Difference between primary and secondary qualities in bodies.

In the last two chapters, we have pointed out general tests of the importance of all truths, which will be sufficient at once to distinguish certain classes of properties in bodies as more necessary to be told than others, because more characteristic, either of the particular thing to be represented, or of the principles of nature.

According to Locke, book ii. chap. 8, there are three sorts of qualities in bodies: first, the “bulk, figure, number, situation, and motion or rest of their solid parts: those” that “are in them, whether we perceive them or no.” These he calls primary qualities. Secondly, “the power that is in any body to operate after a peculiar manner on any of our senses” (sensible qualities). And thirdly, “the power that is in any body to make such a change in another body as that it shall operate on our senses differently from what it did before:” these last being “usually called powers.”

The first are fully characteristic; the second imperfectly so.

Hence he proceeds to prove that those which he calls primary qualities are indeed part of the essence of the body, and characteristic of it; but that the two other kinds of qualities which together he calls secondary, are neither of them more than powers of producing on other objects, or in us, certain effects and sensations.