All original ideas are representations either of the external senses or of the internal sense, or of both. And since, in the case of ideas of sensation, there is a distinction between those which are perceived by a single one of the external senses and those which come from more than one, four classes of simple ideas result: (1) Those which come from one external sense, as colors, sounds, tastes, odors, heat, solidity, and the like. (2) Those which come from more than one external sense (sight and touch), as extension, figure, and motion. (3) Reflection on the operations of our minds yields ideas of perception or thinking (with its various modes, remembrance, judging, knowledge, faith, etc.), and of volition or willing. (4) From both external and internal perception there come into the mind the ideas of pleasure and pain, existence, power, unity, and succession. These are approximately our original ideas, which are related to knowledge as the letters to written discourse; as all Homer is composed out of only twenty-four letters, so these few simple ideas constitute all the material of knowledge. The mind can neither have more nor other simple ideas than those which are furnished to it by these two sources of experience. Locke differs from Descartes again in regard to extension and thought. Extension does not constitute the essence of matter, nor thought the essence of mind. Extension and body are not the same; the former is presupposed by the latter as its necessary condition, but it is the former alone which yields mathematical matter. The essence of physical matter consists rather in solidity: where impenetrability is found there is body, and the converse; the two are absolutely inseparable. With space the case is different. I cannot conceive unextended matter, indeed, but I can easily conceive immaterial extension, an unfilled space Further, if the essence of the soul consisted in thought, it must be always thinking. As the Cartesians maintained, it must have ideas as soon as it begins to be, which is manifestly contrary to experience. Thinking is merely an activity of the mind, as motion is an activity of the body, and not its essential characteristic. The mind does not receive ideas until external objects occasion perception in it through impressions, which it is not able to avert. The understanding may be compared to a mirror, which, without independent activity and without being consulted, takes up the images of things. Some of the simple ideas which have been mentioned above represent the properties of things as they really are, others not. The former class includes all ideas of reflection (for we are ourselves the immediate object of the inner sense); but among the ideas of sensation those only which come from different senses, hence extension, motion and rest, number, figure, and, further, solidity, are to be accounted _primary_ qualities, _i. e_., such as are actual copies of the properties of bodies. All other ideas, on