Space is perceived through all the senses, but by all except kinesthesis, it is perceived by means of signs. Vision and touch, the other most important space perceiving senses, can primarily perceive only extensity or bigness. Stimulation of more receptors in the retina or the skin results, in the perception of more of the sense data, in an extensity (not in intensity or durational) sense. Stimulation of different receptors gives perception of different sense data, but not primarily of the space difference. There is no conceivable way in which the perception of space could be built up except through muscular activity and resulting kinesthesis: and an inherited capacity to perceive space visually or tactually is due to the inheritance of reaction to signs, such as might be built up otherwise through experience.
Suppose that, vision being excluded, two spots on the skin of the arm are stimulated successively. If no tactual space signs have been built up, or inherited, these are felt as different touches, but not as in different places; and stimulation of a large number of spots gives no perception of distance or direction of these spots with reference to each other. But, if these spots are stimulated successively by moving the finger from one to the other: or if they are stimulated successively by some external object, through movements of the arm which bring the different spots successively in contact with the object, the tactual pattern becomes associated with the movements and the amount of movement, and so is woven gradually into a space pattern, in which distance and direction in space are involved. Distance and direction are not kinesthetic sentienda themselves, but are relations, which, first experienced between kinesthetic sentienda, are subsequently experienced between the tactual sentienda. A given tactual sentiendum, such as may be obtained from a certain spot on the skin, then becomes perceptible with space relation to other