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

Physiology and psychology cover, between them, the field of vital phenomena; they deal with the facts of life at large, and in particular with the facts of human life.

We speak of virtue, honour, reason; but our thought does not translate any one of these concepts into a substance.

The materialistic point of view in psychology can claim, at best, only the value of an heuristic hypothesis.

In the animal world, on the other hand, the process of evolution is characterised by the progressive discrimination of the animal and vegetative functions, and a consequent differentiation of these two great provinces into their separate departments.

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