Of all the senses, sight is that which we can least distinguish from the judgments of the mind; as it takes a long time to learn to see. It takes a long time to compare sight and touch, and to train the former sense to give a true report of shape and distance. Without touch, without progressive motion, the sharpest eyes in the world could give us no idea of space. To the oyster the whole world must seem a point, and it would seem nothing more to it even if it had a human mind. It is only by walking, feeling, counting, measuring the dimensions of things, that we learn to judge them rightly; but, on the other hand, if we were always measuring, our senses would trust to the instrument and would never gain confidence. Nor must the child pass abruptly from measurement to judgment; he must continue to compare the parts when he could not compare the whole; he must substitute his estimated aliquot parts for exact aliquot parts, and instead of always applying the measure by hand he must get used to applying it by eye alone. I would, however, have his first estimates tested by measurement, so that he may correct his errors, and if there is a false impression left upon the senses he may correct it by a better judgment. The same natural standards of measurement are in use almost everywhere, the man's foot, the extent of his outstretched arms, his height. When the child wants to measure the height of a room, his tutor may serve as a measuring rod; if he is estimating the height of a steeple let him measure it by the house; if he wants to know how many leagues of road there are, let him count the hours spent in walking along it. Above all, do not do this for him; let him do it himself.