Dianetics: The Evolution of a Science is a book written by L. Ron Hubbard. Originally published in May 1950 as an article in Astounding Science Fiction, and immediately preceding the publication of his book Dianetics: The Modern Science of Mental Health, it was expanded and republished as a 48-page book in 1955 by Hubbard Association of Scientologists International Ltd. In 2007, it was republished by Bridge Publications as a 213-page book — part of the re-release of the "basic books" of Scientology. The book is considered part of Scientology's canon. In Dianetics: The Evolution of a Science, Hubbard describes how he defined the reactive mind and developed the procedures to get rid of it. The book includes Hubbard's account of the reasoning behind his development of Dianetics.