Credits: Aristotle, The National Library of Medicine; Descartes, The National Library of Medicine; Edward Titchener, The National Library of Medicine; Wilhelm Wundt with his colleagues (Friedrich Sander, Otto; Klemm; Ottmar; Dittrich; Wilhelm; Wirth; and Hartmann), circa 1910, © INTERFOTO/Alamy INTERFOTO/Alamy; William James, Pictorial Press Ltd/Alamy; Sigmund Freud, Photo by Time Life Pictures/Mansell/Time Life Pictures/Getty Images; John Watson, © Underwood & Underwood/Corbis; Carl Rogers, Courtesy Everett Collection; Gustav Fechner, Science Source/Photo Researchers, Inc.; George A. Miller, Princeton University, Office of Communications, John T. Miller; Charles Darwin, The National Library of Medicine; Lev Vygotsky, RIA Novosti/Alamy; Plato, © Vladimir Korostyshevskiy/Dreamstime.com