be the ultimate essence of matter and mind there is difference enough between them to justify the distinction which has come to be made between mental and physical processes and has led to the development of two groups of sciences, the mental and the natural. The science of psychology is primarily interested in thoughts, feelings, and volitions; natural science, in material objects. As Professor Münsterberg says: "Psychology examines no body when it analyzes the ideas of bodies, physics examines no ideas when it analyzes the perceived body." These thoughts, feelings, and volitions form a more or less connected series of events, a domain in which we can discover law and order, and are therefore capable of scientific treatment. It is because such an orderly body of unique facts exists that a special science called psychology has grown up and is possible. Now the other philosophical branches, logic, æsthetics, ethics, the theory of knowledge, and metaphysics are likewise fundamentally interested in the mind, and their affiliation with the science of mind is therefore not only historically but logically justifiable.
It is true, the facts of mental life do not appear in isolation, but are somehow related to a physical and biological environment. Hence they may be studied in connection with the occurrences which constitute the special subject matter of the natural scientist. Here the ideal will be to discover the particular material processes with which particular psychic states are connected. But in psychology the interest will always be centered upon mind; the facts of physics and biology will be drawn upon simply in order to throw light upon the inner world. The interest of the natural scientist, on the other hand, is directed toward external nature, and he refers to the inner world only when a proper understanding of this will aid him in understanding the ways of matter. For ages and ages, down through the mediæval period, he believed that mind or soul alone could explain animal or human movements, and therefore introduced it as a principle of explanation. When he felt able to account for all physical occurrences without having recourse to anything mental, he abandoned the principle and ignored mind as lying wholly outside his sphere.
