Primary and Secondary Qualities

The distinction between primary and secondary qualities is the centerpiece of Locke's metaphysics in An Essay Concerning Human Understanding (1689). Primary qualities — extension, motion, figure, solidity, number — are the qualities that bodies actually have, independent of perceivers. Secondary qualities — color, sound, taste, smell, warmth — are not qualities of bodies as they are in themselves but powers in bodies to produce certain ideas in perceivers' minds.

The distinction had Galilean and Cartesian precedents and was central to the early-modern attempt to align metaphysics with the new mathematical physics. Berkeley's most famous argument against Locke is that the distinction collapses on Lockean principles: whatever reasons one has to treat color as mind-dependent apply equally to extension and motion. This argument propelled Berkeley toward his immaterialism and reshaped subsequent debates over realism, idealism, and the metaphysics of perception.

Locke's distinction was rooted in the new mathematical natural philosophy. Galileo had argued that the proper objects of natural science are the geometrical and mechanical properties of bodies; Boyle's corpuscular philosophy treated bodies as collections of geometrically characterized particles. Locke's distinction codified the underlying metaphysical commitment: secondary qualities like color and sound are powers in bodies (grounded in their primary qualities) to produce certain ideas in perceivers, not features of bodies as they are in themselves.

Berkeley's collapse argument has multiple parts. He argues that the same lines of reasoning Locke uses to relativize secondary qualities — they vary across observers, depend on the state of the perceiver, can be conjured by hallucination — apply equally to primary qualities. Apparent extension varies with distance and angle; apparent figure depends on the observer's perspective. If the variability of secondary qualities makes them mind-dependent, the same conclusion follows for primary qualities. Berkeley concludes that material substance, conceived as the bearer of mind-independent primary qualities, is incoherent.