Idea-Expression Divide also called Idea-Expression Dichotomy is embodied in Section 102(b) of the copyright code. Idea-Expression Dichotomy denies protection to the ideas which underlie copyrightable works and only the original "expressions" contained in these works can actually receive copyright protection. The first amendment provides that "Congress shall pass no law abridging the freedom of speech."
The idea-expression dichotomy discharges copyright law's first amendment duties because the application of copyright protection to expressions, but not to ideas, serves to prohibit only speech that is constitutionally valueless. Intellectual property rights are generally designed keeping Idea-Expression Dichotomy in mind to protect the fixed expression of an idea rather than the fundamental idea itself.
There are particular ideas that can be expressed intelligibly only in one or a limited number of ways. Therefore even the expression in these circumstances is unprotected, or extremely limited to verbatim copying only. In the United States this is known as the merger doctrine, because the expression is considered to be inextricably merged with the idea.
Copyright may subsist in the work as a whole, in the particular story or characters involved, or in any artwork contained in the book, but generally not in the idea or genre of the story. The idea-expression divide which is also referred to as idea-expression dichotomy is a concept which explains the appropriate function of intellectual property laws, which are generally designed to protect the fixed expression or manifestation of an idea rather than the fundamental idea itself.
Copyright may not subsist in the idea of a man on an expedition, but may subsist in a book which follows that expedition. Similarly, if the methods or processes described in a technical non-fiction work are patentable they may be the subject of various patent claims, whereas the fundamental underlying idea may not be. An adventure novel provides an illustration of the concept.
Justice Hand once stated that illicit copying need not be word for word "else a plagiarist would escape by immaterial variations." Nichols v. Universal Pictures Corp., 45 F.2d 119, 121 (2d Cir. 1930).
"Unlike a patent, a copyright gives no exclusive right to the art disclosed; protection is given only to the expression of the idea, not the idea itself." Mazer v. Stein, 347 U.S. 201, 217 (1954) - Idea-Expression Dichotomy.
"The person who has clothed the idea in form, whether by means of a picture, a play
or a book" owns the copyright. Donoghue v. Allied Newspapers Limited (1938)
Copyright may not subsist in the idea of a man venturing out on a voyage, but may subsist in a particular story which follows.
"Reporting a person's fall from a bus: the first person to do so could not use the law of copyright to stop other people from announcing this fact." Latham CJ. in Victoria Park Racing and Recreation Grounds Company Limited v. Taylor (1937).