Final answer:
Supreme Court Justice Hugo Black and Justice William J. Brennan, Jr. are figures closely associated with the idea that the First Amendment protects the 'market place of ideas,' emphasizing the importance of free and open discourse for a functioning democracy.
Step-by-step explanation:
The figure most closely associated with the idea that the First Amendment protects the "market place of ideas" is U.S. Supreme Court Justice Hugo Black. He championed the concept that to have a successful representative democracy, a variety of ideas must be allowed to compete freely, so that truth and societal progress may emerge from robust public discourse. This principle is intertwined with the framers' design to curtail tyranny and stimulate change by protecting freedoms essential to democracy - freedoms of religion, speech, press, assembly, and petition.
Justice William J. Brennan, Jr. further solidified this concept in the landmark ruling of New York Times Co. v. Sullivan (1964), where he emphasized that the First Amendment serves as a fortress for "uninhibited, robust, and wide-open" discussion, even of controversial issues. In essence, it is the dissenting voices and unpopular views that are most in need of constitutional shelter, as indicated by the court's stance in cases like Gitlow v. New York where the scope of the First Amendment was extended to state governments through the Fourteenth Amendment.