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.

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