Page:Mind (Old Series) Volume 12.djvu/366

THE PERCEPTION OF SPACE, (ill.) 353 if such a one, actually present, had an immanent and essential space-determination of its own, that might well be added to and over-laid or even momentarily eclipsed by suggestions of its signification, but how could it possibly be altered or completely suppressed thereby? Of actually present sensations, he says, being suppressed by suggestions of expe- rience " We have not a single well-attested example. In all those illusions which are provoked by sensations in the absence of their usually exciting objects, the mistake never vanishes by the better understanding of the object really present, and by insight into the cause of deception. Phos- phenes provoked by pressure on the eye-ball, by traction on the entrance of the optic nerve, after-images, &c., remain projected into their apparent place in the field of vision, just as the image projected from a mirror's surface continues to be seen behind the mirror, although we know that to all these appearances no outward reality corresponds. True enough, we can remove our attention, and keep it removed, from sensations that have no reference to the outer world, those, e.g., of the weaker after-images, and of entoptic objects, &c. . . . But what would become of our perceptions at all if we had the power not only of ignoring, but of transforming into their opposites, any part of them that differed from that outward experience, the image of which, as that of a present reality, accompanies them in the mind" (Physiol Optik, p. 817)? And again : " On the analogy of all other experience, we should expect that the conquered feelings would persist to our perception, even if only in the shape of recognised illusions. But this is not the case. One does not see how the assumption of originally spatial sensations can explain our optical cognitions, when in the last resort those who believe in these very sensations find themselves obliged to assume that they are overcome by our better judgment, based on experience." These words, coming from such a quarter, necessarily carry great weight. But the authority even of a Helmholtz ought not to shake one's critical composure. And the moment one abandons abstract generalities and comes to close quarters with the particulars, I think one easily sees that no such conclusions as those we have quoted follow from the latter. Helmholtz's (and Wundt's) argument in brief is this, that since our spatial interpretation of certain optical sensations is altered by ideas or other sensations alongside of the former, this spatial interpretation could never have been an original element of the sensations as such, but must always have been what it proves itself now to be, an inference, made unconsciously from a number of premisses. Profitably to conduct the somewhat tedious discussion, I must divide the instances into groups. But the room vouchsafed me in this number of MIND is already ex- hausted, and the discussion of the facts relied on by these authors had best form the opening section of my fourth and final article. 23