Routledge History of Philosophy Volume IV

200 RENAISSANCE AND SEVENTEENTH-CENTURY RATIONALISM
transmitted to and from the brain via the nervous system. Such ‘animal spirits’
were purely physical in character, operating in a way very analogous to that in
which gases or fluids are transmitted along systems of pipes and conduits. There
was no need to posit any internal principle such as a ‘nutritive’ or ‘sensitive’
soul in order to explain biological processes like digestion and growth; indeed,
the ordinary laws of matter in motion were quite sufficient to account even for
complex animal behaviour like pursuit and flight. 73 The ways in which the beasts
operate can be explained by means of mechanics, without invoking any
‘sensation, life or soul’; 74 and even in the case of humans,
we have no more reason to believe that it is our soul which produces the
movements which we know by experience are not controlled by our will
than we have reason to think that there is a soul in a clock which makes it
tell the time. 75
Reflection led Descartes to conclude, however, that there were severe limits on
the power of mechanical explanations when it came to accounting for the
characteristically human processes of thought and language. In the Discourse, he
argues that one could in principle construct an artificial automaton which was
indistinguishable from a dog or a monkey. But any such attempt to mimic human
capacities would be doomed to failure. A mechanical android, however complex,
would betray its purely physical origins in two crucial respects: first, it could
never possess genuine language, and second, it could never respond intelligently
to the manifold contingencies of life in the way in which humans do. The first of
these arguments, the argument from language, is a crucial weapon in Descartes’s
strategy of showing that human capacities are not just different in degree from
those of non-human animals, but are radically different in kind:
We can certainly conceive of a machine so constructed that it utters words,
and even utters words which correspond to bodily actions causing a change
in its organs (e.g. if you touch it in one spot it asks what you want of it, and
if you touch it in another it cries out that you are hurting it, and so on). But
it is not conceivable that such a machine should produce different
arrangements of words so as to give an appropriately meaningful answer to
whatever is said in its presence, as even the dullest of men can do. 76
The vital point here is that a mechanical system produces responses in
accordance with a fixed schedule: there is a finite number of possible responses,
each triggered by a specified stimulus. But genuine language is ‘stimulus-free’:
it involves the ability to respond innovatively to an indefinite range of
situations. 77 Hence it is ‘for all practical purposes impossible for a machine to
have enough different organs to make it act in all the contingencies of life in the
way in which our reason makes us act’. 78