Peacocke's characterization of what he calls implicit conceptions recognizes the significance of a subset of contentful states in making rational behavior intelligible. What Peacocke has to offer in this paper is an account of (i) why we need implicit conceptions; (ii) how we can discover them; (iii) what they explain; (iv) what they are; and (v) how they can help us to better understand some issues in the theory of meaning and the theory of knowledge. The rationalist tradition in which Peacocke's project ought to be located is concerned with the nature of understanding. His notion of implicit conceptions is invoked to explain non-straightforwardly inferential but rational patterns of concept-involving behavior. We come to know about implicit conceptions because we treat the thinker's practices as having a certain representational content. They are implicit in what the thinker does.