THE THEODICY
Translation © George MacDonald Ross, 1974, 1999
Theodicy: Essays on the Goodness of God, etc., Part 1, §§60–61
60. Mr. Descartes saw fit to capitulate, and make a part of bodily activity depend on the soul. He believed he knew a law of nature according to which, in his opinion, the same quantity of movement is conserved in the physical universe. He did not consider it possible that the influence of the soul should violate this physical law, but on the other hand he believed that the soul could have the power to change the direction of movements occurring within the body — rather as a horse rider, although he communicates no force to the horse he is riding, still controls it by directing this force from the side he thinks appropriate. But since all this is done by means of the reins, the bit, the spurs, and other material aids, we do have a conception of how it is possible. But there are no instruments which the soul could use to bring this about — nothing, in fact, either in the soul or in the body (that is, either in thought or in mass) that could be used to explain this changing of the one by the other. In a word, that the soul should change the direction in which a force operates is just as inexplicable as that it should change the quantity of the force.
61. Besides, since Mr. Descartes’ time, I have discovered two important truths on this subject. The first is that quantity of absolute force, which is in fact conserved, is different from quantity of movement, as I have demonstrated elsewhere. The second discovery is that, in a system of interacting bodies, in whatever way they may collide with each other, the same direction of motion is still conserved. If Mr. Descartes had known this law, he would have made the direction of motion of bodies as independent of the soul as their force, and I believe that this would have led him straight to the hypothesis of the pre-established harmony, which is where I have been led by these same laws. For, quite apart from the fact that any physical influence of one of these substances on the other is inexplicable, I considered that the soul could not act physically on the body without an utter derangement of the laws of nature. And I did not believe that on this point one could listen to those philosophers, [n.1] who, though very capable in other respects, like playwrights conjure up a Deus ex machina to tie up all the loose ends of the plot, holding that God exerts himself expressly to move bodies as the soul wishes, and to give the soul perceptions as the body demands. For this system, which is called that of occasional causes (since it teaches that God acts on the body on the occasion of the soul, and vice versa), quite apart from its introducing perpetual miracles to bring about interaction between these two substances, does not prevent the same derangement of the natural laws established in each of them, as would be caused by the mutual influence between them supposed in common opinion.
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