The idea-expression divide or idea-expression dichotomy is a concept which explains the appropriate function of copyright laws, which are generally designed to protect the fixed expression or manifestation of an idea rather than the fundamental idea itself. The case of Baker v. Selden was the first U.S. Supreme Court case to fully explain this doctrine, holding that exclusive rights to the "useful art" (in this case bookkeeping) described in a book was only available by patent; the description itself was protectable by copyright. In Harper & Row Publishers, Inc. v. Nation Enters., 471 U.S. 539, 556 (1985), the Supreme Court stated that "copyright's idea/expression dichotomy 'strike[s] a definitional balance between the First Amendment and the Copyright Act by permitting free communication of facts while still protecting an author's expression.'" (internal citation omitted). Additionally, in Mazer v. Stein, 347 U.S. 201, 217 (1954), the Supreme Court stated "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."

Some of the criticism directed at "intellectual property" is based on the confusion between patents, which may confer proprietary rights in relation to general ideas and concepts per se when construed as methods, and copyrights, which cannot confer such rights.

An adventure novel provides an illustration of the concept. 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. Copyright therefore may not subsist in the idea of a man venturing out on a quest, but may subsist in a particular story which follows that pattern. Similarly, if the methods or processes described in a work are patentable, they may be the subject of various patent claims, which may or may not be broad enough to cover other methods or processes based on the same idea. Arthur C. Clarke, for example, sufficiently described the concept of a communications satellite (a geostationary satellite used as a telecommunications relay) in a 1945 paper that it was not considered patentable in 1954 when it was developed (independently) at Bell Labs.

In the English decision of Donoghue v. Allied Newspapers Limited (1938) Ch 106, the court illustrated the concept by stating that "the person who has clothed the idea in form, whether by means of a picture, a play or a book" owns the copyright. In the Australian decision of Victoria Park Racing and Recreation Grounds Company Limited v. Taylor (1937) 58 CLR 479 at 498, Latham CJ used the analogy of 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.

Some courts have recognized that 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. United States courts are divided on whether merger constitutes a defense to infringement or prevents copyrightability in the first place, but it is often pleaded as an affirmative defense to copyright infringement.

See also

• Feist Publications v. Rural Telephone Service
• Functionality doctrine
• Scenes à faire