The enunciation of the summary generalisation, which forms the “New Theory of Vision” (sect. 147-8), may be taken as the Fifth and culminating Part of the Essay.
The closing sections (149-60), as I have said, are supplementary, and profess to determine the sort of extension—visible or tangible—with which Geometry is concerned. In concluding that it is tangible, he tries to picture the mental state of Idominians, or unbodied spirits, endowed with visual perceptions only, and asks what their conception of outness and solid extension must be. Here further refinements in the interpretation of visual perception, and its organic conditions, which have not escaped the attention of latter psychologists and biologists, are hinted at.
Whether the data of sight consist of non-resembling arbitrary Signs of the tactual distances, sizes, and situations of things, is a question which some might prefer to deal with experimentally—by trial of the experience of persons in circumstances fitted to supply an answer. [pg 103] Of this sort would be the experience of the born-blind, immediately after their sight has been restored; the conception of extension and its relations found in persons who continue from birth unable to see; the experience (if it could be got) of persons always destitute of all tactual and locomotive perceptions, but familiar with vision; and the facts of seeing observed in infants of the human species, and in the lower animals.
Berkeley did not try to verify his conclusions in this way. Here and there (sect. 41, 42, 79, 92-99, 103, 106, 110, 128, 132-37), he conjectures what the first visual experience of those rescued from born-blindness is likely to be; he also speculates, as we have seen, about the experience of unbodied spirits supposed to be able to see, but unable to touch or move (sect. 153-59); and in the Appendix he refers, in confirmation of his New Theory, to a reported case of one born blind who had obtained sight. But he forms his Theory independently of those delicate and difficult investigations. His testing facts were sought introspectively. Indeed those physiologists and mental philosophers who have since tried to determine what vision in its purity is, by cases either of communicated sight or of continued born-blindness, have illustrated the truth of Diderot's remark—“préparer et interroger un aveugle-né n'eût point été une occupation indigne des talens réunis de Newton, Des Cartes, Locke, et Leibniz[275].”
Berkeley's New Theory has been quoted as a signal example of discovery in metaphysics. The subtle analysis which distinguishes seeing strictly so called, from judgments about extended things, suggested by what we see, [pg 104] appears to have been imperfectly known to the ancient philosophers. Aristotle, indeed, speaks of colour as the only proper object of sight; but, in passages of the De Anima[276] where he names properties peculiar to particular senses, he enumerates others, such as motion, figure, and magnitude, which belong to all the senses in common. His distinction of Proper and Common Sensibles appears at first to contradict Berkeley's doctrine of the heterogeneity of the ideal visible and the real tangible worlds. Aristotle, however, seems to question the immediate perceptibility of Common Sensibles, and to regard them as realised through the activity of intelligence[277].
Some writers in Optics, in mediaeval times, and in early modern philosophy, advanced beyond Aristotle, in explaining the relation of our matured notion of distance to what we originally perceive in seeing, and in the fifteenth century it was discovered by Maurolyco that the rays of light from the object converge to a focus in the eye; but I have not been able to trace even the germ of the New Theory in these speculations.