THE PEECEPTION OF SPACE. (IV.) 537

mon 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 notion in most minds. The measuring of our space-feelings against each other mainly comes about through the successive arousal of diffe- rent 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 diffe- rent 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 im- mensely the result. In completely educated space-perception, the present sen- sation is usually just what Helmholtz (Pliysiol. 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 dis- criminations. 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. 1 7. Historical. Let us now close with a brief historical survey. The first 1 This shrinkage and expansion of the absolute space-value of the total optical sensation remains to my mind the most obscure part of the whole subject. It is a real optical sensation, seeming introspectively to have nothing to do with locomotor or other suggestions. It is easy to say that " the Intellect produces it," but what does that mean ? The investigator who will throw light on this one point will probably clear up other diffi- culties as well. 35