Sidney Hook (1902–1987) was one of the leading American philosophers of the 20th century and a central figure among those on the left who became disenchanted with the Communist views of their youth and spent much of their lives fighting it instead. Ernest B. Hook is professor emeritus, School of Public Health, University of California, Berkeley. He wishes to thank the Hoover Institution and its archivists and librarians, especially Dale Reed, for their assistance.