But we must own that the amount of freedom which is left to us after strictly complying with the principle of the conservation of energy is rather limited. For, even if this law does not exert a necessitating influence over the course of our ideas, it will at least determine our movements. Our inner life will still depend upon ourselves up to a certain point; but, to an outside observer, there will be nothing to distinguish our activity from absolute automatism. We are thus led to inquire whether the very extension of the principle of the conservation of energy to all the bodies in nature does not itself involve some psychological theory, and whether the scientist who did not possess _a priori_ any prejudice against human freedom would think of setting up this principle as a universal law. [Sidenote: It implies that a system can return to its original state. Neglects duration, hence inapplicable to living beings and conscious states.] We must not overrate the part played by the principle of the conservation of energy in the history of the natural sciences. In its present form it marks a certain phase in the evolution of certain sciences; but it has not been the governing factor in this evolution and we should be wrong in making it the indispensable postulate of all scientific research. Certainly, every mathematical operation which we carry out on a given quantity implies the permanence of this quantity throughout the course of the operation, in whatever way we may split it up. In other words, what is given is given, what is not given is not given, and in whatever order we add up the same terms we shall get the same result. Science will for ever remain subject to this law, which is nothing but the law of non-contradiction; but this law does not involve any special hypothesis as to the nature of what we ought to take as given, or what will remain constant. No doubt it informs us that something cannot come from nothing; but experience alone will tell us which aspects or functions of reality must count for something, and which for nothing, from the point of view of positive science. In short, in order to foresee the state of a determinate system at a determinate moment, it is absolutely necessary that something should persist as a constant quantity throughout a series of combinations; but it belongs to experience to decide as to the nature of this something, and especially to let us know whether it is found in all possible systems, whether, in other words, all possible systems lend themselves to our calculations. It is not certain that all the physicists before Leibniz believed, like Descartes, in the conservation of a fixed quantity of motion in the universe: were their discoveries less valuable on this account or their researches less successful? Even when Leibniz had substituted for this principle that of the conservation of _vis viva,_ it was not possible to regard the law as quite general, since it admitted of an obvious exception in the