Descartes' Dualism

I've been trying to dig through the beginning of the Meditations to find how Descartes necessitates Dualism as opposed to a physical materialism or some other system that's not dualistic, but I can't figure out a way that he does. Does anyone see something I don't? Specifically within the first meditation.

Here is the blog for Modern Philosophy Fall 2015.

We will us this for additional discussion since class sessions are short and we are many.

I will count use of this blog as class participation.