What causes a Supreme Court justice to profoundly change his mind in a short period of time on a crucial issue of constitutional law? For almost a century, constitutional law scholars have puzzled over the apparent dramatic shift in Justice Oliver Wendell Holmes Jr.'s view of the government's ability to punish speech that it deems harmful to a war effort. Seton Hall law professor Thomas Healy, in a wonderful and engaging new book, offers a persuasive account of what happened. In early 1919, Justice Holmes authored three opinions that gave great deference to the government in punishing speech during wartime. In Schenck v. United States (249 U.S. 47 (1919)), the Court ruled that a man could be punished for circulating a leaflet arguing that the draft was unconstitutional. Holmes wrote that the government could punish such speech because there was a clear and present danger of harm, even though there was not a shred of evidence that Schenck's leaflets had the slightest effect on military recruitment. In Frohwerk v. United States (249 U.S. 204 (1919)), the Court affirmed the conviction of the editor of a German-language newspaper for articles criticizing the U.S. government's sending of troops to France during World War I. And in Debs v. United States (249 U.S. 211 (1919)), the Court upheld the conviction of Socialist Party leader Eugene V. Debs for giving a speech in which he told his audience that they were good for more than cannon fodder. In none of these opinions did Holmes or the Court express any sense of the importance of free speech and the First Amendment. But later that year, in Abrams v. United States (250 U.S. 616 (1919)), he wrote an eloquent dissent in which he spoke movingly about the First Amendment, in terms that are invoked to this day. The Abrams case involved individuals who were convicted for circulating leaflets in 1918 that urged the United States not to intervene or oppose the Bolshevik revolution in Russia. Even though the leaflets did not directly concern the war in Europe, the Court affirmed the convictions and rejected the First Amendment arguments of the defendants. Holmes dissented and sharply disagreed with the majority's allowing individuals to be punished for such speech without any evidence that it was intended to harm the war effort or that it would have any effect whatsoever. He explained that "the best test of truth is the power of the thought to get itself accepted in the competition of the market." In teaching these cases, I always point to the seeming inconsistency in Holmes's position and always have assumed that it was based on his seeing the facts of the cases differently. The speech at issue in Abrams had much less to do with the United States' war effort than with what was said in Schenck, Frohwerk, and Debs. But Healy offers a very different account. He tells of how a group of young men-federal district court Judge Learned Hand, Harvard University lecturer and intellectual Harold J. Laski, and Harvard law professor Zechariah Chafee Jr.-changed Holmes's mind about free speech through their personal interactions with the justice and their writings. In meticulous detail, Healy documents his original and persuasive thesis. At the same time, Healy paints a rich and compelling portrait of Holmes and the world in which these cases arose. Ultimately, The Great Dissent is about one man, Oliver Wendell Holmes, and what caused him to change his mind. But the book should be of interest to all who care about the First Amendment and constitutional law, because it tells the story of a dissenting opinion that is the foundation of much First Amendment analysis over the past century. More generally, the book is a wonderful exploration of how justices think and arrive at their positions. Erwin Chemerinsky is the dean and Raymond Pryke Professor of First Amendment Law at UC Irvine School of Law.