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.

62.  Being on other considerations already convinced of the principle of Harmony in general, I was in consequence convinced likewise of the preformation and the Pre-established Harmony of all things amongst themselves, of that between nature and grace, between the decrees of God and our actions foreseen, between all parts of matter, and even between the future and the past, the whole in conformity with the sovereign wisdom of God, whose works are the most harmonious it is possible to conceive.  Thus I could not fail to arrive at the system which declares that God created the soul in the beginning in such a fashion that it must produce and represent to itself successively that which takes place within the body, and the body also in such a fashion that it must do of itself that which the soul ordains.  Consequently the laws that connect the thoughts of the soul in the order of final causes and in accordance with the evolution of perceptions must produce pictures that meet and harmonize with the impressions of bodies on our organs; and likewise the laws of movements in the body, which follow one another in the order of efficient causes, meet and so harmonize with the thoughts of the soul that the body is induced to act at the time when the soul wills it.