The Cartesian philosopher Franz Brentano was Freud's teacher when Freud was a university student. Recent scholarship has clarified Brentano's theory of the mental and illuminated the surprisingly extensive relationship between Freud and Brentano, provoking renewed assessment of Brentano's influence on Freud. In this chapter, I reconstruct Brentano's argument that intentionality is the essence of both consciousness and the mental, and I identify the central tenets of Brentano's version of Cartesianism that confronted Freud. I analyze Brentano's primary argument against unconscious mental states based on his theory of the self-reference of intentional states and conclude that it is invalid and was properly ignored by Freud.