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This chapter analyzes what Reid says about the nature of our awareness of sensations in order to identify the degree of contact between our minds and the world. It begins with a summary of the reasons for thinking that Reid's theory brings mind very close to world. It then discusses an interpretation according to which our awareness of the sensations that signify primary qualities renders our perception of primary qualities indirect. A handful of texts offer prima facie support for this view; in them Reid says we perform cognitive operations on those sensations by ‘interpreting’, ‘conceiving’, and ‘believing’ them. These passages and their context are analyzed to show that evidence on behalf of this interpretation fails fully to justify its attribution to Reid. It is argued that determining the degree of directness of Reidian perception is intractable due to his coarse discussion of our awareness of sensations. However, we are justified in thinking that his theory of perception is more direct than indirect.

There are three areas of particular concern to mechanists, concerns which highlight the legitimatory aspects of the mechanist project in natural philosophy. The first is the question of how mechanists deal with the explanatory load placed on their systems by a combination of minimal explanatory resources and ambitious explanatory aims. In particular, the role that the doctrine of primary and secondary qualities plays in this respect, especially in Malebranche's reworking of Cartesianism. Second, there is the attempt to extend mechanism into the realms of vital and cognitive functions, phenomena that were treated as part of natural philosophy in the early-modern period, and which generated a great deal of controversy. Finally, there is the question of the relation between natural philosophy expanded into the biological realm and the traditional practice of clinical medicine, which harbours a very different model of understanding biological processes as they relate to illness and health.

This chapter explores Russell’s idea that reflection on the nature of scientific inquiry prompts a version of the ignorance hypothesis, an idea much discussed in contemporary philosophy. As interpreted here, Russell’s idea is that empirical inquiry acquaints only with relational or dispositional features of physical objects, rather than their categorical or intrinsic features. A version of the epistemic view based on this idea is explicated.

Besides distinguishing between primary and secondary qualities, Locke implicitly draws a distinction between two types of primary quality, those of macroscopic objects and those of what he calls the insensible parts of matter. This chapter explores three main interpretive results that come from attending to the distinction as it figures in the Essay. First, it helps to reconcile several seemingly conflicting statements that Locke makes about primary qualities. Second, it makes better sense of Locke's claims that ideas of primary qualities are resemblances of bodies and that such qualities are ‘really in’ them, whereas (ideas of) secondary qualities are not. Third, it enables us to uncover the structure of Locke's argument for the primary–secondary quality distinction itself, which puts us in a better position to appreciate the limits on the justificatory force and ontological import that Locke can claim for this argument.

The chapter promotes the idea that properties, ways substances are, are powerful qualities. The familiar distinction between categorical and dispositional properties is grounded in a confusion inherited from philosophers hoping to ‘analyze away’ talk of dispositions. A power or disposition is for a particular kind of manifestation with particular sorts of reciprocal manifestation partner, but a disposition need never be manifested. The idea that dispositions are relations stems from imagining that because dispositions are typically characterized ‘relationally’ — by reference to what they are for — it follows that dispositions are relations. Unger’s criticism of ‘scientiphicalism’ is examined and shown to be partly right, partly wrong. An account of secondary qualities as arrangements of primaries is advanced, and the idea that claims concerning contingency require no defense is questioned.

Properties of phenomena are, like Locke's secondary qualities, relational; and, like Locke's primary qualities, objective and attributable by science. These ‘primary’ qualities include spatial and space‐filling, i.e. geometrical and dynamical, features, and matter is constituted by a conflict of forces. Kant substitutes impenetrability for solidity, thereby substituting relational powers for intrinsic properties. His argument bears on our contemporary view that powers are contingently grounded in intrinsic properties, a view that must concede the possibility of solid things being penetrable, and acknowledge, for different reasons, the unknowability of those intrinsic properties. Kant's ‘primary’ qualities anticipate Faraday's field theory, and show, pace Evans and Bennett, that qualities do not need to be intrinsic if they are to be scientific and objective.

The distinction between two kinds of qualities, primary and secondary, is one of the core doctrines of Scholastic natural philosophy. Far from being an invention of the modern era, it is something to which any student of Aristotelian philosophy—which is to say anyone who studied philosophy in a European university up until the eighteenth century—would have been introduced at a tender age. The distinction is, moreover, every bit as important for Scholastic philosophers as it would become in the seventeenth and eighteenth centuries, since the primary qualities, on the Aristotelian tradition, are the fundamental causal agents of the natural world.

What is the basis of the distinction between primary and secondary qualities? Is it that primary qualities reside in external objects and secondary qualities in the mind only? Reid says no. Is it that primary qualities resemble ideas in our minds and secondary qualities do not? Reid again says no. Positively, he says we have a direct conception of primary qualities and only a relative conception of secondary qualities as powers to cause certain sensations. Is his own way of drawing the distinction then merely epistemological? This chapter says no: for Reid there is a distinction in the nature of the properties themselves, primary qualities being intrinsic and secondary qualities extrinsic. Relatedly, secondary qualities for Reid are better thought of as dispositions than as causal bases of dispositions. Finally, the chapter brings out the significance of Reid's views by calling attention to four other views that are wrong if he is right.

