NO knowledgeable scholar of American political thought would dispute the importance and influence of Herbert Croly’s 1909 book The Promise of American Life. In the book’s own day, Felix Frankfurter extolled it as “the most powerful single contribution to progressive thinking,” while Walter Lippmann crowned Croly the “first important [American] political philosopher” of the century. It was the right book at the right time. Not only did it ride the wave of reformist energy that swept American life at the turn of the century, embodied in such towering figures as Theodore Roosevelt, Woodrow Wilson, and Robert La Follette, but it also provided the era’s scattered reform impulses with a coherent philosophical basis. The book’s success offers potent evidence of the enduring power of ideas in history.