The central problem with Chicago's ordinance is that it describes permissible picketing in terms of its subject matter. Peaceful picketing on the subject of a school's labor-management dispute is permitted, but all other peaceful picketing is prohibited. The operative distinction is the message on a picket sign. But, above all else, the First Amendment means that government has no power to restrict expression because of its message, its ideas, its subject matter, or its content. [Citations.] To permit the continued building of our politics and culture, and to assure self-fulfillment for each individual, our people are guaranteed the right to express any thought, free from government censorship. The essence of this forbidden censorship is content control. Any restriction on expressive activity because of its content would completely undercut the 'profound national commitment to the principle that debate on public issues should be uninhibited, robust, and wide-open. New York Times Co. v. Sullivan, supra, at 376 U. S. 270.