Routledge History of Philosophy Volume IV

206 RENAISSANCE AND SEVENTEENTH-CENTURY RATIONALISM
The notion that two different substances can unite to form a single thing is not, in
itself, obscure or problematic. We are familiar nowadays, for example, with the
idea that hydrogen and oxygen can unite to form water; furthermore, this
‘substantial union’ generates ‘emergent’ properties—water has properties such
as that of being drinkable which were not present in its constituent elements—
and this (though it is not of course Descartes’s own example) might be thought to
give some grip on the Cartesian notion that events like sensations emerge or
‘arise’ when mind and body are united, even though they are not part of the
essence of either res cogitans or res extensa. Nevertheless, Descartes himself
clearly felt that his notion of the ‘substantial union’ of mind and body presented
problems. For mind and body, as defined throughout his writings, are not just
different, but utterly incompatible substances: in terms of their essential
characteristics, they mutually exclude one another, since mind is defined as nonextended
and indivisible, whereas matter is by its nature extended and divisible.
And it is not easy to see how incompatible items can be, in any intelligible sense,
‘united’. As Descartes rather ruefully put it:
it does not seem to me that the human mind is capable of conceiving at the
same time the distinction and the union between body and soul, because
for this it is necessary to conceive them as a single thing and at the same
time to conceive them as two things, and this is absurd. 98
CAUSAL INTERACTION AND OCCASIONALISM
The idea of the union of utterly heterogeneous items is not the only problematic
feature of Descartes’s theory of the mind and its relation to the body. Descartes
frequently talks in a way which suggests both that the mind has causal powers
vis-à-vis the body (e.g. it can cause the body to move), and that the body has
causal powers with respect to the soul (e.g. passions and feelings are ‘excited’ by
corporeal events in the blood and nervous system). A great deal of Descartes’s
last work, the Passions of the Soul, is devoted to examining the workings of this
two-way causal flow between body and mind. The following is his account of
memory:
When the soul wants to remember something, this volition makes the
[pineal] gland lean first to one side and then to another, thus driving the
animal spirits [the tiny, fast moving particles which travel through the
nervous system] towards different regions of the brain until they come
upon the one containing traces left by the object we want to remember.
These traces consist simply in the fact that the pores of the brain through
which the spirits previously made their way owing to the presence of this
object have thereby become more apt than the others to be opened in the
same way when the spirits again flow towards them. The spirits thus enter
these pores more easily when they come upon them, thereby producing in