Locke seems to hold that we have knowledge of the existence of external objects through sensation. Two problems face Locke's account. The first problem concerns the logical form of knowledge of real existence. Locke defines knowledge as the perception of the agreement or disagreement between ideas. However, perceiving agreements between ideas seems to yield knowledge only of analytic truths, not propositions about existence. The second problem concerns the epistemic status of sensitive knowledge: How could the senses yield certain knowledge? This (...) essay argues that the key to solving both problems is Locke's claim that simple ideas are all real, adequate, and true. This explains why, on Locke's view, we have certain knowledge through the senses. It also explains how sensitive knowledge can consist in perceiving an agreement between ideas: perceived agreements among simple ideas of sensation are guaranteed to track relations that obtain among really existing things. (shrink)