The marketplace of ideas is a rationale for freedom of expression based on an analogy to the economic concept of a free market. The marketplace of ideas holds that the truth will emerge from the competition of ideas in free, transparent public discourse, and concludes that ideas and ideologies will be culled according to their superiority or inferiority and widespread acceptance among the population. The concept is often applied to discussions of patent law as well as freedom of the press and the responsibilities of the media in a liberal democracy.
The marketplace of ideas metaphor is founded in the philosophy of John Milton in his work "Areopagitica" in 1644 and also John Stuart Mill in his book, On Liberty in 1859 (although neither use the term "marketplace"). It was later used in opinions by the Supreme Court of the United States. The first reference to the "free trade in ideas" within "the competition of the market" appears in Justice Oliver Wendell Holmes, Jr.'s dissent in Abrams v. United States. The phrase "marketplace of ideas" first appears in a concurring opinion by Justice William O. Douglas in the Supreme Court decision United States v. Rumely in 1953: "Like the publishers of newspapers, magazines, or books, this publisher bids for the minds of men in the market place of ideas." The Court's 1969 decision in Brandenburg v. Ohio enshrined the marketplace of ideas as the dominant public policy in American free speech law (that is, against which narrow exceptions to freedom of speech must be justified by specific countervailing public policies). It has not been seriously questioned since in U.S. jurisprudence.
The general idea that free speech should be tolerated because it will lead toward the truth has a long history. The English poet John Milton suggested that restricting speech was not necessary because "in a free and open encounter", truth would prevail. U.S. President Thomas Jefferson argued that it is safe to tolerate "error of opinion ... where reason is left free to combat it". Fredrick Siebert echoed the idea that free expression is self-correcting in Four Theories of the Press: "Let all with something to say be free to express themselves. The true and sound will survive. The false and unsound will be vanquished. Government should keep out of the battle and not weigh the odds in favor of one side or the other." These writers did not rely on the economic analogy to a market.
If beliefs such as religions are considered as ideas, the marketplace of ideas concept favors a marketplace of religions rather than forcing a state religion or forbidding incompatible beliefs. In this sense, it provides a rationale for freedom of religion.
In recent years questions have arisen regarding the existence of markets in ideas. Several scholars have noted differences between the way ideas are produced and consumed and the way more traditional goods are produced and consumed. It has also been argued that the idea of the marketplace of ideas as applied to religion "incorrectly assumes a level playing field" among religions. In addition, the idea of a marketplace of ideas has been applied to the study of scientific research as a social institution.