Mechanism, resemblance and secondary qualities: From Descartes to Locke

Locke’s argument for the primary-secondary quality distinction is compared with Descartes’s argument (in the Principles of Philosophy) for the distinction between mechanical modifications and sensible qualities. I argue that following Descartes, Locke’s argument for the primary-secondary quality distinction is an essentially a priori argument, based on our conception of substance, and the constraints on intelligible bodily interaction that this conception of substance sets.