The unreflective man regards his percepts, such as they appear to his immediate apprehension, as things having a wholly independent existence. When he sees a tree he believes that it stands in the form which he sees, with the colours of all its parts, etc., there on the spot towards which his gaze is directed. When the same man sees the sun in the morning appear as a disc on the horizon, and follows the course of this disc, he believes that the phenomenon exists and occurs (by itself) exactly as he perceives it. To this belief he clings until he meets with further percepts which contradict his former ones. The child who has as yet had no experience of distance grasps at the moon, and does not correct its first impression as to the real distance until a second percept contradicts the first. Every extension of the circle of my percepts compels me to correct my picture of the world. We see this in everyday life, as well as in the mental development of mankind. The picture which the ancients made for themselves of the relation of the earth to the sun and other heavenly bodies, had to be replaced by another when Copernicus found that it contradicted percepts which in those early days were unknown. A man who had been born blind said, when operated on by Dr. Franz, that the idea of the size of objects which he had formed before his operation by his sense of touch was a very different one. He had to correct his tactual percepts by his visual percepts. How is it that we are compelled to make these continual corrections in our observations?