Page:Mind (Old Series) Volume 12.djvu/529

 II. THE PEKCEPTION OF SPACE. (IV.) 1 By Professor WILLIAM JAMES. 5. The Intelledualist Theory of Space (continued). LET me remind the reader of where we left off. I had spoken of the difference which frequently obtains between the form and size of an optical sensation and the form and size of the reality which it suggests ; and I had tried to make it clear that, in all common cases, the form and size which we attribute to the reality mean nothing more than certain other optical sensations, now absent, but which would be present under different conditions of observation. I then referred to a residual class of cases on which much stress is laid by such authors as Helmholtz and Wundt. These are cases of illusion, cases in which a presented form and size are not felt at all, the only thing cognised being what these authors consider to be a demonstrably inferred form and size. Were the presented form and size themselves sensations, the authors say, they could not be annulled by inferences ; no instance of the suppression of a real sensa- tion by the inferred image of an absent one being known. I am utterly unconvinced of the truth of this thesis, and of the theory which would explain most of the illusions in point by inferences unconsciously performed. But profitably to conduct the discussion we must divide the alleged instances into groups. (a) With Helmholtz, colour-perception is equally with space- perception an intellectual affair. The so-called simultaneous colour-contrast, by which one colour modifies another along- side of which it is laid, is explained by him as an unconscious inference. This chapter on space is not for the discussion of the colour-contrast problem ; but I mention it here, because maybe the principles which apply to its solution will prove also applicable to part of our own problem. Heririg's treatment of colour-contrast seems, in fact, to have con- clusively convicted Helmholtz of error. In my opinion, Her- ing has definitively proved that, when one colour is laid beside another, it modifies the sensation of the latter, not by virtue of any mere mental suggestion, as Helmholtz would have it, 1 Concluded from MIND, Nos. 45, 46, 47.