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 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 erroneously 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 and the brightness of the blaze, the redness, the pleasant taste, and the aromatic odor of the strawberry, exist in these bodies merely as the power to produce such sensations in us by stimulation of the skin, the eye, the palate, and the nose.  If we remove the perceptions of them, they disappear as such, and their causes alone remain—­the bulk, figure, number, texture, and motion of the insensible particles.  The ground of the illusion lies in the fact that such qualities as color, etc., bear no resemblance to their causes, in no wise point to these, and in themselves contain naught of bulk, density, figure, and motion, and that our senses are too weak to discover the material particles and their primary qualities.—­The distinction between qualities of the first and second order—­first advanced by the ancient atomists, revived by Galileo and Descartes on the threshold of the modern period, retained by Locke, and still customary in the natural science of the day—­forms an important link in the transition from the popular view of all sense-qualities as properties of things in themselves to Kant’s position, that spatial and temporal qualities also belong to phenomena alone, and are based merely on man’s subjective mode of apprehension, while the real properties of things in themselves are unknowable.