René Descartes (1596-1650)
Principia philosophiæ
Amsterdam: Elzevier, 1644

We know from dated manuscripts how thorough was Newton’s reading of Descartes’ works, whose theory of circling ‘vortices’ in space was in the mid-seventeenth century at the forefront of ideas about the explanation of the orbital motion of the planets. Descartes gave an explanation of ‘why’ the planets were in orbit.

Forty years after this publication, Newton pointedly chose his title Philosophiæ naturalis principia mathematica, the ‘Mathematical principles of natural philosophy’, as a distinction between his work and Descartes’s simple Principles of philosophy. Newton’s theory gave the mathematics of ‘how’ gravitational force acts but we had to await Einstein for an explanation of ‘why’ gravity operates.

M.5.26(1), p. 124