Descartes: A Biography

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 Descartes: A Biography

or pronounce certain words,’ relied on a complex but hidden hydraulic

system of pipes. Descartes invites his readers to imagine that the human

body is similar, with animal spirits substituted for water as the dynamic

fluid. Despite the connotations of the name, animal spirits were nothing

more than ‘a very lively and very pure flame’ (xi.), that is, a type of

subtle matter that was similar to the matter found in flames.

This subtle matter was generated in the heart from blood and was

then distributed throughout the body by circulation through the veins.

ForDescartes, all those features of animals that included movement and

that required an explanation – such as the transmission of sensations to

the brain and the responses that they trigger, and the controls exercised

automatically by any animal over its own body – are explained by variations

in the movement of animal spirits throughout the body.

I want to tell you first about the composition of the nerves and the muscles, and to show

youhow,from the sole fact that the spirits in the brain are ready to enter into certain

nerves, they have the power to move particular bodily parts at the same instant. Then,

after touching briefly on respiration and other similar, simple, and normal movements,

I shall say how external objects act upon the sense organs. After that, I shall explain

in detail all that happens in the cavities and pores of the brain, what route the animal

spirits follow there, and which of our functions this machine can imitate by these

means. (xi.)

With this plan in place, Descartes offers schematic explanations of how

the muscles move and thereby control bodily movements, how animals

breathe, how they swallow food and drink and convert them to nutrients,

and how ‘this machine is able to sneeze, yawn, cough, and to cause the

motions needed to expel various excretions’ (xi.).

The same model can explain how ‘external objects that strike the sense

organs can cause the machine to move its members’ (xi.), and how

the machine of the body can register these sensations in the brain. The

mechanical explanations of hearing, feeling, smelling, and so on were

sketched only in outline. In the case of sight, however, the work done in

the years immediately prior toprovided enough detail to hypothesize

how an impression is formed on the back of an animal’s eye by the optic

lens, and how this pattern is communicated to a part of the brain in which

inputs from various sensory organs are synthesized. This synthesizing

feature of the brain was called the ‘common sense’, in deference to its

function, although its exact location was a matter of speculation.