The secondary qualities are those qualities of objects that bear a certain relation to our sensory powers: roughly, they are those qualities that we can readily detect only through a certain distinctive phenomenal experience. Contrary to what is sometimes supposed, there is nothing about the world itself (independent of our minds) that determines the distinction between primary and secondary qualities. Instead, a theory of the secondary qualities must be grounded in facts about how we conceive of these qualities, and ultimately in facts about human perception.