which the other atoms exert upon it. And the mathematician who knew the position of the molecules or atoms of a human organism at a given moment, as well as the position and motion of all the atoms in the universe capable of influencing it, could calculate with unfailing certainty the past, present and future actions of the person to whom this organism belongs, just as one predicts an astronomical phenomenon.[1] [Sidenote: If principle of conservation of energy is universal, physiological and nervous phenomena are necessitated, but perhaps not conscious states.] We shall not raise any difficulty about recognizing that this conception of physiological phenomena in general, and nervous phenomena in particular, is a very natural deduction from the law of the conservation of energy. Certainly, the atomic theory of matter is still at the hypothetical stage, and the purely kinetic explanations of physical facts lose more than they gain by being too closely bound up with it. We must observe, however, that, even if we leave aside the atomic theory as well as any other hypothesis as to the nature of the ultimate elements of matter, the necessitating of physiological facts by their antecedents follows from the theorem of the conservation of energy, as soon as we extend this theorem to all processes going on in all living bodies. For to admit the universality of this theorem is to assume, at bottom, that the material points of which the universe is composed are subject solely to forces of attraction and repulsion, arising from these points themselves and possessing intensities which depend only on their distances: hence the relative position of these material points at a given moment--whatever be their nature--would be strictly determined by relation to what it was at the preceding moment. Let us then assume for a moment that this last hypothesis is true: we propose to show, in the first place, that it does not involve the absolute determination of our conscious states by one another, and then that the very universality of the principle of the conservation of energy cannot be admitted except in virtue of some psychological hypothesis. Sidenote: To prove conscious states determined, we should have to show a necessary connexion between them and cerebral states. No such proof.