This chapter explores some aspects of Descartes' reactions to materialism. It argues and turns on a certain feature of the doctrine of substance that Descartes presented in the Meditations and in the Appendix to the Second Replies. Descartes suggests that there are two kinds of substances, thinking substances and extended substances, which support two kinds of faculties: faculties for certain special ways of thinking and faculties that involve changing position, taking on various shapes. Descartes' reply to the portion of Hobbes' objections that relate to Meditation II are rather too long to discuss in their entirety. Descartes examines the different kinds of accidents (actus) that pertain to finite substances. Descartes' reply to Antoine Arnauld involves an interesting elaboration of his notion of substance, indeed, important changes over what he had written in earlier texts. Descartes radically revised his view of the nature of substance.