Routledge History of Philosophy Volume IV

164 RENAISSANCE AND SEVENTEENTH-CENTURY RATIONALISM
any degree of activity once activity has been ascribed to the former. His
contemporaries were especially concerned to restrict all causal efficacy to the
transfer of motion from one body to another in impact but Descartes goes
further. While he recognizes the need for such a description at the phenomenal
level, at the metaphysical level he does not allow any causation at all in the
natural realm. 17 Motion is conceived as a mode of a body just as shape is, and
strictly speaking it is not something that can be transferred at all, any more than
shape can be. The power that causes bodies to be in some determinate state of
rest or motion is a power that derives exclusively from God, and not from impact
with other bodies. Moreover, this power is simply the power by which God
conserves the same amount of motion that he put in the corporeal world at the
first instant. 18 On Descartes’s account of the persistence of the corporeal world,
God is required to recreate it at every instant, because it is so lacking in any
power that it does not even have the power to conserve itself in existence. As he
puts it in the replies to the first set of objections to the Meditations, we can find
in our own bodies, and by implication in other corporeal things as well, no power
or force by which they could produce or conserve themselves. Why, one may ask,
is such a force or power required? The answer is that causes and their effects
must be simultaneous: ‘the concept of cause is, strictly speaking, applicable only
for as long as it is producing its effect, and so is not prior to it.’ 19 My existence at
the present instant cannot be due to my existence at the last instant any more than
it can itself bring about my existence at the next instant. In sum, there can be no
causal connections between instants, so the reason for everything must be sought
within the instant: 20 and since no such powers are evident in bodies, they must be
located in God. Such an inert corporeal world certainly contains none of the
powers that naturalists saw as being immanent in nature, it is not a world in
which God could be immanent, and it is, for Descartes and virtually all of his
contemporaries (Hobbes and Gassendi being possible exceptions), a world quite
distinct from what reflection on ourselves tells us is constitutive of our natures,
which are essentially spiritual. And it also has another important feature: a world
without forces, activities, potentialities and even causation is one which is easily
quantifiable.
This brings us to the second ingredient in Descartes’s natural philosophy: his
commitment to quantitative explanations. This is often seen as if it were a
necessary concomitant of mechanism, but in fact mechanism is neither sufficient
nor necessary for a mathematical physics. Most mechanists in the early to midseventeenth
century offered mechanist explanations which were almost
exclusively qualitative: Hobbes and Gassendi are good cases in point. Moreover,
Kepler’s thoroughly Neoplatonic conception of the universe as being ultimately a
mathematical harmony underlying surface appearances could not have been
further from mechanism, yet it enabled him to develop a mathematical account in
areas such as astronomy and optics which was well in advance of anything else
at his time. But it cannot be denied that the combination of mechanism with a