The philosophy of primary and secondary qualities is in a state of some confusion. There is no agreement as to the basis upon which the two classes of quality may be distinguished—a host of features, as diverse as perceptible by more than one sense and belonging to the definition of matter, are offered as the mark of the primary. There is not even agreement on which qualities belong to which group. Shape, size and solidity are generally held to be primary, while colours, smells, and the like (I) are favoured secondary candidates. But for large numbers of qualities, for example being acidic, malleable, rust-proof—or, among perceptible qualities, glistening and vibrating—we are offered no effective guidance.
Inevitably, in such a situation, we are without clear answers to the questions; Why should any distinction be made between primaries and secondaries? Must all qualities be the one or the other? To the solution of which problems does the distinction serve as a preliminary step? What special relationship is there between primary qualities and scientific theory, or between secondary qualities and peculiarities in perception?