Descartes: A Biography

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 Descartes: A Biography

The most contentious issue that Descartes had to face, at the beginning

of Part III, was whether to offer any opinion about Galileo’s heliocentric

theory and, if so, how to phrase his views without risking the wrath of the

Catholic Church. He had very carefully avoided public discussion of this

topic since, and he continued to repeat his original explanation of his

silence – that he was waiting for signs of a change of mind in Rome. He

provides a context for the Cartesian solution by warning readers against

reducing God’s power, and the universe He created, to the limits of human

intelligence. He also rejects as ‘clearly ridiculous and inappropriate’ any

kind of anthropocentrism that would imply that God created the whole

universe only for human beings.

Descartes was convinced that, among the various models of the plane-

tary system, Galileo’s was most consistent with observational evidence

and with the laws of nature that he had outlined in Part II. He also

thought that it was not enough for an astronomical theory todescribethe

motions of the planets, that is, to say which planets move, what paths

they describe, and so on. It also had to provide some explanation of

howthe planets move or what makes them move as they apparently do.

Most of all, he wished to avoid conflict with Rome over its prohibition

onteaching that ‘the Sun is in the centre of the universe and is immo-

bile, and that the Earth moves’. He took care of this restriction easily.

According to the relativist definition of motion proposed in Part II, para-

graph,abody is said to be in motion when it is transferred from being

close to the bodies in its immediate vicinity to being in the vicinity of

other bodies.

According to Descartes, all the matter of the planetary system is whirling

about in a vortex around the Sun, and it is carried along by the vortex of

which it is a floating part. This allows him to ‘save the phenomena’ by

having the Sun at the centre and the planets in orbit around the Sun,

as Galileo claimed. It also explains how such large bodies move, without

invoking gravity or action at a distance. Finally, Descartes could maintain

that, despite its motion around the Sun, there is a genuine sense in which

each planet is at rest relative to the immediately contiguous matter that

moves in the same vortex as itself.

The conclusion of this convoluted redefinition of terms and of his

dynamical theory of planetary motion, he claimed, was that he had

‘resolved every scruple about the Earth’s motion’ and that ‘all the matter

of the heavens in which the planets are located turns constantly like a