Routledge History of Philosophy Volume IV

116 SCIENCE AND MATHEMATICS UP TO DESCARTES
MOTION AND MECHANICAL PHILOSOPHY
The new cosmology necessitated a new theory of motion, for, as had been
obvious from the time of Copernicus and even before, Aristotelian ‘mechanics’
could not accommodate a moving Earth. Some have even seen cosmological
reform as providing the prime motivation for reform in mechanics, but the old
system also had many strains of its own. Aristotle made a sharp distinction
between the celestial and sublunary regions. The former, which we have already
considered in summary fashion, was the province of the fifth element and of
universal circular motions, but the latter was far more chaotic; as also was
Aristotle’s account of it, seeing that his usual technique was to start from the
situation in front of him and try to impose some semblance of order on it, rather
than develop a new science axiomatically from first principles.
All bodies were composed of a mixture of the four elements, earth, water, air
and fire, and their basic behaviour was dominated by the doctrines of natural
places and natural motions. The natural place of earth was at the centre of the
universe and that of fire at the periphery of the elementary regions, with the
other two elements being in between. All bodies aspired towards their natural
places, so that a heavy, or predominantly earthy, body would tend to move
downwards and a predominantly fiery one upwards. These motions were
conceived mainly in terms of their final causes, and less attention was paid to the
question of their efficient causes. This was not the case with violent motions, in
which a body was moving against its own nature. If I am lifting a heavy body, I
am clearly the efficient cause of its motion, but if I throw it upwards the situation
is more difficult, since there is no obvious mover once it has parted company
with my hand. Aristotle proved himself a model of consistency. The projectile
has nothing in contact with itself except the air surrounding it: therefore the air
must be the mover, and in the process of throwing it I must have communicated a
power of continuing the motion to the air, which would then, besides moving the
projectile, pass the power on to the succeeding parts of itself. Despite the internal
coherence of this scheme, it understandably drew much criticism from different
cultural areas, and we find many thought experiments, such as those concerning
the efficacy of shooting arrows by means of flapping the air behind them. Along
with other Italians, the young Galileo saw these considerations as providing a
fine stick with which to beat Aristotle. Like many of their predecessors these
‘radicals’ replaced the power communicated to the air with an internal moving
force communicated to the projectile itself, which in the later Western Middle
Ages was often known as ‘impetus’.
But Galileo was different in that he came to realize that, even though he was
giving anti-Aristotelian answers, he was still asking Aristotelian questions, and
this led to his imposing a self-denying ordinance whereby he did not consider
causes in his discussions of motion. This new attitude took root from around the
beginning of the century, and received its most mature public expression in what
is arguably his greatest work, the Discorsi e dimostrazioni matematiche, intorno