Like the mere powers of producing change in other objects...

Like the mere powers of producing change in other objects, these supposed qualities are nothing other than effects produced by the genuine (primary) qualities of bodies themselves. [Essay II viii 10] Notice that all of our ideas of secondary qualities are produced in us only by a single sense; their perception clearly depends upon the normal operation of our sensory apparatus in response to the merely dispositional properties of things.

Thus, our customary supposition that such qualities really exist in nature is nothing more than a reification of the ideas that are actually produced in our minds by the primary qualities of things. [Essay II viii 14-17] Locke's arguments in defence of this thesis may be familiar from their prior statement in Galileo, Descartes, and Boyle or from their later appropriation by Berkeley and Hume.

The color and taste of manna (a natural laxative) are less like its powdery texture than like its effect on the digestive tract; although things themselves remain the same in darkness as in light, their color appears only through the mediation of rays of light; contrapositively, the mechanical process of grinding an almond can change only its primary qualities, yet the secondary qualities are thereby modified; and a single body of tepid water can seem both warm and cool at the same time, if our fingers have been differentially chilled and heated beforehand.

[Essay II viii 18-21] In every case, the proper conclusion is that the ideas of secondary qualities are produced in us only by the mechanical operation of the primary qualities of bodies. From this distinction, Locke concluded that there is a significant difference in the representational reliability of primary and secondary qualities.

Our ideas of primary qualities resemble the qualties themselves; the mind-independent features of material objects are (at least at the microscopic level) exactly as we perceive them to be. But our ideas of secondary qualities and powers resemble nothing in the material world; they are the causal consequences, in our minds, of the mechanical action upon our sensory organs of corpuscular primary qualities.