Descartes's conception of matter changed the account of physical nature in terms of extension and related quantitative terms. Plants and animals were turned into species of machines, whose natural functions can be explained mechanistically. This article reflects on the consequences of this transformation for the psychology of human soul. In so far the soul is rational it lacks extension, yet it is also united with the body and affected by it, and so it is able to act on extended matter. The article examines Descartes's concept of scientia and his different uses of nature, and argues that there is much more continuity between Aristotelian and Cartesian psychology than is usually recognized when it comes to an explanation of the functions of the embodied human soul. If this makes psychology unfit for inclusion in the new science of nature, its object is still a natural phenomenon and has an important place within scientia as Descartes conceived of it.