 In the fourth book of An Essay Concerning Human Understanding Locke tells us what knowledge is and what humans can know and what they cannot (not simply what they do and do not happen to know). Locke defines knowledge as “the perception of the connexion and agreement or disagreement and repugnancy of any of our Ideas” (IV.1.1, N: 525). This definition of knowledge contrasts with the Cartesian definition of knowledge as any ideas that are clear and distinct. Locke’s account of knowledge allows him to say that we can know substances in spite of the fact that our ideas of them always include the obscure and relative idea of substance in general. Still, Locke’s definition of knowledge raises in this domain a problem analogous to those we have seen with perception and language. If knowledge is the “perception of … the agreement or disagreement … of any of our Ideas”—are we not trapped in the circle of our own ideas? What about knowing the real existence of things? Locke is plainly aware of this problem, and very likely holds that the implausibility of skeptical hypotheses, such as Descartes’ Dream hypothesis (he doesn’t even bother to mention Descartes’ malin genie or Evil Demon hypothesis), along with the causal connections between qualities and ideas in his own system is enough to solve the problem. It is also worth noting that there are significant differences between Locke’s brand of empiricism and that of Berkeley that would make it easier for Locke to solve the veil of perception problem than Berkeley. Locke, for example, makes transdictive inferences about atoms where Berkeley is unwilling to allow that such inferences are legitimate. This implies that Locke has a semantics that allows him to talk about the unexperienced causes of experience (such as atoms) where Berkeley cannot. (See Mackie’s perceptive discussion of the veil of perception problem, in Problems from Locke, 1976: 51 through 67.)