Now it is a significant fact that the single exception to this rule consists in, and is constituted by, _life_, or vital energy, which is constantly building lower forms of energy into higher forms. Life is certainly the highest form of energy which we know in this world, and all energies are below this in rank--as may readily be proved by an appeal to the facts of nutrition and metabolism. And, as life is constantly being added to or infused into the world (as the population increases), it is certainly true that there is here a definite increase of the sum-total of the highest form of energy of which we have any knowledge. Life thus occupies not only an important but a unique position--in that it is constructive instead of destructive; and this fact alone should give us pause, and make us ask whether life is, in its totality, subject to and included within the law of Conservation of Energy. The establishment of the fact that the human will is a definite physical energy is of importance also, because of its bearing upon the problem of the connection or inter-relation of mind and matter. Theories as to this bond or connection have been propounded since the dawn of philosophy. Aristotle and others wrote and thought deeply upon this subject. As is well known, this question formed one of the central points of debate in the works of Hobbes, Berkeley, Hume, Descartes, Leibnitz, Spinoza, Kant, Hegel, Lotze, and many other philosophical writers--all of whom wrote and speculated at length upon this subject. The theories which have been advanced in the past are briefly as follows:[19] _1st. Crude Materialism._--This doctrine contends that consciousness is merely matter, or energy, or matter in motion. It is not necessary to discuss this theory here, as it is not held today by any scientist of the first rank. _2nd. Epiphenomenalism._--This doctrine found its foremost champion in Huxley. It contends that the important happenings are the brain-changes--which are causally connected--and that our thoughts, or corresponding states of consciousness, merely accompany the brain-changes, just as the shadow of a horse may be said to accompany the horse. The objections of this doctrine are:-- (_a_) That it is just as inconceivable to believe or imagine that brain-changes generate consciousness as it is to imagine that consciousness generates brain-changes. (_b_) The law of Conservation is preserved at the expense of the law of Causality. For, if no part of the cause passed over into the effect (the state of consciousness), the law of Causality would be violated. (_c_) The appearance of consciousness, at some definite point in the course of the evolution of the animal kingdom constitutes a breach of continuity. For these and other reasons epiphenomenalism is today held by few, if any, philosophers.