For a century now, scholars have searched for the source of Marcel Proust's startlingly innovative novel A la recherche du temps perdu. Some have pointed to Henri Bergson, Sigmund Freud, or Paul Sollier. Others have referenced the novels of Henry James. But no one has focused on the more significant influence of the writings of Henry's older brother, the psychologist and Harvard professor William James. A close comparison reveals the degree to which Proust's novel stems from James's psychological and philosophical theories. William James was a prominent member of the scientific, medical and philosophical communities in Proust's Paris and was close friends with two men well known to Proust. His works were translated into French and reviewed in French journals and newspapers. This book discloses how Proust likely became familiar with William James and illustrates how James's writings were key to Proust's ability to craft the book he had been trying to write, extending even to his use of