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In this paper I shall focus attention on a principle which lies at the heart of Locke's distinction between primary and secondary qualities. It is to be found explicitly or implicitly stated at many places in the Essay , but its clearest expression is at E.II.viii.11, where Locke writes that ' Impulse [is] the only way which we can conceive Bodies operate in'. Let us call it 'the impulse principle'. The first task is to describe what exactly the term impulse means here and to what the principle amounts. Next, I shall consider the kind of role the principle plays in the Essay and whether Locke changed his mind about it in the fourth edition. Then, in the main part of the paper, I shall try to show how the impulse principle helps make possible Locke's distinction between primary and secondary qualities. In the course of my discussion I shall refer to some of Locke's pre- Essay writings: the Epitome, the Abr g , his review of Newton's Principia and Draft C.1 It is a subsidiary aim of the paper to show how these writings - particularly the Abr g which ran to over ninety pages of the Biblioth que universelle and was published in 1688 - can be of help in disentangling the main line of argument in Locke's Essay.

Introduction in chapter viii of book ii of An Essay Concerning Human Understanding, John Locke provides various putative lists of primary qualities. Insofar as they have considered the variation across Locke's lists at all, commentators have usually been content simply either to consider a self-consciously abbreviated list (e.g., "Size, Shape, etc.") or a composite list as the list of Lockean primary qualities, truncating such a composite list only by omitting supposedly co-referential terms. Doing the latter with minimal judgment about what terms are co-referential gives us the following list of eleven qualities (in the order in which they appear in this chapter of the Essay): solidity, extension, figure, mobility, motion or rest, number, bulk, texture, motion, size, and situation. Perhaps surprisingly given the attention to the primary/secondary distinction since Locke, Locke's primary qualities themselves have received little more than passing mention in the bulk of the subsequent literature. In particular, no discussion both offers an interpretation of Locke's conception of primary qualities and makes sense of Locke's various lists as lists of primary qualities. A central motivation for this paper is the idea that these two tasks are not independent.

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.

Locke denied that ideas of secondary qualities resemble their causes. It has been suggested that Locke denied this because he accepted a mechanical corpuscular hypothesis about the constitution of objects. This paper shows that this and other usual explanations of Locke's denial are mistaken. Further, it suggests an alternative relationship between the scientific account and Locke's philosophical views, and finally it provides Locke's real justification for his claim that ideas of secondary qualities do not resemble their causes.

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.

Locke asserts that “the Ideas of primary Qualities of Bodies, are Resemblances of them, and their Patterns do really exist in the Bodies themselves; But the Ideas, produced in us by these Secondary Qualities, have no resemblance of them at all.”1 On an unsophisticated way of taking his words, he means that ideas of primary qualities are like the qualities they represent and ideas of secondary qualities are unlike the qualities they represent.2 I will show that if we take his assertions in this unsophisticated way, our reward will be a straightforward and satisfying interpretation of the central arguments of his chapter on primary and secondary qualities. With these arguments, Locke attempts to justify his assertions about resemblance. Some may be skeptical, thinking that the assertions, interpreted literally, are either too absurd or too obvious to have reasons supporting them. I take this skepticism to rest on deep foundations of charity, so half of the paper will be devoted to undermining these foundations by giving a sympathetic and historical exposition of Locke’s positive thesis that primary qualities resemble the ideas that represent them. I criticize rival interpretations of Lockean resemblance, say what it means to believe that ideas resemble qualities, explain the plausibility of the belief in Locke’s environment, and examine his descriptions of how ideas resemble particular primary qualities. Once I establish that we should take Locke’s resemblance theses literally, I can describe their place in his theory of representation. After that, I can describe his reasons for believing the resemblance theses. I will conclude by showing how Locke’s belief that ideas of secondary qualities do not resemble anything in bodies leads him to conclude that secondary qualities are merely powers to produce ideas. Let me begin near the end, however. Before defending a literal interpretation of the resemblance theses, I want to criticize three rival interpretations..

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.

Descartes and Boyle were the most influential proponents of strict mechanist accounts of the physical world, accounts which carried with them a distinction between primary and secondary (or sensible) qualities. For both, the distinction is a piece of natural philosophy. Nevertheless the distinction is quite differently articulated, and, especially, differently grounded in the two thinkers. For Descartes, reasoned reflection reveals to us that bodies must consist in mere extension and its modifications, and that sensible qualities as we conceive of them based on sense perception can pertain only to the mind. Just how we are supposed to arrive at this realization is, this essay will argue, a deep puzzle that brings us to the basic assumptions of Descartes' metaphysics. For Boyle, by contrast, while reflection can reveal the unique explanatory status of mechanism, and, thus, the primary/secondary quality distinction, only experience can confirm its truth. Our central focus will be on Descartes, and on the question: How does he intend to remove the sensible qualities from the physical world, how does he strip them from bodies? I will try to show that Descartes has an argument that he takes to show a priori that sensible qualities cannot be attributed to the material world (as foundational qualities, or, as we conceive of them based on sense experience). The argument fails, however, leaving him with at best a partly empirical case for removing the sensible qualities, based on the purported explanatory success of his physics.

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?

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).