Physiological psychology, on the other hand, is competent to investigate the relations that hold between the processes of the physical and those of the mental life.

The distinguishing characteristics of mind are of a subjective sort; we know them only from the contents of our own consciousness.

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

Philosophical reflection could not leave the relation of mind and spirit in the obscurity which had satisfied the needs of the naive consciousness.

We know, from ordinary life, that we are not able to direct our attention perfectly steadily and uniformly to one and the same object... At times the attention turns towards the object most intensely, and at times the energy flags.

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.

Thus ordered thinking arises out of the ordered course of nature in which man finds himself, and this thinking is from the beginning nothing more than the subjective reproduction of the regularity according to the law of natural phenomena. On the other hand, this reproduction is only possible by means of the will that controls the concatenation of ideas.