METAPHYSICS matter.3 As a consequence of this, thinkers like Aquinas had argued that if there were substances that were immaterial—angels, say—then there could be only one of each kind, since there was no matter to distinguish one member of a species from another Leibniz’s doctrine of individual notions or essences forced him to generalize this: all substances, and not just Aquinas’ angels, were unique specimens of their kind He argued that if there were in nature two beings indiscernible from each other, then God would act without suYcient reason in treating one diVerently from the other (G VII 393) Is the principle of the identity of indiscernibles itself necessary or contingent? Leibniz does not seem to have made up his mind Since, to establish it, he appeals to the principle of suYcient reason, not to that of non-contradiction, it appears contingent; and in a letter he wrote that it was possible to conceive two indiscernible substances, even though it was false to suppose they existed (G VII 394) In his New Essays, however, he says that if two individuals were perfectly alike and indistinguishable there would not be any distinction between them; and he goes on to draw the conclusion that the atomic theory must be false It was not enough to say that one atom was at a diVerent time and place from another: there must be some internal principle of distinction or there would be only one atom, not two (G V 214) Berkeley’s Idealism Leibniz’s philosophy is the Wrst systematic presentation since Antiquity of idealism, the theory that reality consists ultimately of mental entities, that is to say immaterial perceivers along with their perceptions During his lifetime another version of idealism was propounded by Bishop Berkeley The two systems resemble each other, but there are important diVerences between them: Leibniz’s idealism is a rationalist idealism; Berkeley’s is an empiricist idealism The diVerences arise from the diVerent starting points of the two philosophers Before comparing the systems in detail, therefore, we should follow the track of argument by which Berkeley arrives at his destination Vol II, pp 204–6 199