Locke on Primary and Secondary Qualities

In Book II, Chapter viii of the Essay, Locke develops a distinction, borrowed in its essentials from the "mechanical philosophy" of Robert Boyle and, more distantly, from Galileo Galilei's Il Saggiatore (1623), between primary and secondary qualities of bodies. Primary qualities — solidity, extension, figure (shape), motion or rest, and number — are, Locke argues, utterly inseparable from a body in whatever state it is, and our ideas of them genuinely resemble qualities that actually exist in the object itself: a grain of wheat, however finely divided, retains solidity, extension, figure, and mobility down to its smallest imaginable parts. Secondary qualities — colors, sounds, tastes, smells, and tactile qualities such as warmth — are, by contrast, nothing in the objects themselves but powers to produce various sensations in us by means of the primary qualities of their imperceptibly small, insensible parts, chiefly their bulk, figure, texture, and motion. On this view, the idea of red that a ripe tomato produces in a perceiver does not resemble any actual redness residing in the tomato; the tomato possesses only a microstructural arrangement of primary qualities capable of causing that sensation in a suitably constituted observer.