METAPHYSICS For the Aristotelians, there were many diVerent kinds of substances, each speciWed by a particular substantial form—humans by the form of humanity and so on According to Descartes there were no such things as substantial forms, and there were only two kinds of substance: mind, or thinking substance, and body, or extended substance These did not have substantial forms, but they did have essences: the essence of mind was thought and the essence of body was extension How particular substances of these two kinds are individuated remains unclear in Descartes’ system, and in the case of body he sometimes writes as if there was only one single, cosmic, substance, of which the objects we encounter are simply local fragments engaging in local transactions (AT VIII.54, 61; CSMK I.233, 240) The Aristotelians believed that substances were visible and tangible entities, accessible to the senses, even though it took the intellect to work out the nature of each substance When I look at a piece of gold, I am genuinely seeing a substance, though only science can tell me what gold really is Descartes took a diVerent view ‘We not have immediate awareness of substances,’ he wrote in the Fourth Replies, ‘rather, from the mere fact that we perceive certain forms or attributes, which must inhere in something in order to have existence, we name the thing in which they exist a substance’ (AT VII 222; CSMK II.156) So substances are not perceptible by the senses—not only their underlying nature, but their very existence, is something to be established only by intellectual inference Locke took much further the thesis that substances are imperceptible The notion of substance, he says, arises from our observation that certain ideas constantly go together If, to some idea of substance in general, we join ‘the simple Idea of a certain dull whitish colour, with certain degrees of Weight, Hardness, Ductility and Fusibility, we have the Idea of Lead’ The idea of any particular kind of substance always contains the notion of substance in general; but this is not a real idea, certainly not a clear and distinct one, but only a ‘supposition of we know not what support of such qualities which are capable of producing simple Ideas in us; which are commonly called Accidents’ (E, 295) The operative part of our idea of a distinct kind of substance, then, will be a complex idea made up of a number of simple ones The idea of the sun, for instance, is ‘an aggregate of those several simple Ideas, Bright, Hot, Roundish, having a constant regular motion, at a certain distance from us, and, perhaps some other’ (E, 299) The ideas of kinds of 188