  Intelligibility is at the heart of the revival of the innateness doctrine in the modern period. From the very start of the Western tradition, it has been widely accepted that we not only sense the world, but that we also make sense of it; that it is not only sensible but also intelligible. Platonism explains intelligibility in terms of the innate forms that we are reminded of by sense-experience. Aristotelianism explains intelligibility in terms of a richer theory of perception – viz., it holds that we receive the intelligible nature of things from the things themselves. The scientific revolution of the 17th century, with its stress on the distinction between primary and secondary qualities and its materialist conception of the physical world, undercut the Aristotelian view of perception completely. The problem can be illustrated with the new account of vision. What happens when we see, according to the new scientific picture, is that light bouncing off an object carries a pattern of motion to the eye. These motions cause motions in the brain, and (somehow) conscious visual experience results. How can we explain the richness of our conception of the world given that only motions reach the sense organs? There seems to be no natural way to incorporate the transmission of Aristotelian intelligible forms into this account, so the problem of intelligibility returns. In the Second Meditation, Descartes exposes this gap between what the senses receive and what the mind knows and understands. Considering a piece of wax, which presents itself to us as a contingent set of actual and possible sensory images, he asks how we come to understand it in the way scientists must – as an inherently colorless, odorless, persistent object with an underlying nature that is subject to mathematical laws? The answer, for Descartes, is that we all have an abstract, non-sensory idea of a physical object. Sense-perception makes it possible for us to ‘fill out’ this abstract idea with the contingent details of our actual situation. But intelligibility, our most general understanding of the world – of physical objects, of space, of causality, of God, and so on – is grounded in these abstract ideas. But where do these general abstract ideas come from, given the ‘poverty’ of sense? Nativism now seems an attractive answer. We must come into the world pre-loaded with the categories (concepts, principles, general ideas, etc) that will enable us to make sense of what we actually see, hear, etc. Reason can mine this innate endowment to arrive at an apriori understanding of things.