  There may be several different ways of accounting for Locke’s distinction between primary and secondary qualities. The important aspect of the distinction for the present discussion is that on Locke’s view we ought always to explain a secondary quality in terms of a primary quality by which it produces the relevant sensation in us. Colors, sounds, smells and so on are on Locke’s account powers that a body has as a result of its particular shape, motion, and so on which produce the relevant sensations in observers: “[Secondary qualities] are only Powers to act differently upon other things, which Powers result from the different Modifications of those primary qualities” (Essay 2.8.23). For example, in arguing that some, apparently more genuine properties of fire, such as heat, are really on the same footing with qualities such as the tendency to melt wax that are more obviously relational, Locke holds that each of these qualities alike are powers that a body has in virtue of its primary qualities to produce certain effects[my emphasis added]: “…the power in fire to produce a new color, or consistency in wax or clay by its primary qualities is as much a quality in fire, as the power it has to produce in me a new idea or sensation of warmth or burning” (Essay 2.8.10). For Locke, then, all bodies have primary qualities, and any secondary qualities that they have are to be understood in terms of the primary qualities that produce them.