This common mind is, indeed, a fertile field for psycholovical cultivation. The mind of a people is, from one point of view, nothing more than the aggregate of the minds of the indi viduals composing the people. There are no new processes in the common mind : common mental processes are impossibilities. From an other point of view, however, the common mind offers a distinct problem to psychology. The mental productions of men in society differ from the mental productions of the individual. No single man would ever have invented language in order to talk with himself ; language is a product of the common mind. One mind may take on a habit; it requires at least two to make a law or a custom. These products of the com mon mind—mythology, and religious observ ances, language, law and custom—are treated by social psychology, which approaches them by analytic, comparative and genetic methods, and seeks to explain them in part from the nature of the individual mind, in part from the cir cumstances in which man, as a social being, has at various epochs of his history been placed.
Lastly, we may contrast normal with ab normal psychology. This deals with such tran sitory abnormalities as sleep and hypnosis, or with the chronic derangements of insanity. In every case, the key to the abnormal mind is to be found in the status of attention. In dreams, we have a limited and irregular distribution of attention; in hypnosis, an exaggerated passive attention, with its accompanying suggestibility. And we have misdirection, inertia and decay of attention variously exhibited in the various forms of mental disorder.
Here our list of the principal departments of psychology may come to an end.• All through, introspection is the special psychological method, and the dictum of introspection is the final appeal. Animals and infants and the insane cannot, of course, be called upon to introspect. Nor, for that matter, can a *common') mind. But we observe the conduct of children and animals, and interpret it in the light of our own adult introspections; we search for parallels to the abnormal in our own normal conscious nesses; and we base our study of social psy chology upon the laws of the individual mind.
Mind and The question of the rela tion of mind to body is an old one in the history of philosophy; but it is a question that still awaits its answer. Psychology cannot avoid it; and we may, therefore, give here a brief indica tion of current views.
The two rival theories now most in vogue among systematic psychologists are those of psychophysical parallelism and of interaction. The former declares that mental and bodily processes run side by side, without mutual influ ence or interference. Every mental process is
paralleled by a nervous process. But the nerv ous processes are links in the unbroken chain of physical causation; and this is complete in itself. Mind is wholly unable to work changes in matter. The latter theory affirms, on the con trary, that mental process influences and is in turn influenced by bodily process: our grief makes us cry, our idea that it is late makes us run to catch a train, as truly as our fit of indi gestion makes us gloomy, or a cold in the head makes us stupid.
Common sense is, naturally enough, on the side of interaction. Nevertheless, the theory presents grave difficulties. We must either admit that mind is a special mode of energy, capable of give-and-take with the various forms of energy familiar to us in the natural world; or we must assume that the mind can, without expenditure of energy, deflect the course of physical molecules. Parallelism, while it sins against common sense by making mind causally inefficient, epiphenomenal, has at least the ad vantage that it leaves the order of the natural universe intact. It has, indeed, further advan tages. It has proved to be an admirable work ing hypothesis for scientific psychology; and it is a purely empirical and non-committal state ment of the relationship of mind and body,— a statement that may be reconciled with diverse forms of philosophical belief. It leads, per haps, most directly to the philosophical doc trine of panpsychism, according to which con sciousness is the reality and the concomitant brain-process a phenomenal symbol of this real ity. This doctrine serves to reconcile the con flict between parallelism and interactionism, seeing that it guarantees the efficiency of mind while it insists upon the parallel relation of mind and body. It may be noted that Gustav Theodor Fechner (1801-87), the father and founder of experimental psychology, combined an acceptance of psychophysical parallelism, as a working hypothesis for the laboratory, with a belief in panpsychism as the only tenable metaphysics.
The Relation of Psychology to Other Sci ences and to Philosophy.— Psychology comes into manifold relation with the natural sciences. As a quantitative science, it requires the aid of mathematics; as an experimental science, it requires the aid of physics. In all its forms, it presupposes a knowledge of the physiology of sense-organs and nervous system: if it seek its explanations in physiology, this knowledge must be accurate and detailed. Insanity can be understood only in the light of pathological histology; comparative psychology rests upon comparative anatomy and comparative physiol 08Y.