Abstract:  This paper argues that Locke has a representative theory of sensitive knowledge. Perceivers are immediately aware of nothing but sensory ideas in the mind; yet perceivers think of real external substances that correspond to and cause those ideas, and they are warranted in believing that those substances exist (at that time). The theory poses two questions: what warrants the truth of such beliefs? What is it in virtue of which sensory ideas represent external objects and how do they make perceivers think of those objects? Both the epistemic and semantic issues need to be addressed. This paper urges that Locke's basic account of warrant is roughly reliabilist. The causal origin of sensory ideas assures that, in general, sense based beliefs are true. Locke defines the limit of this warrant by the theoretical point that we cannot discuss skeptical doubt without assuming the truthfulness of our perceptual faculty. Turning to the semantic question, the paper argues that ideas are mental modifications or entities. They are not intrinsically representative (satisfiable), but rather represent only by virtue of their causal origin. They merely “track” the presence of substances and their qualities. Ideas nevertheless prompt perceivers to think of their causes. This is roughly because sensory ideas have a specific mental role, namely, to serve as marks for distinguishing substances and their respective qualities for purposes of action. The paper suggests that, for Locke, the challenge posed by the semantic veil of ideas is to explain this externally directed marking function within bounds of his anti-innatism. But it concludes that his answers to the twin questions fit together reasonably well.