Many have been the philosophical controversies over the nature of space and our perception of it. The psychologists have even quarreled concerning whether we possess an _innate_ sense of space, or whether it is a product of experience and training. Fortunately, for our present purpose we shall not need to concern ourselves with either of these controversies. For our discussion we may accept space for what common sense understands it to be. As to our sense of space, whatever of this we may possess at birth, it certainly has to be developed by use and experience to become of practical value. In the perception of space we must come to perceive _distance_, _direction_, _size_, and _form_. As a matter of fact, however, size is but so much distance, and form is but so much distance in this, that, or the other direction. THE PERCEIVING OF DISTANCE.--Unquestionably the eye comes to be our chief dependence in determining distance. Yet the muscle and joint senses give us our earliest knowledge of distance. The babe reaches for the moon simply because the eye does not tell it that the moon is out of reach. Only as the child reaches for its playthings, creeps or walks after them, and in a thousand ways uses its muscles and joints in measuring distance, does the perception of distance become dependable. At the same time the eye is slowly developing its power of judging distance. But not for several years does visual perception of distance become in any degree accurate. The eye's perception of distance depends in part on the sensations arising from the muscles controlling the eye, probably in part from the adjustment of the lens, and in part from the retinal image. If one tries to look at the tip of his nose he easily feels the muscle strain caused by the required angle of adjustment. We come unconsciously to associate distance with the muscle sensations arising from the different angles of vision. The part played by the retinal image in judging distance is easily understood in looking at two trees, one thirty feet and the other three hundred feet distant. We note that the nearer tree shows the _detail_ of the bark and leaves, while the more distant one lacks this detail. The nearer tree also reflects more _light_ and _color_ than the one farther away. These minute differences, registered as they are on the retinal image, come to stand for so much of distance. The ear also learns to perceive distance through differences in the quality and the intensity of sound. Auditory perception of distance is, however, never very accurate. THE PERCEIVING OF DIRECTION.--The motor senses probably give us our first perception of direction, as they do of distance. The child has to reach this way or that way for his rattle; turn the eyes or head so far in order to see an interesting object; twist the body, crawl or walk to one side or the other to secure his bottle. In these experiences he is gaining his first knowledge of direction.