=Taste, smell, as well as hunger, thirst, nausea, and other so-called 'common' sensations= need not be touched on in this book, as almost nothing of psychological interest is known concerning them. CHAPTER VI. SENSATIONS OF MOTION. I treat of these in a separate chapter in order to give them the emphasis which their importance deserves. They are of two orders: 1) Sensations of objects moving over our sensory surfaces; and 2) Sensations of our whole person's translation through space. =1) The Sensation of Motion over Surfaces.=--This has generally been assumed by physiologists to be impossible until the positions of _terminus a quo_ and _terminus ad quem_ are severally cognized, and the successive occupancies of these positions by the moving body are perceived to be separated by a distinct interval of time. As a matter of fact, however, we cognize only the very slowest motions in this way. Seeing the hand of a clock at XII and afterwards at VI, I judge that it has moved through the interval. Seeing the sun now in the east and again in the west, I infer it to have passed over my head. But we can only _infer_ that which we already generically know in some more direct fashion, and it is experimentally certain that we have the feeling of motion given us as a direct and simple _sensation_. Czermak long ago pointed out the difference between _seeing the motion_ of the second-hand of a watch, when we look directly at it, and noticing the fact that it has _altered its position_, whilst our gaze is fixed upon some other point of the dial-plate. In the first case we have a specific quality of sensation which is absent in the second. If the reader will find a portion of his skin--the arm, for example--where a pair of compass-points an inch apart are felt as one impression, and if he will then trace lines a tenth of an inch long on that spot with a pencil-point, he will be distinctly aware of the point's motion and vaguely aware of the direction of the motion. The perception of the motion here is certainly not derived from a preëxisting knowledge that its starting and ending points are separate positions in space, because positions in space ten times wider apart fail to be discriminated as such when excited by the compass-points. It is the same with the retina. One's fingers when cast upon its peripheral portions cannot be counted--that is to say, the five retinal tracts which they occupy are not distinctly apprehended by the mind as five separate positions in space--and yet the slightest _movement_ of the fingers is most vividly perceived as movement and nothing else. It is thus certain that our sense of movement, being so much more delicate than our sense of position, cannot possibly be derived from it.