Online research in philosophy

QUESTIONS Objects seem to have some properties in themselves (like shape), and some other properties that depend on other things around them (like being alone or accompanied). The distinction between primary and secondary qualities is a special case of this more general contrast: what, according to Locke, is the basis for the distinction? Is there more than one way to understand Locke’s argument: what is the best reading of Locke? What wider significance does the distinction between primary and secondary qualities have?

Locke’s argument for the primary-secondary quality distinction is compared with Descartes’s argument (in the Principles of Philosophy) for the distinction between mechanical modifications and sensible qualities. I argue that following Descartes, Locke’s argument for the primary-secondary quality distinction is an essentially a priori argument, based on our conception of substance, and the constraints on intelligible bodily interaction that this conception of substance sets.

John Locke’s distinction between primary and secondary qualities of objects has meet resistance. In this paper I bypass the traditional critiques of the distinction and instead concentrate on two specific counterexamples to the distinction: Killer yellow and the puzzle of multiple dispositions. One can accommodate these puzzles, I argue, by adopting Thomas Reid’s version of the primary/secondary quality distinction, where the distinction is founded upon conceptual grounds. The primary/secondary quality distinction is epistemic rather than metaphysical. A consequence of Reid’s primary/ secondary quality distinction is that one must deny the original version of Molyneux’s question, while one must affirm an amended version of it. I show that these two answers to Molyneux’s question are not at odds with current empirical research.

Reid offers an under-appreciated account of the primary/secondary quality distinction. He gives sound reasons for rejecting the views of Locke, Boyle, Galileo and others, and presents a better alternative, according to which the distinction is epistemic rather than metaphysical. Primary qualities, for Reid, are qualities whose intrinsic natures can be known through sensation. Secondary qualities, on the other hand, are unknown causes of sensations. Some may object that Reid's view is internally inconsistent, or unacceptably relativistic. However, a deeper understanding shows that it is consistent, and relative only to normal humans. To acquire this deeper understanding, one must also explore the nature of dispositions, Reid's rejection of the theory of ideas, his distinction between sensation and perception, and his distinction between natural and acquired perceptions.

The first half of this review article on Locke on primary and secondary qualities leads up to a fairly straightforward reading of what Locke says about the distinction in Essay II.viii, one that, in its general outlines, represents a sympathetic understanding of Locke’s discussion. The second half of the paper turns to consider a few of the ways in which interpreting Locke on primary and secondary qualities has proven more complicated. Here we take up what is sometimes called the Berkeleyan interpretation of Locke (section 6), the understanding of Locke’s resemblance thesis (section 7), and Locke’s views of qualities and their relationship to powers (section 8).

In this paper, I argue that Book II, Chapter viii of Locke' Essay is a unified, self-consistent whole, and that the appearance of inconsistency is due largely to anachronistic misreadings and misunderstandings. The key to the distinction between primary and secondary qualities is that the former are, while the latter are not, real properties, i.e., properties that exist in bodies independently of being perceived. Once the distinction is properly understood, it becomes clear that Locke's arguments for it are simple, valid and (in one case) persuasive as well.