Gravity

Summary

Descartes’ theory of gravity, which is an integral component of his vortex theory of planetary motion, proved to be one of his most important contributions to natural philosophy in the early modern period. The “Cartesian” account, which includes the many hypotheses adapted from Descartes’ views by later Cartesians, was the dominant theory of gravity toward the end of the seventeenth century, especially on the European continent (see Cartesianism). However, Cartesian gravity subsequently relinquished its leading role to Newton's theory, and by the mid-eighteenth century it had largely been abandoned.

At the beginning of the seventeenth century, the Aristotelian-based Scholastic explanation of gravity still held sway among natural philosophers, an account that categorized “heaviness” as a substantial form – that is, as a sort of internal, goal-oriented property of a body (see form, substantial). In the Sixth Replies, Descartes acknowledges that he had earlier accepted a similar view of gravity, an account that he now criticizes as falsely derived from the mental properties of the human mind: “What makes it especially clear that my idea of gravity was taken largely from the idea I had of the mind is the fact that I thought that gravity carried bodies towards the centre of the earth as if it had some knowledge of the centre within itself. For this surely could not happen without knowledge, and there can be no knowledge except in a mind” (AT VII 442, CSM II 298). Descartes developed his mechanical explanation of gravity, which relies only on the extension and motion of bodies, in large part to counter this misleading conception. As he claims in The World (1633), all of the “forms of inanimate bodies can be explained without the need to suppose anything in their matter other than motion, size, shape, and arrangement of its parts” (AT XI 25–26, G 18).