324 w. JAMES :

of absolute size now, it must later be taken up in a thorough way. Let us confine ourselves to the way in which the three dimensions seen get their values fixed, relatively to each other. Reid, in his Inquiry into the Human Mind, has a section " Of the Geometry of Visibles," in which he assumes to trace what the perceptions would be of a race of ' Idomenians ' reduced to the sole sense of sight. Agreeing with Berkeley that sight alone can give no knowledge of the third dimension, he humorously deduces various ingenious absurdities in their interpretations of the material appearances before their eyes. Now I firmly believe, on the contrary, that one of Reid's Idomenians would frame precisely the same conception of the external world that we do, if he had our intellectual powers. 1 Even were his very eyeballs fixed and not movable like ours, that would only retard, not frustrate, his education. For the same object, by alternately covering in its lateral movements different parts of his retina, would determine the mutual equivalencies of the first two dimensions of the field of view ; and by exciting the physiological cause of his perception of depth in various degrees, it would establish a scale of equivalency between the first two and the third. First of all, one of the sensations given by the object is chosen to represent its * real ' size and shape, in accord- ance with the principles laid down on pp. 191 and 193. One sensation measures the * thing ' present, and the ' thing ' then measures the other sensations. The peri- pheral parts of the retina are equated with the central by receiving the image of the same object. This needs no elucidation in case the object does not change its distance or its front. But suppose, to take a more complicated case, that the object is a stick, seen first in its whole length, and then rotated round one of its ends ; let this fixed end be the one near the eye. In this movement the stick's image will grow progressively shorter ; its farther end will appear less and less widely separated from the fixed near end ; soon it will be screened by it, and then re-appear on the opposite side, and finally on that side resume its original length. Suppose this movement to become a familiar experience ; 1 " In Froriep's Notizen, 1838, July, No. 133, is to be found a detailed account, with a picture, of an Esthonian girl, Eva Lauk, then 14 years old, born with neither arms nor legs, which concludes with the following words : ' According to the mother, her intellect developed quite as fast as that of her brother and sisters ; in particular, she came as quickly to a right judgment of the size and distance of visible objects, although, of course, she had no use of hands'." (Schopenhauer, Well als Wille, ii. 44.)