 Locke defines the ‘quality’ of any substance as “the Power to produce any Idea in our mind” (II.viii.8). There are three categories of qualities according to the mechanical natural philosophy. Primary qualities are the inseparable features of a body, e.g., its solidity, extension, figure, motion or rest, number, situation, etc. (II.viii.9 &amp; 23). Secondary qualities are the colors, sounds, smells, tastes, etc., of any object. These are secondary because these features of objects do not exist in the objects themselves, but rather are nothing but the power of the primary qualities of the object to produce an idea in us of a certain kind (II.viii.10). So, for example, the color of the table is not in the table, rather, it is part of how the matter and texture of the table (i.e., its primary qualities), as it reflects, absorbs and refracts light, has the power to produce ideas in us of that color. Finally, the tertiary qualities of a body are those powers in it that, by virtue of its primary qualities, give it the power to produce observable changes in the primary qualities of other bodies, e.g., the power of the sun to melt wax is a tertiary quality of the sun (II.viii.23).