more than psychology can overlook the results established by biological research.
This becomes more evident in view of the fact that both mental and bodily life may and too often does vary from the course of normal development. Those modifications which, either suddenly or gradually, reduce the mind to chaos, have in the past been studied chiefly on their practical side. They possess none the less an interest for psychology. By contrast at least they throw light on the normal activities of mind.
It should be noted, however, that in mental pathology as in psychophysics and in the study of normal development, comparison is simply a means of getting new information concerning those processes which appear in the normal individual consciousness. To render this information available and to coordinate the multitude of facts which it contains or suggests, some sort of theory is necessary; and there is no lack of theorizing in modern psychology.
Owing, perhaps, to the predominance of comparative methods, the theory of psycho-physical parallelism is just now in vogue. It is agreed, in general, that mental processes and physical processes take place simultaneously, and to this extent the one series may be said to parallel the other. But beyond this, discussion is rife and the end is not yet. If, on the more orthodox view, it is claimed that the two series are merely parallel, i.e., without any causal nexus, it is urged on the other hand, especially in these latter times, that there must be interaction, mind producing effects in the organism and vice versa. While, again, it is maintained that the conscious processes have causal relations of their own—a specific psychical causality; it is also asserted that the causal relation exists for the physical processes only, and that mind gets its connectedness as an accompaniment of the bodily series.
The final adjustment of these claims may depend, in a measure, upon the eventual acceptance or rejection of the hypothesis, now favorably regarded, that every conscious state has for its organic basis the passage from centripetal effects in the brain to centrifugal effects, and consequently has its efferent or motor phase as well as its afferent or sensory phase. This hypothesis, however, leaves untouched the inquiry as to the precise way in which mind acts on body and body on mind. The latter problem, in fact, is essentially metaphysical. It points at once to the remoter region in which epistemology and criticism hold sway.
More than any other science, psychology finds itself obliged to canvass its results in the light of philosophy. Of late the statement has been made quite frankly that the way to psychology has its starting-point in philosophy. Be this as it may, psychology is, for the time being, at the stage of self-examination, taking an account of its titles to existence as a science. Not that it doubts of its proper object;