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; William James, Pictorial Press Ltd/Alamy; Sigmund Freud, Photo by Time Life Pictures/Mansell/Time Life Pictures/Getty Images; John Watson, © Underwood & Underwood; Carl Rogers, Courtesy Everett Collection; Gustav Fechner, Science Source/Photo Researchers, Inc.; George A. Miller, Mike Lien/New York Times/File 1969/Redux; Charles Darwin, The National Library of Medicine; Lev Vygotsky, RIA Novosti/Alamy; Plato, © Vladimir Korostyshevskiy/Dreamstime.com