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THEORY OF KNOWLEDGE. 159 sion and thought. Extension does not constitute thei y essence of matter, nor thought the essence of mind. Exten- ' sion and body are not the same ; the former is presupposed i 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 impene- trability is found there is body, and the converse ; the two i 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 man- ifestly contrary to experience. Thinking is merely an ac- tivity 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 understand- ing may be compared to a mirror, which, without inde- pendent 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 sensa- tion those only which come from different senses, hence extension, motion and rest, number, figure, and, further, solidity, are to be accounted /rzw^r;' qualities, /. e., such as are actual copies of the properties of bodies. All other ideas, on the contrary, have no resemblance to properties of bodies; they represent merely the ways in which things act, and are not copies of things. The ideas of secondary or derivative qualities (hard and soft, warm and cold, colors and sounds, tastes and odors) are in the last analysis caused — as are the primary — by motion, but not perceived as such. Yellow and warm are merely sensations in us, which we erro- neously ascribe to objects ; with equal right we might ascribe to fire, as qualities inherent in it, the changes in form and color which it produces in wax and the pain which it causes in the finger brought into proximity with it. The warmth