Discusses the theories of Socrates, Plato, Aristotle, Plotinus, St. Augustine, St. Thomas Aquinas, John Locke, George Berkeley, David Hume, David Hartley, Alexander Bain, Herbert Spencer, Immanuel Kant, Johann Herbart, Galen, Marshall Hall, Sir Charles Bell, Franz Joseph Gall, Hermann von Helmholtz, Ewald Hering, Ernest H. Weber, Gustav Theodor Fechner, Charles Darwin, Sir Francis Galton, Wilhelm Maximilian Wundt, William James, John Dewey, James Rowland Angell, Harvey A. Carr, Ivan Petrovich Pavlov, Vladimir Bekhterev, Edward Lee Thorndike, McDougall, Woodworth, John Broadus Watson, Albert P. Weiss, Karl Lashley, Walter S. Hunter, Edwin B. Holt, Edwin R. Guthrie, Clark L. Hull, J. R. Kantor, Burrhus Frederic Skinner, Albert Bandura, Immanuel Kant, John Stuart Mill, Franz Brentano, Carl Stumpf, Ernst Mach, Christian von Ehrenfels, Max Wertheimer, Lewin, Tolman, Jean Piaget, Noam Chomsky, Leibnitz, Schopenhauer, Nietzsche, Sigmund Freud, Alfred Adler, Carl Jung, Karen Horney, Erich Fromm, Harry Stack Sullivan, Erik Erikson, Carl Rogers, Soren Kierkegaard, Martin Heidegger, Jean-Paul Sartre, Karl Jaspers, Adrian von Kaam, Ludwig Binswanger, Medard Boss, Rollo May, and others.