Every student of philosophy knows how to draw up the lists of primary and secondary qualities: on the left go extension, size, shape or figure, solidity, motion or rest, and number; on the right go color, sound, scent, taste, heat and cold. But what is the principle of the distinction? Does it have to do with objective versus subjective? Categorical versus dispositional? Intrinsic versus extrinsic? Or several or none of these?
To summarize this section, the basis of the primary/secondary quality distinction for Reid cannot be that primary qualities reside in objects and secondary qualities do not (as Berkeley’s Hylas has it), since both sets of qualities reside in objects. Nor can it be that primary qualities resemble their associated ideas or sensations and secondary qualities do not (as Locke has it), since in neither case is there any resemblance. So what, if anything, is the basis for the distinction?
– James Van Cleve, Problems from Reid