Abstract: Cartesian substances are ultimate subjects of predication, not things in themselves (substrata) apart from their properties. Our knowledge of substances is limited to knowledge of their qualities. Since the concept of a material substance cannot explain anything about the properties of material things, so likewise the concept of an immaterial substance cannot explain anything about the mind or God. Substances may be combined if their properties are compatible.

Keywords: immaterial substance, material substance, predication, substance, substratum,