MIND AND SOUL The notions of extension and motion, Leibniz went on to argue, were not as distinct as Descartes thought: the notions of these primary qualities contained a subjective element no less than secondary qualities such as colour and heat This was a theme later to be developed by Berkeley.6 Leibniz had two main arguments against the identiWcation of matter with extension First, if there were nothing in matter but size and shape, he argued, bodies would oVer no resistance to each other A rolling pebble colliding with a stationary boulder would put the boulder into motion without losing anything of its own force Second, if matter was mere extension, we could never identify individual bodies at all, for extension is inWnitely divisible At whatever point we stop in our division we meet only an aggregate—and an aggregate (e.g the pair formed by the diamond of the Great Mogul and the diamond of the Grand Duke) is only an imaginary object, not a real being Only something resembling a soul can confer individual unity on a body and give it a power of activity (D, 21; G II 97) For these reasons Leibniz felt compelled to re-admit into philosophy the substantial forms which were so despised by fashionable philosophers, and he adopted a name for them which advertised their Aristotelian origin, namely ‘entelechy’ But he diVered from contemporary Aristotelians in two ways First, he thought that while substantial forms were necessary to explain the behaviour of bodies, they were not suYcient; for the explanation of particular phenomena one must have recourse to the mathematical and mechanical theories of current corpuscular science If asked how a clock tells the time, he said, it would be futile to say they had a horodictic faculty rather than explaining how the weights and wheels worked (D, 10; G V 61) Second, he thought that in a human being there was not just one substantial form but an inWnite number: each organ of the body had its own entelechy, and each organ was, he told Arnauld, ‘full of an inWnite number of other corporeal substances endowed with their own entelechies’ (G II.120) The great gap which Leibniz saw in Descartes’ system was the lack of the notion of force ‘The idea of energy or virtue,’ he wrote in 1691, ‘called by the Germans Kraft and by the French la force, to explain which I have projected a special science of dynamics, throws a lot of light on the true See above, p 147 232