than the soul, since it is inmost to us, that is to say inmost to itself. [156] 60. M. Descartes wished to compromise and to make a part of the body's action dependent upon the soul. He believed in the existence of a rule of Nature to the effect, according to him, that the same quantity of movement is conserved in bodies. He deemed it not possible that the influence of the soul should violate this law of bodies, but he believed that the soul notwithstanding might have power to change the direction of the movements that are made in the body; much as a rider, though giving no force to the horse he mounts, nevertheless controls it by guiding that force in any direction he pleases. But as that is done by means of the bridle, the bit, the spurs and other material aids, it is conceivable how that can be; there are, however, no instruments such as the soul may employ for this result, nothing indeed either in the soul or in the body, that is, either in thought or in the mass, which may serve to explain this change of the one by the other. In a word, that the soul should change the quantity of force and that it should change the line of direction, both these things are equally inexplicable. 61. Moreover, two important truths on this subject have been discovered since M. Descartes' day. The first is that the quantity of absolute force which is in fact conserved is different from the quantity of movement, as I have demonstrated elsewhere. The second discovery is that the same direction is still conserved in all bodies together that are assumed as interacting, in whatever way they come into collision. If this rule had been known to M. Descartes, he would have taken the direction of bodies to be as independent of the soul as their force; and I believe that that would have led direct to the Hypothesis of Pre-established Harmony, whither these same rules have led me. For apart from the fact that the physical influence of one of these substances on the other is inexplicable, I recognized that without a complete derangement of the laws of Nature the soul could not act physically upon the body. And I did not believe that one could here listen to philosophers, competent in other respects, who produce a God, as it were, _ex machina_, to bring about the final solution of the piece, maintaining that God exerts himself deliberately to move bodies as the soul pleases, and to give perceptions to the soul as the body requires. For this system, which is called that of _occasional causes_ (because it teaches that God acts on the body at the instance of the soul, and _vice versa_), besides introducing perpetual miracles to establish communication [157] between these two substances, does not obviate the derangement of the natural laws obtaining in each of these same substances, which, in the general opinion, their mutual influence would cause.