Descartes: A Biography

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 Descartes: A Biography

One of them, Henri Reneri (–),asked Descartes for his expla-

nation of the phenomenon, apparently in the hope of comparing it to

what had already been promised by Gassendi. This sequence of events

prompted Descartes to write to Mersenne, onOctober, asking for

confirmation of the observation:

My mind is not strong enough to study a number of different things at the same

time....Ihavetoconcentrate exclusively on one subject when I wish to examine any

part of it. This is what I found recently, in looking for the cause of the phenomenon

that you wrote to me about. For it is more than two months since one of my friends

showed me a reasonably adequate description of it and asked my opinion about it. I

had to interrupt what I was working on to examine, in order, all the meteors, before I

could satisfy myself about it. However, I think now that I can offer some explanation of

it, and I have decided to turn it into a little treatise which will include an explanation

of the colours of the rainbow – which cause me more trouble than everything else –

and generally of all sublunar phenomena. (i.)

This lengthy letter reflects the diversity of natural phenomena that

Mersenne had asked him to explain, and it includes one of the earliest

statements by Descartes of a principle of inertia.During the next three

years, Descartes wrote frequently to Mersenne about the extensive range

of topics to be included in his treatise, the slow progress he was making,

and the eventual reasons for suppressing its publication.

Within one month of writing to Mersenne about parhelia, Descartes

extends the scope of his project to ‘all the phenomena of nature’ and

expresses more enthusiasm about it than about any previous study:

Since I wrote to you a month ago, I have done nothing at all about it apart from

outlining its structure, and instead of explaining only one phenomenon I have decided

to explain all the phenomena of nature, that is, the whole of physics. I am more satisfied

with my plan than with any other that I have ever had, because I think I have found a

way of expressing my thoughts in such a way that they will satisfy some people while

no one else will have an opportunity to contradict them. (i.)

The extension in scope also meant that he would need more time for the

‘little treatise’ and that it would not be completed for ‘more than a year’

(i.).

One of the central principles on which the whole project relied was a

principle of inertia for bodies in motion. Although Descartes wrote this

letter to Mersenne in French, when he reached the point at which he

introduced a principle of inertia he switched languages to Latin, so that

the same idea is repeated in both languages. ‘First of all I assume that the