Augustine famously employs cogito-style reasoning analogous to that later found in Descartes in order to defeat global scepticism. This has been the subject of substantial scholarly attention. However, what has received less attention is that Augustine, like Descartes, also employs cogito-style reasoning to ascertain the nature of the mind and to argue against materialist theories of mind. In this paper, I examine Augustine's central argument to this end in De Trinitate 10.