Subdivisions, once discriminated, acquire definite relations of position towards each other within the total space. These 'relations' are themselves feelings of the subdivisions that intervene. When these subdivisions are not the seat of stimuli, the relations are only reproduced in imaginary form. The various sense-spaces are, in the first instance, incoherent with each other; and primitively both they and their subdivisions are but vaguely comparable in point of bulk and form. The _education_ of our space-perception consists largely of two processes--reducing the various sense-feelings to a common _measure_, and _adding them together_ into the single all-including space of the real world. Both the measuring and the adding are performed by the aid of _things_. The imagined aggregate of positions occupied by all the actual or possible, moving or stationary, things which we know, is our notion of 'real' space--a very incomplete and vague conception in all minds. The _measuring_ of our space-feelings against each other mainly comes about through the successive arousal of different ones by the same _thing_, by our selection of certain ones as feelings of its _real_ size and shape, and by the degradation of others to the status of being merely _signs_ of these. For the successive application of the same thing to different space-giving surfaces motion is indispensable, and hence plays a great part in our space-education, especially in that of the eye. Abstractly considered, the motion of the object over the sensitive surface would educate us quite as well as that of the surface over the object. But the self-mobility of the organ carrying the surface _accelerates_ immensely the result. In completely educated space-perception, the present sensation is usually just what Helmholtz (Physiol. Optik, p. 797) calls it, 'a sign, the interpretation of whose meaning is left to the understanding.' But the understanding is exclusively reproductive and never productive in the process; and its function is limited to the recall of previous space-sensations with which the present one has been associated and which may be judged more real than it. Finally, this reproduction may in the case of certain visual forms be as vivid, or almost so, as actual sensation is. The third dimension forms an original element of all our space-sensations. In the eye it is subdivided by various discriminations. The more distant subdivisions are often shut out altogether, and, in being suppressed, have the effect of diminishing the absolute space-value of the total field of view.[259] HISTORICAL.