In this article the author puts forward an interpretation of Locke’s famous chapter on primary and secondary qualities, chapter 8 of Book II of The Essay concerning Human Understanding. It is argued that the key to the distinction is the thought that the causal processes of perception must conform to what the author calls Locke’s impulse principle. This principle, found at E.II.viii.11, as well as at many other places in the Essay, states that we can understand causation in nature only as operating by impulse, where impulse involves movement and contact. The author argues that, despite cosmetic changes to E.II.viii.11 as a result of his dispute with Stillingfleet, Locke still maintains the principle, and effectively rules out action-at-a-distance in perception, even in the final edition of the Essay he oversaw. The principle plays the same crucial role in chapter 8 as it did in the first edition of the Essay, distinguishing (primary) qualities, that reside in objects as they are perceived, from (secondary) qualities, which are merely powers of those objects to produce ideas in us.
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