Abstract: In the Prolegomena, Kant says he is like Locke, except that he makes all the qualities secondary—a position which makes him look uncomfortably like Berkeley, if one takes secondary qualities to be mind-dependent. Elsewhere, Kant is a scientific realist, endorsing a primary–secondary quality distinction within the phenomenal world: phenomenal qualities include unobservable properties of e.g. magnetic matter, but exclude e.g. colours and tastes. But how can the qualities of phenomenal objects be exclusively secondary (the Lockean comparison), and exclusively primary (the scientific realism)? For Locke, primary qualities are mind-independent, and intrinsic, which suggests a new possibility. Qualities of phenomenal objects are secondary not because they are mind-dependent, but because they are relational powers: roughly, Locke's tertiary qualities.

Keywords: Berkeley, intrinsic, Kant, Locke, primary quality, Prolegomena, relational powers, scientific realism, secondary quality, tertiary quality, unobservability,