but rather the idea of red. Secondary qualities are used to classify similar ideas produced by an object. That is why when we see something “red” it is only “red” in our minds because they produce the same idea as another object. So, going back to the color of the apple, it produces an idea of red, which we classify and identify with other red ideas. Again, secondary qualities do not exist inside the mind; they are simply the powers that allow us to sense a certain object and thus ‘reflect’ and classify similar ideas. According to the theory, primary qualities are measurable aspects of physical reality; secondary qualities are subjective. History "By convention there are sweet and bitter, hot and cold, by convention there is color; but in truth there are atoms and the void" —Democritus, Fragment 9. "I think that tastes, odors, colors, and so on are no more than mere names so far as the object in which we locate them are concerned, and that they reside only in the consciousness. Hence if the living creature were removed, all these qualities would be wiped away and annihilated" —Galileo Galilei, The Assayer (published 1623). "[I]t must certainly be concluded regarding those things which, in external objects, we call by the names of light, color, odor, taste, sound, heat, cold, and of other tactile qualities, [...]; that we are not aware of their being anything other than various arrangements of the size, figure, and motions of the parts of these objects which make it possible for our nerves to move in various ways, and to excite in our soul all the various feelings which they produce there." —René Descartes, Principles of Philosophy (published 1644/1647). "For the rays, to speak properly, are not coloured. In them there is nothing else than a certain power and disposition to stir up a sensation of this or that colour." —Isaac Newton, Optics (3rd ed. 1721, original in 1704). Criticism Leibniz Gottfried Leibniz was an early critic of the distinction, writing in his 1686 Discourse on Metaphysics that "[i]t is even possible to demonstrate that the ideas of size, figure and motion are not so distinctive as is imagined, and that they stand for something imaginary relative to our perceptions as do, although to a greater extent, the ideas of color, heat, and the other similar qualities in regard to which we may doubt