  Descartes introduced this new theory of sensory qualities in the first six chapters of the World. There, he defended it by arguing that his explanation of qualities in bodies in terms of size, shape, and motion are clearly understood by comparison with the Aristotelian qualities (11:33). Subsequently, in the Meditations and Principles, he defended this account by appeal to the metaphysical result that body possesses only geometrical modes of extension. Real qualities are ruled out because they are not themselves instances of size, shape, or motion (even if patches of color have a size and a shape, and can be moved about).