PHYSICS gravity was something above and beyond the mere motion of extended matter which was all that was allowed in Cartesian physics Descartes had considered the notion of attraction between bodies, but had rejected it as too like Aristotelian Wnal causes, and as involving the attribution of consciousness to inert masses What is it, Newton asks, that glues together the parts of homogeneous hard bodies? Descartes tells us that it is nothing but lack of motion; Gassendi talks of the hooks and eyes of atoms The Wrst answer explains nothing; the second merely puts the question back ‘I had rather infer from their cohesion’, Newton said, ‘that their particles attract one another by some force, which in immediate contact is exceedingly strong.’ It was this same power of attraction which, operating upon bodies not in immediate contact, was the force of gravity Was this then a case of action at a distance? At Wrst Newton denied this; but by the time of his Opticks (1706) he seemed to be willing to accept that gravity, magnetism, and electricity were indeed forces or powers by which the particles of bodies could act at a distance He seems to have remained agnostic whether the laws that he had discovered could eventually be explained without appeal to action across a vacuum— e.g by the postulation of some medium such as an aether.3 By accepting the existence of forces in nature which may, for all we know, have no explanation in terms of matter and motion, Newtonian physics made a complete break with the mechanism of Descartes And by bringing under a single law not only the motion of falling bodies on earth, but also the motion of the moon round the earth and the planets round the sun, Newton put to rest for ever Aristotle’s idea that terrestrial and celestial bodies were totally diVerent from each other His physics was quite diVerent from the competing systems it replaced, and for the next two centuries physics simply was Newtonian physics The Labyrinth of the Continuum The separation of physics from the philosophy of nature, set in train by Galileo, was now complete However, Newton left one problem for philosophers to chew upon for a century or more: the nature of space On See Steven Nadler, ‘Doctrines of Explanation’, in CHSCP, pp 342–6 174