GENERAL SURVEY OF THE SCIENCE General Psychology.—In the foregoing exposition an attempt has been made to present the central problem of the science, and to indicate certain representative lines along which its topics may be treated. It need scarcely be said, however, that the sub ject is controversial, and that even fundamental issues are still matters of dispute. In particular, we have noted two sharply opposed tendencies which have contended for priority throughout the history of the science. On the one hand the distinctive nature of conscious life has given encouragement to the project of con structing a pure Psychology, a science of mind treated in abso lute separation from its physical and physiological connections. This conception of the science has been fostered not so much perhaps by psychologists themselves as by the development of the physical science through which Nature has come to be treated as a system "closed to mind," a doctrine inevitably suggesting that Mind is equally closed to Nature. On the other hand, mental life has a bodily counterpart and in a variety of ways appears to be bound up with the more elementary functions of the or ganism. Hence the tendency to approach the problem of mind from a biological point of view.
On each side of this broad division subsidiary schools are aligned. It is important to avoid each of these extremes, and more particularly important to avoid the assumptions upon which the antithesis is based. What is revealed to "introspection" is not wholly exclusive of the world revealed to external observa tion. Without assuming two separate and independent fields of investigation, Psychology may be comprehensively treated as the science of individual experience, its conditions and its expressions in behaviour. In practice it is generally so treated whatever the view professed. Primarily it is concerned with the analysis of ex perience, and with the laws of psychical process. Herein its most distinctive concepts emerge, and for the conduct of the inquiry introspective observation is essential. In determining the bodily processes with which mental life is connected it is in the main committed to the procedure of the biological sciences. These
studies are psychological so far as the facts revealed are related to mental life. The two methods are systematically combined in the use of experiment. The distinctive feature of the psycho logical experiment is that it involves a two-fold process of ob servation : external observation by the experimenter who arranges the physical setting, and introspection on the part of the subject of the experiment.
More specifically, Psychology is concerned with mental process in relation to a certain cycle of events in which five phases may be conveniently distinguished: I. A process of stimulation commencing externally to the body, II. Receptive physiological processes, III. Central processes, IV. Responsive physiological processes, V. Physical products and other changes, again external to the body.
The fundamental relations between body and mind call for formulation primarily with regard to mental process and the third phase of the cycle. It is doubtful if these be causal in the ordinary sense of the term. But there is a considerable mass of evidence that a close parallelism holds between mental and cen tral physiological process. How close it is as yet impossible to say; but the further we pass from central to peripheral events the less exact it becomes. Nevertheless it is possible to establish certain correlations between stimuli and experiences and between experiences and expressions. In fact upon such relations depends in very large measure our knowledge of minds other than our own.
Traditional Psycho-physics is concerned with the precise rela tions which hold between experience and the phase of the process, though a complete solution to the problems raised involves some reference to phases II. and III. In this connection there arose the first important attempts at mental measurement. (Cf. EXPERI MENTAL PSYCHOLOGY and PSYCHOLOGY, HISTORY OF.) The central problem here is to determine the quantitative relations between stimulus and sensation.