LXXVII. The cause of the progressive motion, is also very difficult
to be accounted for. It is hard to comprehend, how the motion of a
stone thrown from the hand subsists, after the action of throwing it
ceases. Who moves the stone after the hand is still? What many of the
Aristotelites say, is, that the action of the hand produces a quality
in the stone, which they call impetus, and that this quality causes the
stone to move after it is thrown from the hand. But this solution, is
destitute of all appearance of truth. If every violent motion, as the
Aristotelites maintain, proceeds from an extrinsic cause, how can the
motion of the stone thrown in a mounting direction, which is a violent
one, proceed from an intrinsic quality communicated to the said stone?
If all generation, or accumulation, according to the doctrine of the
same school, supposes corruption, what quality or accidental form in the
stone was corrupted, to prepare it for engendering that new quality,
which they call impetus? What dispositions preceded this generation? Or
what time is there for their preceding it, when a globe of great bulk,
with its motion impels a little one? It being certain, that the contact
of the two, does not continue longer than an instant, what property then
has that quality, to occasion so quick a corruption? Is it perchance, the
gravity of the stone itself? This however, as it subsisted at the time
of receiving the impulse, if it is a property opposite to that quality,
would at the instant, have prevented its generation, as it is afterwards
said to impede its conservation. We might make many other reflexions, to
prove, that that quality is chimerical. Others recur to the air, for the
means by which the motion is continued, which they say, being violently
divided by the fore part of the stone, takes a sudden turn to the hinder
part, and impels the stone forward. But omitting many other objections,
which would render this mode of philosophizing totally improbable, I
shall just remark, that it would follow from hence, that a stone could
not move through a void space, give it what impulse you will, which is
what I imagine nobody can believe. Descartes, reconciles this difficulty,
with his maxim respecting the general law of the communication of
motion, established by the Author of nature at the creation, which we
shall not combat minutely, in order not to waste time; but shall content
ourselves with observing, that that maxim applied to the present matter,
and throughly investigated and explained, would signify, that the stone
thrown from the hand, moves because God has ordained that it should move;
but to solve difficulties in this way, it is not necessary to study
philosophy.