PHYSICS vacuum, he also opposed the atomic hypothesis Matter, being identical with extension, must be inWnitely divisible, and there was no such thing as a void for the atoms to move about in Descartes sought to explain away the evidence for the existence of a vacuum that had been provided in 1643 by Evangelista Torricelli’s invention of the barometer The Atomism of Gassendi When Descartes published his Principles, atomism was being revived by Pierre Gassendi, on the model of the ancient theories of Democritus and Epicurus, whose ideas had recently become familiar to the learned world through the discovery and wide dissemination of Lucretius’ great Epicurean poem, De Rerum Natura.2 A Catholic priest, who held both a professorship of mathematics and the deanship of a cathedral, Gassendi sought to show that the philosophy of the pagan Epicurus was no more diYcult to reconcile with Christianity than was the philosophy of the pagan Aristotle Both pagan philosophers had erred in teaching that the world was eternal and uncreated; but from a philosophical point of view the explanation of physical phenomena in terms of the behaviour of atoms was to be preferred to an account in terms of substantial forms and real accidents Gassendi attacked Aristotle in his earliest treatise, and in a series of works between 1647 and his death in 1655 Gassendi defended not only the atomism, but also the ethics and character, of Epicurus Natural bodies, said Gassendi, following Epicurus, are aggregates of small units of matter These units are atoms, that is to say, they are indivisible They possess size, shape, and weight, and solidity or impenetrability These atoms, according to Gassendi, possess motion under the constant inXuence of the divine prime mover: they move in a straight line unless they collide with other atoms or get incorporated into a larger unit (which he called a ‘molecule’) All bodies of whatever size are composed of molecules of atoms, and the motions of atoms are the origin and cause of all motions in nature Philosophical objections against atomism, Gassendi argued, rested on a confusion between physics and metaphysics One could accept that any See vol I, pp 179–80 172