A key figure of the French 17th century, Mersenne studied, like Descartes, at La Flèche, and subsequently taught in Nevers and Paris. Mersenne was a correspondent of all the great mathematicians and scientists of the time, and was described by Hobbes as ‘the pole round which revolved every star in the world of science’. Mersenne’s principal philosophical preoccupation was the refutation of Pyrrhonian scepticism, and the correct acceptance and interpretation of scientific and mathematical knowledge. His attitude to science and mathematics is a forerunner of modern instrumentalism, for although the Pyrrhonist is right that knowledge of reality is unattainable, science suffices to serve as the guide of our actions. Mersenne’s writings include the La Vérité des sciences contre les sceptiques ou pyrrhoniens (‘The Truth of Science against the Sceptics or Pyrrhonians’, 1625), and he is the author of the 2nd and 6th sets of Objections to Descartes’s Meditations.