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

There are other sources of psychological knowledge, which become accessible at the very point where the experimental method fails us.

The attitude of physiological psychology to sensations and feelings, considered as psychical elements, is, naturally, the attitude of psychology at large.

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.

If we take an unprejudiced view of the processes of consciousness, free from all the so-called association rules and theories, we see at once that an idea is no more an even relatively constant thing than is a feeling or emotion or volitional process. There exist only changing and transient ideational processes ; there are no permanent ideas that return again and disappear again.

Now, there are a very large number of bodily movements, having their source in our nervous system, that do not possess the character of conscious actions.

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.