||Berkeley was an early defender of a relational conception of space and time. In his 1709 Essay Toward a New Theory of Vision, Berkeley defended what has come to be known as the Heterogeneity Thesis, which states that there are no ideas common to two sense modalities. An important corollary, which Berkeley himself emphasizes, is that, contrary to Descartes and Locke, there is no one idea of extension which is to be found both in vision and in touch. Instead, Berkeley argued, visible distance (or magnitude) and tangible distance (or magnitude) are two entirely different features of our perception which we learn by experience to correlate with one another. Visual distance is a matter of how far apart two features on the visual field are; tangible distance is a matter of how far one must walk (or move one's hand) to get from touching one object to touching another. In the Principles, Berkeley also gives a relational account of time as the succession of ideas in a mind. Berkeley's understanding of space, and its relation to Newtonian physics, are further developed in his 1721 De Motu (On Motion).