Routledge History of Philosophy Volume IV

RENAISSANCE AND SEVENTEENTH-CENTURY RATIONALISM 121
a continuous plenum without empty spaces, although for most scientific purposes
one could focus on the three different sorts of particles that were separated out, in
a manner similar to relatively self-subsistent eddies, by the vortical motion.
These were in a way a replacement for the Aristotelian elements. The first
comprised extremely subtle and mobile matter, the second small spherical
globules, and the third larger particles which were less apt for motion.
And from these three we show that all the bodies of this visible world are
composed, the Sun and the fixed stars from the first, the heavens from the
second, and the Earth together with the planets’ and the comets from the
third. 45
In this way one is relieved from the almost impossible task of thinking all the time
starkly in terms of the motions of fundamentally undifferentiated continuous
matter.
A perhaps more important difference from ancient atomism was that the
motions of matter were by no means random, and Descartes’s system was also
more effectively rule-governed than those of Empedocles, with his guiding
principles of Love and Strife, and of Anaxagoras with that of Mind. Descartes
believed that any system of matter created by God would obey certain laws of
motion that followed from God’s unchangeableness. The first is that every body
would remain always in the same state unless this was changed by the action of
external forces, and Descartes was adamant that there be included in this
(contrary to the Aristotelian tradition) a body’s state of motion. The second 46 is
that the motion of any part of matter always tends to be in a straight line, and this
when added to the first gives a fair approximation to Newton’s first law of
motion, the principle of rectilinear inertia, which may rightly be regarded as the
foundation stone of classical mechanics. The third law is rather more
complicated: it concerns the collisions of bodies, and asserts inter alia that in
these the amount of motion is conserved. From it Descartes deduces some
hideously incorrect laws of impact, which older fashioned histories of mechanics
used often to crow over, but this was to neglect the fact that, as regards historical
influence, the form of Descartes’s discussion was far more important than its
exact content.
Descartes’s first two laws in particular led to some novel questions about
forces. For instance, the acceleration of falling bodies implied the existence of a
force to bring about this change from its preferred state of uniform rectilinear
motion, and similarly did the deviation of the planetary orbits from this state.
Armed with hindsight, the reader will recall how important these questions were
for Newton on his journey towards the principle of universal gravitation, but this
serves to show up important differences between the two men. Whereas Newton
asked typically how big the forces were (at least as a preliminary to deeper
explanation), Descartes sought for their causes, and in both cases specified these