The idea–expression distinction or idea–expression dichotomy is a legal doctrine in the United States that limits the scope of copyright protection by differentiating an idea from the expression or manifestation of that idea.

Unlike patents, which may confer proprietary rights in relation to general ideas and concepts per se when construed as methods, copyrights 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 that 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 was not considered patentable in 1954 when it was developed at Bell Labs.

Legal origins and status.

Philosophically, there is disagreement about the distinction between thought and language. In the past it was often thought that the two could not be separated, and so a paraphrase could never exactly reproduce a thought expressed in different words. At the opposite extreme is the view that concepts and language are completely independent, so there is always a range of ways in which a concept can be expressed.

In the United States, the doctrine originated from the 1879 Supreme Court case of Baker v Selden. The Supreme Court held in Selden that, while exclusive rights to the "useful arts" (in this case bookkeeping) described in a book might be available by patent, only the description itself was protectable by copyright. In later cases, the Supreme Court has stated that "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," and that "copyright's idea or expression dichotomy 'strike a definitional balance between the First Amendment and the Copyright Act by permitting free communication of facts while still protecting an author's expression.'"

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), 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.

Today, Article 1.2 of the European Union Software Directive expressly excludes from copyright ideas and principles that underlie any element of a computer program, including those that underlie its interfaces. As stated by the European Court of Justice in SAS Institute Inc v World Programming Limited, "to accept that the functionality of a computer program can be protected by copyright would amount to making it possible to monopolize ideas, to the detriment of technological progress and industrial development."

Scènes à faire.

Some courts have recognized that particular ideas can be expressed effectively only by using certain elements or background. The French name for this doctrine is Scènes à faire. Therefore, even the expression in these circumstances is unprotected, or extremely limited to verbatim copying only. This is true in the United Kingdom and most Commonwealth countries.