^{page 53 note 1} “Simple natures” axe not ideas but essential ‘ontal’ elements, constitutive and explanatory, presupposed throughout the whole of Descartes's Metaphysics and Natural Philosophy. The theory of representative perception is, indeed, indirect evidence of this, for, eliminate simple natures, and there is nothing left to be represented in distinct idea, or misrepresented in confused ones. Commentators appear to think it sufficient to discuss simple natures when treating of his Method, and omit all reference to them thereafter. This seems to me regrettable, even though they but follow Descartes's example. They are mentioned in no other work than the Regulae. Yet the doctrine of innate ideas and the epistemology of the Meditations presuppose the existence of simple natures. Descartes's failure to carry them explicitly right through his metaphysic and to work out his theory of them in sufficient fulness seems to be mainly due to his later interest being dominated by difficulties of an epistemological rather than an ontological character.

^{page 58 note 1} The ultimacy claimed for this metaphysical conclusion has, for its epistemological counterpart, the view that the sciences of pure mathematics and rational mechanics are parts of Philosophy. The facts that the laws of these sciences are adequate to explain are not phenomenal, but ‘ontal’; to know these laws is thus to know the ultimate constitution of one part of the universe. Descartes's position is not that of an ‘epistemological idealist’, for though he holds that we are never directly apprehensive of Nature (in perception or in conception), there is a Nature independent of those ideas for its character and existence, to which they refer. The fact that there exists an independent Nature in this sense it was one of the principal aims of his metaphysic to establish.