Like all other natural phenomena, the psychic processes are subject to the supreme, all-ruling law of substance; not even in this province is there a single exception to this highest cosmological law (compare chap. xii.). The phenomena of the lowly psychic life of the unicellular protist and the plant, and of the lowest animal forms--their irritability, their reflex movements, their sensitiveness and instinct of self-preservation--are directly determined by physiological action in the protoplasm of their cells--that is, by physical and chemical changes which are partly due to heredity and partly to adaptation. And we must say just the same of the higher psychic activity of the higher animals and man, of the formation of ideas and concepts, of the marvellous phenomena of reason and consciousness; for the latter have been phylogenetically evolved from the former, and it is merely a higher degree of integration or centralization, of association or combination of functions which were formerly isolated, that has elevated them in this manner. The first task of every science is the clear definition of the object it has to investigate. In no science, however, is this preliminary task so difficult as in psychology; and this circumstance is the more remarkable since logic, the science of defining, is itself a part of psychology. When we compare all that has been said by the most distinguished philosophers and scientists of all ages on the fundamental idea of psychology, we find ourselves in a perfect chaos of contradictory notions. What, really, is the "soul"? What is its relation to the "mind"? What is the inner meaning of "consciousness"? What is the difference between "sensation" and "sentiment"? What is "instinct"? What is the meaning of "free will"? What is "presentation"? What is the difference between "intellect" and "reason"? What is the true nature of "emotion"? What is the relation between all these "psychic phenomena" and the "body"? The answers to these and many other cognate questions are infinitely varied; not only are the views of the most eminent thinkers on these questions widely divergent, but even the same scientific authority has often completely changed his views in the course of his psychological development. Indeed, this "psychological metamorphosis" of so many thinkers has contributed not a little to the _colossal confusion of ideas_ which prevails in psychology more than in any other branch of knowledge.