I. THE PHYSIOLOGICAL MECHANISM OF MENTAL LIFE
Psychologists no longer question that the product of mind, which we call behavior, by which mind is judged, is in some way intimately connected with the sensitivity and integrity of the nervous system. The proof of this has often been set forth, and will merely be taken for granted here. Any organ or substance which reacts upon this sensitivity or integrity may then indirectly influence mental life in certain respects. For instance, the glandular system of the body, especially that part of it which comprises the glands of internal secretion, may affect behavior by affecting the growth or function of the nervous system. Drugs may influence mental processes, because they act upon the neurones. However, all present knowledge points to the conclusion that if the nervous tissue could be isolated from such influences, mental life would be immune from their effects. Mental life is but indirectly subject to such influences, in so far as nervous tissue is affected in a particular manner by them.