Descartes: A Biography

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 Descartes: A Biography

This was a key text to which he returned, years later, in a very public

controversy between Regius and his university rector.Descartes did

not really believe that his approach to explanation was compatible with

that of scholastic philosophers, a point that became clear in subsequent

discussions and was obvious even to his earliest readers. Here he is merely

trying to avoid controversy, but the seeds of future acrimonious disputes

are barely hidden in this somewhat wishful peace token.

Much of what is explained in theMeteorsderives from specific expe-

riences that are dated by Descartes himself. Thus he refers to the shapes

of snowflakes that he observed in Amsterdam, during the winter of

(vi.); to the avalanches he observed when crossing the Alps in May

(vi.); to the coronas around a lighted candle that he had recently

seen while crossing the Zuiderzee (vi.–); and to parhelias observed

in Rome in March(vi.). It was not merely a coincidence that

Descartes tried to explain various phenomena that he had witnessed him-

self. He often argued that observations made by others are unreliable;

accordingly, he conceded in theMeteorsthat he could speak only conjec-

turally about ‘things that happen on the oceans, which I have never seen

and about which I have only very imperfect descriptions’ (vi.). Apart

from explaining very unusual phenomena, therefore, he also constructed

explanations of familiar things such as clouds, winds, rainfall, thunder,

and so on.

The focal point of theMeteors,however, was the explanation of the rain-

bow that presupposed the sine law of refraction presented in theDioptrics.

Descartes acknowledged some of the familiar features of rainbows that

require explanation, for example, that they are bow-shaped, that they

occur at a definite angle in the sky relative to the observer, that there is

often a secondary bow that is less bright than the primary bow, that the

colours appear in reverse order in the two bows, and so on. The explana-

tion he offered relied on an assumption that light is a tendency to move

imparted by the Sun to the subtle matter that fills up all the apparently

empty spaces between larger parts of matter, and that our perception of

colours results from the different ways in which those subtle particles

modify their spins when they tend to pass from one medium to another.

Even though this ‘assumption’ was very wide of the mark, Descartes could

still successfully calculate the ways in which light is refracted as it passes

through raindrops, because that depended only on the refractive index

of the rain rather than on any more general theory of what light is. In an

ingenious suggestion, Descartes explains how the two bows appear. If light