Psychology was for centuries a part of philosophy, the happy, undisputed hunting-ground of the speculative metaphysician. With the beginning of the nineteenth century a new era dawned gradually for psychology. Anatomists, physiologists and physicists began to invade this reserve of the metaphysician. These early attempts of Gall and Spurzheim, Johannes Müller, E. H. Weber, Du Bois Reymond, Helmholtz, Lotze, Fechner, Thomas Young and many others are too well known to detain us. They lead up to the work of Wilhelm Wundt and the establishment of psychology as one of the natural sciences, from speculation without facts to speculation allied with experiment. However, as usually happens, the swing of the pendulum goes from one extreme to the other. For whilst the metaphysical psychologist would ignore physiology and neurology, maintaining that what is not itself a phase of consciousness cannot be used to explain consciousness, some of the physiological psychologists would overshoot the mark and attempt a psychology without consciousness. Thus we find the behaviourists (Watson) and the objective psychologists (Bechterew). Pavlov appears to belong to the latter school. But as Prof. Brett, in his History of Psychology, rightly says: “Modern psychology lies between two points: it emerges from anatomy and physiology, and terminates in a region where those sciences cease to guide.”