We have seen that empiricism holds two theses: all ideas and beliefs are acquired; and all knowledge of real existence rests on experience. The first is an alternative to Descartes's innatism and the second a claim about the source of evidence about reality. The first three chapters focused on the major historical empiricists: Locke, Berkeley and Hume. We discussed Locke's defence of both theses and evaluated some of Leibniz's criticisms of his rejection of innatism. Descartes and Locke were reacting against the Aristotelian-medieval conception of science in light of the new science initiated by Galileo, Descartes himself and the English scientists, Newton and Boyle. In Chapter 2, we discussed Berkeley's claim that Locke's physical realism leads to skepticism and has to be replaced by a radically new metaphysics, idealism or what he labelled immaterialism. Much of the discussion centred on his criticisms of Locke and so was an extension of Chapter 1. The chapter on Hume sketched his attempt to elaborate Locke's theory of ideas as the foundation of a new science of human nature in his early book, A Treatise of Human Nature, but focused on his more strictly epistemological work, An Enquiry Concerning Human Understanding. We considered his defence of empiricism's second thesis in terms of his distinction between relations of ideas and matters of fact, his argument that induction cannot be justified non-circularly but must be assumed to be reliable and his theory of the nature of causation.