Descartes: A Biography

C CUNYB/Clarke     December, :

AFabulous World (–) 

movement that is once impressed on some body remains in it perpetually,

unless it is taken away by some other cause; that is, [in Latin]whatever

begins to move in a vacuum is moved for ever with the same speed’ (i.–). It

might seem trivial, in retrospect, that Descartes adopted this as a primary

insight of his physics. However, that would seriously underestimate the

significance of his discovery. Philosophers had tried to explain for centuries

why bodies in motion continue to move as they do or, more usually, to slow

down. They had speculated that bodies in motion acquired an impetus

that continued to push them along from one moment to the next, or

that they displaced the air in front of them in such a way that it rushed

around behind them, nudging them forward incrementally. By adopting a

principle of inertia, Descartes radically changed the focus of the question.

Rather than ask: why does a body in motion continue to move? he asked:

since bodies in motion remain naturally in motion, why do they slow

down or otherwise change their condition? This change in perspective

represented a revolutionary change in physics, the significance of which

was minimally illustrated by the linguistic change in Descartes’ letter.

Instead of trying to explain why bodies in motion continue to move, it was

assumed that a continuation of motion was ‘natural’, and that the reality

that required explanation was any change in motion, in either its quantity

or direction.

By December, Descartes was already expressing premature anxiety

about publication of his treatise in the making. He first decided to with-

hold his name as the author from the title page, and to get advice from

Mersenne and other reliable friends about its contents. The main source

of worry was not his physical theories, but their possible implications for

theology:

I wish to do this principally because of theology, which has been so ruled by Aristotle

that it is almost impossible to explain any other philosophy without it seeming, initially,

to be contrary to the faith. In this context, please tell me if there is anything decided in

religion about the extension of created things, namely, whether it is finite or infinite, and

whether there are created and real bodies in all those regions that are called imaginary

spaces. Although I did not wish to discuss that question, I think nevertheless that I

will be forced to investigate it. (i.–)

Apart from theological concerns, Descartes signalled other reasons for a

potential delay in completing the ‘little treatise.’ One reason was that he

had decided to widen its scope even more and ‘to begin studying anatomy’