This chapter finds two versions of the distinction between primary and secondary qualities in Galileo, Descartes, Boyle, and Locke. Although agreeing that primary qualities are physically basic properties of extended particles (including size, shape, position, and motion), these authors differed on whether secondary qualities such as color exist only in the mind as sensations or belong to bodies as powers to cause sensations. Kant was initially a metaphysical realist about primary qualities as spatialized forces (vs bare extended particles), before placing space among the appearances in his critical period. Space becomes the subjective form in which transcendently real forces and relations appear. Kant viewed color as a subjective sensation in the mind, whereas Helmholtz treated color as a power to cause sensations. Helmholtz was initially a realist about primary qualities as spatialized masses and forces, but he later adopted the epistemically modest view of space as the subjective form in which forces and relations appear.

In the Prolegomena, Kant says he is like Locke, except that he makes all the qualities secondary—a position which makes him look uncomfortably like Berkeley, if one takes secondary qualities to be mind‐dependent. Elsewhere, Kant is a scientific realist, endorsing a primary–secondary quality distinction within the phenomenal world: phenomenal qualities include unobservable properties of e.g. magnetic matter, but exclude e.g. colours and tastes. But how can the qualities of phenomenal objects be exclusively secondary (the Lockean comparison), and exclusively primary (the scientific realism)? For Locke, primary qualities are mind‐independent, and intrinsic, which suggests a new possibility. Qualities of phenomenal objects are secondary not because they are mind‐dependent, but because they are relational powers: roughly, Locke's tertiary qualities.

This chapter argues that the primary–secondary quality distinction can be found in ancient Greek philosophers in the form of two distinctions, one between the intrinsic qualities of basic matter and the derivative qualities of composites, and another between appearance and reality. The first ancient Greek thinkers to consider the place of sensible qualities such as colors and smells in the natural world were Plato and Democritus. Both draw a kind of distinction between the intrinsic qualities of basic matter and the non-intrinsic qualities of composites and aggregates. Aristotle offers his own version of this kind of distinction, emphasizing that sensible qualities have causal efficacy in their powers to affect perceivers in certain ways. Epicurus went even further in arguing that sensible qualities are relational powers, and like relational powers are real properties of things, not to be dismissed as merely subjective appearances.

Gassendi explicates the various qualities of bodies in atomist terms. But unlike most of his contemporaries, he does not distinguish primary and secondary qualities or argue that the size, shape, and motion of macroscopic bodies have a privileged metaphysical status their colors and tastes lack. This is because all qualities, for Gassendi, are textures—structures of atoms vibrating in locked patterns. Qualities like motion and taste are only epistemically different: we have a better grasp on what texture of bodies constitutes macro-level shape than taste. This chapter suggests that this epistemic difference is not the primary quality–secondary quality distinction—although this does depend on what philosophical work you expect the distinction to do. The chapter also suggests that Gassendi does not distinguish primary and secondary qualities because his physics is qualitative rather than quantitative. Hence, macro-level size, shape, and motion are mere explananda.

The aim of this chapter is to understand better Hume's response to what he calls the "fundamental principle" of the modern philosophy, the opinion that "colors, sounds, tastes, smells, heat and cold" are "nothing but impressions in the mind." Does he endorse the principle? Can he rely on it when he compares virtue and vice to sensible qualities? Can he accept the suggestion, made before him by Ralph Cudworth, that passionate engagement in the world of our senses survives intellectual entry into the world of modern philosophy? In trying to make progress on these questions and others, the chapter examines three ways in which Hume uses perceptual relativity, and consider a difficulty concerning objectivity into which one of those uses leads him. The chapter concludes, bleakly, that Hume probably cannot be completely satisfied by any philosophy of the sensible qualities that he is in a position to provide.

A key part of the world-view that Berkeley tried to overturn is the theory of primary and secondary qualities, and the classic exposition of this theory is in Locke's Essay Concerning Human Understanding. This chapter attempts to extract from Locke's ambiguous exposition what the author regards as the strongest version of the theory, so as to see later how it stands up against Berkeley's attack on the theory. Although Locke never wavers from his definition of secondary qualities as powers in objects to produce sensations of color, taste, smells, sound, and temperature, he oscillates inconsistently between equating colors etc. with secondary qualities and equating them with the sensations or ideas produced by those qualities. The chapter explores the reasons for this oscillation and proposes a modernized version of Locke's theory that uses a two-term theory of perception and that distinguishes between the dispositional and the manifest aspect of secondary qualities.

This chapter argues that while Berkeley's arguments against the theory of primary and secondary qualities may count against certain versions of the theory, they don't refute Locke's version, especially when modernized as proposed in Chapter 1. Berkeley's first argument is that since (a) one cannot abstract a primary quality (e.g., shape) from a secondary quality (e.g., color), and (b) secondary qualities are only ideas in the mind, so are primary qualities. Locke would reject (b), since for him secondary qualities are “powers” in objects. But there are complications, since the manifest aspect of a secondary quality is not a mere power. Dicker argues that ultimately, Berkeley's argument shows only that the manifest aspect of a color cannot exist apart from the visual manifest aspect of a shape, and doesn't support idealism. Berkeley's second, relativity argument is invalid and shows only that primary qualities would be mind-dependent if secondary qualities were.