case of the direct impact of two inelastic bodies. Thus science has done for a very long time without a universal conservative principle. In its present form, and since the development of the mechanical theory of heat, the principle of the conservation of energy certainly seems to apply to the whole range of physico-chemical phenomena. But no one can tell whether the study of physiological phenomena in general, and of nervous phenomena in particular, will not reveal to us, besides the _vis viva_ or kinetic energy of which Leibniz spoke, and the potential energy which was a later and necessary adjunct, some new kind of energy which may differ from the other two by rebelling against calculation. Physical science would not thereby lose any of its exactitude or geometrical rigour, as has lately been asserted: only it would be realized that conservative systems are not the only systems possible, and even, perhaps, that in the whole of concrete reality each of these systems plays the same part as the chemist's atom in bodies and their combinations. Let us note that the most radical of mechanical theories is that which makes consciousness an _epiphenomenon_ which, in given circumstances, may supervene on certain molecular movements. But, if molecular movement can create sensation out of a zero of consciousness, why should not consciousness in its turn create movement either out of a zero of kinetic and potential energy, or by making use of this energy in its own way? Let us also note that the law of the conservation of energy can only be intelligibly applied to a system of which the points, after moving, can return to their former positions. This return is at least conceived of as possible, and it is supposed that under these conditions nothing would be changed in the original state of the system as a whole or of its elements. In short, time cannot bite into it; and the instinctive, though vague, belief of mankind in the conservation of a fixed quantity of matter, a fixed quantity of energy, perhaps has its root in the very fact that inert matter does not seem to endure or to preserve any trace of past time. But this is not the case in the realm of life. Here duration certainly seems to act like a cause, and the idea of putting things back in their place at the end of a certain time involves a kind of absurdity, since such a turning backwards has never been accomplished in the case of a living being. But let us admit that the absurdity is a mere appearance, and that the impossibility for living beings to come back to the past is simply owing to the fact that the physico-chemical phenomena which take place in living bodies, being infinitely complex, have no chance of ever occurring again all at the same time: at least it will be granted to us that the hypothesis of a turning backwards is almost meaningless in the sphere of conscious states. A sensation, by the mere fact of being prolonged, is altered to the point of becoming unbearable. The