Hume on Primary and Secondary Qualities

125. HUME ON PRIMARY AND SECONDARY QUALITIES Hume's view of the primary/secondary quality distinction is, I believe, a matter of considerable interest. It bears upon Hume's position in relation to Locke and Berkeley, and has important implications for general features of his epistemology and metaphysics . The central part of my discussion will therefore be taken up with a consideration of those passages from his writings in which Hume refers to the primary/ secondary quality distinction. Hume's treatment of this aspect of the 'modern philosophy' may be seen against the background of his discussion of perception and its objects, and it is therefore this with which I begin. HUME'S SYSTEM I refer to Hume's system in relation to perception and the manner in which objects affect the senses . Hume's avowed purpose here is to discuss the way in which objects appear to the senses (in particular, those of sight and touch), rather than to account for their real nature and operations. Our ignorance of the nature of bodies reflects the fact that we are limited in experience to those properties of objects which discover themselves 2 to the senses . In determining the nature of Hume's system there are, I think, two (related) factors to be taken into account: (i) Hume's distinction between perceptions and objects; and (ii) Hume's critique of the Naive or Direct Realist theory of perception. (i) Perceptions and Objects The distinction which Hume wishes to make between perceptions and objects, and his general account of the relation between them, is embodied in his 126. discussion of the idea of external existence (T 6) . We may observe, that 'tis universally allow'd by philosophers , and is besides pretty obvious of itself, that nothing is ever really present with the mind but its perceptions or impressions and· ideas, and that external objects become known to us only by the perceptions they 3 occasion . We should note that what Hume is describing here is, in essence, the Indirect Realist account of perception, i.e. that we are acquainted with an external reality only in the form of perceptions ('impressions') which arise from sensory contact with it. That Hume is prepared to speak elsewhere of per4 ceptions in their relation to external objects as images is indicative of his commitment to a Representative theory. There are, of course, severe limitations upon our understanding of the relation between perceptions and objects. As every idea is deriv'd from a preceding perception, 'tis impossible our idea of a perception, and that of an object or external existence can ever represent what are specifically different from each other . In so far as external objects are supposed to differ from perceptions in their specific identity, we can form only a relative idea of them (T 68) . Nevertheless the idea of bodies or objects is clearly to be distinguished from that of perceptions; while being specifically the same, like ideas and impressions, these items, or rather the ideas involved, are attended with the supposition of a difference , that is unknown and incomprehensible (T 244) . As different existences perceptions and objects may be conceived to differ in respect of their relational properties, for example, but otherwise whatever is true of the one must also be true of the other. These two sets of distinctions — between, on the one hand, ideas and impressions and, on the other, perceptions and objects — may usefully be compared and 127. contrasted. In each case the items with which we are concerned are numerically different but specifically identical; and, furthermore, the relation between the items which make up each pair is a causal one. The obvious difference between the two cases is that while both impressions and ideas are objects of experience, the causal dependence of the one upon the other being revealed by the order of their appearance before the mind, we do not directly experience external objects as opposed to perceptions. Questions concerning the 'real nature' of objects or bodies belong to the province of natural - rather than 'moral' - philosophy, which indeed has reasonable hypotheses to offer (T 48) . As for our idea of body, this comprises nothing more than the ideas...