud, or crap, it can be argued that 90% of film, literature, consumer goods, etc. are crap. In other words, the claim (or fact) that 90% of science fiction is crap is ultimately uninformative, because science fiction conforms to the same trends of quality as all other artforms.
According to Philip Klass (William Tenn), Sturgeon made this remark in about 1951, at a talk at New York University at which Tenn was present. The statement was subsequently included in a talk Sturgeon gave at a session of the World Science Fiction Convention in Philadelphia, held over the Labor Day weekend of 1953.
Sturgeon's Law and Sturgeon's Revelation 
Sturgeon had originally deemed Sturgeon's Law to be
Nothing is always absolutely so
in the story "The Claustrophile" in a 1956 issue of Galaxy. The second adage, variously rendered as "ninety percent of everything is crud" or "ninety percent of everything is crap", was originally known as "Sturgeon's Revelation", formulated as such in his book review column for Venture in 1957. However, almost all modern uses of the term "Sturgeon's Law" actually refer to the second, including the definition listed in the Oxford English Dictionary.
See also