 In his eventual argument, Descartes’ strategy for proving an external material world has two main parts: first, he argues for the externality of the causes of sensation; second, he argues for the materiality of these external causes. (Calling the ideas in question “sensations” can seem circular, if one conceives of sensations as having a physiological component. But note that for Descartes, “what is called ‘having a sensory perception’,” strictly encompasses only a mental aspect. Med. 2, AT 7:29, CSM 2:19) From these two steps, it follows that there exists an external material world. Let’s consider each phase of the argument.