The idea-expression divide or 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. However, in the United States a patent is not restricted to a specific embodiment of an underlying idea. E.g., Harper & Row Publishers, Inc. v. Nation Enters., 471 U.S. 539, 556 (1985) ("[C]opyright'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); Mazer v. Stein, 347 U.S. 201, 217 (1954) ("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 incorrectly based on the notion that intellectual property laws confer proprietary rights in relation to general ideas and concepts per se. The concept originated in copyright law.

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 technical non-fiction work are patentable they may be the subject of various patent claims, whereas the fundamental underlying idea may not be.

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 infringement.