By primary qualities he meant extension, weight, motion and number, and so on. When it is a question of qualities such as these, we can be certain that the senses reproduce them objectively. But we also sense other qualities in things. We say that something is sweet or sour, green or red, hot or cold. Locke calls these secondary qualities. Sensations like these – color, smell, taste, sound – do not reproduce the real qualities that are inherent in the things themselves. They reproduce only the effect of the outer reality on our senses.’
