Thomas Reid's primary/secondary quality distinction has proven challenging. Reid scholars offer impressively divergent readings from one another. Chapter 4's approach may be unique among these. So some additional precision in interpretation is warranted. If you are impatient to see how Reid's objective theory of secondary qualities handles philosophical scrutiny, try proceeding to Chapter 7 and returning here later. But if your interest is in understanding Reid's writing, exploring his historical significance or grasping the theoretical landscape, Chapter 6 addresses a number of exegetical concerns. These serve to extend or reiterate the interpretation in Chapter 4, minimising the confusion generated by Reid's leaving these details for the reader to sort out.

Epistemic or Metaphysical?

It is not unusual for Reid interpreters to apply Reid's talk of relative secondary quality conceptions to the qualities themselves, positing a metaphysical distinction where there is none. They suggest that primary qualities are intrinsic qualities of bodies whereas secondary qualities are mere relational ‘powers to produce certain characteristic sensations in us in normal circumstances’. For example, James Van Cleve holds that secondary qualities are dispositions. From there, he argues, ‘If secondary qualities are dispositions, they will differ metaphysically from primary qualities – they will be relational or extrinsic properties … whereas primary qualities are intrinsic’. Thus, in addition to whatever distinction arises as a result of original conceptions of perceivable qualities, Van Cleve understands Reid as positing a metaphysical distinction between primary and secondary qualities. Others see Reid's description of secondary qualities as ‘that which occasions such a sensation’ and his mention of power and virtue, as evidence for the metaphysical distinction. However, these interpreters have misunderstood Reid on this issue. His distinction is purely conceptual.

Van Cleve's argument, which focuses on Reid's secondary qualities as causes of sensations, suggests two ways to take the primary/ secondary quality distinction as metaphysical. First, dispositional/ categorical: secondary qualities are dispositions, abilities, tendencies or powers, best understood in terms of stimulus conditions and manifestations, while primary qualities are categorical. Fragility, for example, is the disposition of an object to shatter when struck. Solubility is the disposition of an object to dissolve in water. Primary qualities, on the other hand, are categorical, like being round or being made of plastic.