Just as we find that wherever there is living substance there is Mind, so do we find that we are unable to intelligently consider Mind unless as _embodied_ in living substance. The idea of Mind, independent of its substantial embodiment, becomes a mere abstraction impossible of mental imaging--something like color independent of the colored substance, or light without the illuminated substance. And just as we find that Mental States influence Physical Conditions, so do we find that Physical Conditions influence Mental States. And, so the problem of Life, Health and Disease once more loses its simplicity, and the mystery again deepens. The deeper we dig into the subject, the more do we become impressed with the idea of the universal principle of Action and Reaction so apparent in all phenomena. The Mind acts upon the Body; the Body reacts upon the Mind; cause and effect become confused; the reasoning becomes circular--like a ring it has no beginning, no end; its beginning may be any place we may prefer, its ending likewise. The only reconciliation is to be found in the fundamental working hypothesis which holds that both Mind and Body--both Mental States and Physical Conditions--are _the two aspects of something greater than either--the opposing poles of the same Reality_. The radical Materialist asserts that the Body is the only reality, and that Mind is merely its "by-product." The Mentalist asserts that the Mind is the only reality, and that the Body is merely its grosser form of manifestation. The unprejudiced philosopher is apt to stand aside and say: "You are both right, yet both wrong--each is stating the truth, but only the half-truth." With the working hypothesis that Mind and Body are but varying aspects of the Truth--that Mind is the inner essence of the Body, and Body the outward manifestation of the Mind--we find ourselves on safe ground. We mention this fundamental principle here, for in the body of this book we shall not invade the province of metaphysics or philosophy, but shall hold ourselves firmly to our own field, that of psychology. Of course, the very nature of the subject renders it necessary that we consider the influence of psychology upon physiology, but we have remembered that this book belongs to the general subject of the New Psychology, and we have accordingly emphasized the psychological side of the subject. But the same material could have been used by a writer upon physiology, by changing the emphasis from the psychological phase to the physiological.