Imaginary spaces

STEVEN NADLER

In November 1633, the French philosopher René Descartes, now living in the Netherlands, was searching in the bookstores of Amsterdam and Leiden for a copy of Galileo’s recently published Dialogue on the Two Chief World Systems. Descartes’s friend Marin Mersenne, a Minim friar in Paris who was the centre of a wide European intellectual network, had explained to him Galileo’s view that all bodies fall at the same rate, regardless of their material or weight (and assuming their fall is not affected by the friction of any medium). Descartes replied to Mersenne that this cannot be right. “According to my philosophy there will not be the same relation between two spheres of lead, one weighing one pound and the other a hundred pounds, as there is between two spheres of wood, one weighing one pound and the other a hundred pounds . . . he [Galileo] makes no distinction between these cases, which makes me think that he cannot have hit upon the truth.”
Descartes’s interest was piqued, however, and he wanted to know what Galileo had to say about the ebb and flow of the tides, which he knew was also discussed in the Dialogue. Mersenne must also have filled Descartes in on the most important and controversial feature of the Dialogue: a detailed account and defence of the Copernican cosmology. Galileo’s views on this topic were of immediate relevance to Descartes’s own current project, a treatise titled Le Monde in which he would explain “all the phenomena of nature” and which also included a heliocentric account of the cosmos.