This paper examines Descartes's third primary notion and the distinction between different kinds of knowledge based on different and mutually irreducible primary notions. It discusses the application of the notions of clearness and distinctness to the domain of knowledge based on that of mind-body union. It argues that the consequences of the distinctions Descartes is making with regard to our knowledge of the human mind and nature are rather different from those that have been attributed to Descartes due to the influential Rylean picture of Cartesian mind-body dualism.