Physiology, in its analysis of the physiological functions of the sense organs, must use the results of subjective observation of sensations; and psychology, in its turn, needs to know the physiological aspects of sensory function, in order rightly to appreciate the psychological.

From the standpoint of observation, then, we must regard it as a highly probable hypothesis that the beginnings of the mental life date from as far back as the beginnings of life at large.

Our mind is so fortunately equipped, that it brings us the most important bases for our thoughts without our having the least knowledge of this work of elaboration. Only the results of it become unconscious.

The results of ethnic psychology constitute, at the same time, our chief source of information regarding the general psychology of the complex mental processes.

Physiology seeks to derive the processes in our own nervous system from general physical forces, without considering whether these processes are or are not accompanied by processes of consciousness.

Hence, wherever we meet with vital phenomena that present the two aspects, physical and psychical there naturally arises a question as to the relations in which these aspects stand to each other.

Experimental psychology itself has, it is true, now and again suffered relapse into a metaphysical treatment of its problems.