Descartes famously claims that mind and body are really distinct substances, a view known as substance dualism (AT VII 78, 169–70; CSM II 54, 119–20). This claim is part of a larger theory of distinctions, which has both a metaphysical and an epistemological side. On the one hand, it explains what kinds of entities there are in the world and how they are related to each other; on the other, it also spells out how we can gain knowledge of these relations and how we are thereby able to provide an adequate description of the world – a description that fits not only human beings but also purely material substances (e.g., plants and animals) and purely mental substances (God and angels) as well as their attributes and modes.

The theory of distinctions has Scholastic roots. It was originally developed to resolve various theological puzzles (e.g., the connection between the three divine persons and the relation between God's essence and his attributes) but soon became a general instrument for distinguishing different types of metaphysical relations (Adams 1987, 16–29). Descartes came to know it through late Scholastic authors, among them Francisco Suárez and Eustachius a Sancto Paulo (Ariew 2011), and used it primarily in a nontheological context (for an application to some theological problems, see Armogathe 1977). On his view, which he sets forth in detail in Principles I.60–62, there are three types of distinction.

First, there is a real distinction. It obtains between two or more substances, and we can recognize it “from the fact that we can clearly and distinctly understand one apart from the other” (AT VIIIA 28, CSM I 213). This is precisely the distinction that exists between mind and body, but also between two (or more) minds and, should there be more than one, between two (or more) bodies (see individuation and substance).