John Locke’s silence on so many of the questions and issues central to early modern theorizing about the role that causation plays in cognition is striking. Daniel Garber suggested in conversation, that Locke’s setting aside the physical consideration of the mind is not really puzzling at all, since Locke is engaged in a different sort of project. Locke’s largely a priori account of how sensation works is justified by the functional role that sensation is intended to fill. This allows him to show that sensation, conceived in the right way, must be reliable, in a way that entirely bypasses any sort of physiological story about how the senses operate. The chapter explains how and why Locke argues that our sensation is reliable in a way that bypasses entirely any sort of physical or physiological story about how our senses work.