Oliver Wendell Holmes is known today for many things: his decades of service on the Supreme Court, his discerning analogies (fire in a crowded theater, anyone?), his magnificent mustache (rivaling that of the almighty Salovey). But his greatest legacy transcends these popular images, found instead in his trailblazing defense of free speech.

He wasn’t always such a civil libertarian. In fact, for the first 78 years of his remarkable life, Holmes was of a decidedly authoritarian bent. He trusted the government and granted it broad powers in a way that, today, would seem absurd. Could a policeman be fired for merely expressing his political views? Yup. Could the government imprison someone for expressing these political beliefs? Yeah, probably. Was there a right to public expression of controversial opinions, beliefs, or profanity? Nope. Could the government suppress the writings of someone trying to criticize it? Absolutely.

So what happened? Well, Holmes changed his mind. As law professor Thomas Healy describes in “The Great Dissent: How Oliver Wendell Holmes Changed His Mind — and Changed the History of Free Speech in America,” a series of chance meetings and the relentless arguing of his friends convinced Holmes that he had been wrong. So in 1919, in Abrams v. United States, Holmes wrote a dissenting opinion that changed the course of American (and perhaps world) history.

Before Holmes’s dissent in Abrams, which would soon be adopted by a majority of the Court and become firmly ingrained in the American ethos, the right to free speech was narrow, failing to include the right to dissent that we as Americans pride today. In 1915, Holmes himself wrote a unanimous decision that upheld the conviction of a newspaper owner who wrote in support of nude swimming in areas far from the public view — scandalous stuff.

Then, in 1918, a young Russian-American anarchist named Jacob Abrams, along with four others, tossed leaflets criticizing America’s involvement in World War I from the roof of a New York office building. Abrams was arrested, tried, and sentenced to 20 years in prison for bringing the government into “contempt, scorn, contumely, and disrepute” and for attempting to hinder the war effort.

Meanwhile, Holmes was on his way to his New England summer home when he happened upon a young New York judge with the delightful and erudite name of Learned Hand. Hand, nervously, confronted Holmes about the Abrams case (and a few others) and raised the idea that the defendants’ speech might be covered by the First Amendment. Holmes quickly dismissed Hand and returned to his seat.

But, according to Healy, that chance encounter spurred Holmes to begin talking to many of his friends about Hand’s then radical idea that controversial speech should be protected. These friends pushed back against Holmes’s conception of free speech, and urged him to consider it from the perspective of the minority. A year of lobbying and cajoling eventually paid off when Holmes published his dissent in Abrams.

The friends who changed Holmes’s mind were all bright young men nearly 50 years Holmes’s junior. The childless Holmes maintained intimate friendships — almost erotic in their closeness — with several of the brightest young intellectuals of the day, including radical Harvard professors Felix Frankfurter (later on the Supreme Court) and Harold Laski, Rhode Island brahman Zechariah Chafee, and Hand (who would later become one of the most prominent lower court judges in American history).

Healy goes to great pains to reconstruct the correspondence and meetings between Holmes and these men, and to trace Holmes’s cautious evolution. Holmes was neither a radical nor an ideologue, and his shift on free speech was tentative. Yet by the time Abrams v. United States was handed down, Holmes was ready to write the now immortal words: speeches and writings cannot be restricted or denied “unless they so imminently threaten immediate interference with the lawful and pressing purposes of the law that an immediate check is required to save the country.”

There are, to be sure, flaws with “The Great Dissent,” Healy’s first book. There seems to be an exultant sense of inevitability with Holmes’s decisions, when, in fact, things could have gone the other way. The United States has the broadest definition of free speech in the developed world — few other countries would include wearing Nazi paraphernalia in public schools or burning the nation’s flag as protected speech. By ignoring the chance that history could have gone the other way, Healy ignores the fragility of rights we now take for granted.

Constitutional protections should never be dismissed as inevitable, and it is terrifying that the most controversial decisions of the day will often be decided by a single aging man in a silly robe. From Oliver Wendell Holmes to Anthony Kennedy, the lesson of “The Great Dissent” seems to hold up. The arc of history is a delicate thing. It so often comes down to chance, folly, and one person’s whim.