PSYCHOLOGY is the study of the mind, or of "mental phe nomena," or of the higher functions of beings "endowed with mind." The last definition emphasizes the fact that ordinarily the term is restricted to the study of mental functions connected with living organisms.
Though psychological studies may be said to have begun with the De Anima of Aristotle they were merged in what would now be regarded as purely biological questions. An absolute separation of the two sciences only came about through the general acceptance among scientists, twenty centuries later, of the Cartesian Dualism.
A sharp distinction was drawn between matter and mind, as radically different substances with mutually incompatible attri butes; psychology became then the science of mind and biology was assimilated to the group of studies concerned with material things. With this ontological dualism, a dualism of method was combined. To the study of mind was allowed a special source of information—"introspection"—the use of distinctive concepts, and explanation by reference to ends. On the other side of the chasm a rigid discipline was established. In the material sciences, facts are only admitted when guaranteed by common observation (in contrast to the alleged privacy of introspection), descriptions must be couched in purely physical terms, and the phenomena must be given a physicochemical or "mechanistic" explanation. From the success of this discipline there has come about the general leftward trend of the sciences; chemistry is reduced to physics, biology to chemistry, and psychology has tended to play the part of the poor relation to the natural sciences.
The culmination of this movement in the twentieth century has taken the form of an attempt to establish a purely "objective" psychology—a science of the higher functions of an organism physically conceived. Since, then, both in its origin and in one of its latest developments, the postulate of a distinctive mental substance has been absent it is undesirable to embalm in the defini tion of psychology any reference to "the mind." To begin with, the conception of "vital functions" will cover the relevant facts. Further enquiry will show which of these may be regarded as of distinctively "mental" nature.
The wider field has come to be embraced in the psychologist's sphere of investigation in the course of a tendency to retreat psy chological problems only as part of another science which might be better described as general psycho-physics. Originally, this latter
term applied only to the study of certain relations obtaining be tween sensations and their physical stimuli. The term, however, admits of application, with equal propriety, to the study of all the inter-relations between psychical phenomena, on the one hand, and their physical and physiological antecedents, concomitants and consequents, on the other. Whatever further relations may be involved there is certainly an extensive parallelism between mental process and certain kinds of physical process. Sensations are generally paralleled by certain modes of stimulation, and probably quite invariably by certain cerebral processes. Simi larly pleasure, desire and aversion correspond to certain cerebral conditions and have their outward manifestations. One conse quence of this parallelism is that almost every mental state has an obvious "behaviouristic" equivalent; and where direct intro spective evidence is uncertain it is natural to seek for informa tion from the external manifestations. But the facts of behaviour admit of independent systematization from the objective point of view, and terms originally applied to the mental state come to be used for the connected bodily process in relative isolation. Sensation comes to stand simply for receptivity, perception is identified with the behaviour which constitutes its external cri terion, desire means restlessness of a certain kind, and purpose is defined in terms of the behaviour by which it is expressed. In this way there has arisen a systematic duplication of the mean ings of psychological terms. So far has this tendency gone that a complete presentation of contemporary psychology must involve a corresponding duplication in exposition—an account of mental process in itself, and an account of the correlative bodily life from an external point of view. In what follows we shall explore first the possibilities in the purely objective standpoint, but the reader may proceed at once to the exposition from the traditional point of view which opens in the section on Psychology as the Science of Individual Experience.