328 w. JAMES :

This passage of Helmholtz's has obtained, it seems to me, an almost deplorable celebrity. The reader will please observe its very radical import. Not only would he, and does he, for the reasons we have just been ourselves con- sidering, deny distance to be an optical sensation ; but, extending the same method of criticism to judgments of size, shape and direction, and finding no single retinal or muscular process in the eyes to be indissolubly linked with any one of these, he goes so far as to say that all optical space-perceptions whatsoever must have an intellectual origin, and a content that no items of visual sensibility can account for. 1 As Wundt and others agree with Helmholtz here, and as their conclusions, if true, are irreconcilable with all the sen- sationalism which I have been teaching hitherto, it clearly devolves upon me to defend my position against this new attack. The wisest order of procedure seems this : first, Reid's and Helmholtz's principle for distinguishing between what is sensible and what is intellectual, must be disproved by showing cases of other senses than sight in which it is violated; secondly, we must review the further facts of vision to which the principle is supposed to apply ( this will be the longest segment of our task) ; and thirdly, it must be shown that the facts admit of another interpretation com- pletely in accordance with the tenor of the space-theory we have ourselves defended hitherto. I think we shall, without extreme difficulty, make good all the parts of this perhaps presumptuous-sounding program. 2 of size, shape and place, and ought by parity of reasoning to be called in- tellectual products and not sensations. In other places he does treat colour as if it were an intellectual product. 1 It is needless at this point to consider what Helmholtz's views of the nature of the intellectual space-yielding process may be. He vacillates we shall later see how. - Before embarking on this new topic it will be well to shelve, once for all, the problem of what is the physiological process that underlies the distance-feeling. Since one-eyed people have it, and are only inferior to the two-eyed in measuring its gradations, it can have no exclusive con- nexion with the double and disparate images produced by binocular parallax. Since people with closed eyes, looking at an after-image, do not usually see it draw near or recede with varying convergence, it cannot be simply constituted by the convergence-feeling. For the same reason, the feeling of accommodation cannot be identical with the feeling of distance. The differences of apparent parallactic movement between far and near objects as we move our head, cannot constitute the distance-sensation, for such dif- ferences may be easily reproduced experimentally (in the movements of visible spots against a background) without engendering any illusion of perspective. Finally, it is obvious that visible faintness, dimness and