Locke believes that every mental act, event or state essentially involves ideas and that our only access to the world is through ideas. The theory of ideas, thus, is central to his philosophy, and understanding the epistemological and metaphysical positions developed in the Essay requires addressing it. This chapter discusses the importance of recognizing the theory's status as a proposed contribution to natural science. It emphasizes that ideas are representational contents of states of consciousness; this does not imply that they are something distinct from those states; rather, they are those aspects or features of mental states which point beyond themselves. The chapter focuses on Locke's thesis that all of one's ideas are derived from experience. It highlights that the operation of comparing is dependent upon the brute ability of discernment, one of the simplest mental operations.