=Conservation considered as physical residua.=—Now the theory of memory which offers a satisfactory explanation of the mode in which registration, conservation, and reproduction occur postulates the conserved residua as physical in nature. Whenever we have a mental experience of any kind—a thought, or perception of the environment, or feeling—some change, some “trace,” is left in the neurons of the brain. I need not here discuss the relation between brain activity and mind activity. It is enough to remind you that, whatever view be held, it is universally accepted that every mental process is accompanied by a physical process in the brain; that, parallel with every series of thoughts, perceptions, or feelings, there goes a series of physical changes of some kind in the brain neurons. And, conversely, whenever this same series of physical changes occurs the corresponding series of mental processes, that is, of states of consciousness, arises. In other words, physical brain processes or experiences are correlated with corresponding mind processes or experiences, and vice versa.[56] This is known as the doctrine of psycho-physical parallelism. Upon this doctrine the whole of psycho-physiology and psycho-pathology rests. Mental physiology, cerebral localization, and mental diseases excepting on its assumption are unintelligible—indeed, the brain as the organ of the mind becomes meaningless. We need not here inquire into the nature of the parallelism, whether it is of the nature of dualism, e. g., a parallelism of two different kinds of facts, one psychical and the other physical; or whether it is a monism, i.e., a parallelism of two different aspects of one and the same fact or a parallelism of a single reality (mind) with a mode of apprehending it (matter)—mind and matter in their inner nature being held to be practically one and the same. The theory of memory is unaffected whichever view of the mind-brain relation be held.