V. (Sect. 147-48.) The explanation of the tactual significance of the visible and visual Signs, upon which human experience proceeds, is offered in the Theory that all visible phenomena are arbitrary signs in what is virtually [pg 099] the Language of Nature, addressed by God to the senses and intelligence of Man.
VI. (Sect. 149-60.) The true object studied in Geometry is the kind of Extension given in Touch, not that given in Sight: real Extension in all its phases is tangible, not visible: colour is the only immediate object of Sight, and colour being mind-dependent sensation, cannot be realised without percipient mind. These concluding sections are supplementary to the main argument.
The fact that distance or outness is invisible is sometimes regarded as Berkeley's contribution to the theory of seeing. It is rather the assumption on which the Essay proceeds (sect. 2). The Essay does not prove this invisibility, but seeks to shew how, notwithstanding, we learn to find outness through seeing. That the relation between the visual signs of outness, on the one hand, and the real distance which they signify, on the other, is in all cases arbitrary, and discovered through experience, is the burden of sect. 2-40. The previously recognised signs of “considerably remote” distances, are mentioned (sect. 3). But near distance was supposed to be inferred by a visual geometry—and to be “suggested,” not signified by arbitrary signs. The determination of the visual signs which suggest outness, near and remote, is Berkeley's professed discovery regarding vision.
An induction of the visual signs which “suggest” distance, is followed (sect. 43) by an assertion of the wholly sensuous reality of colour, which is acknowledged to be the only immediate object of sight. Hence visible extension, consisting in colour, must be dependent for its realisation upon sentient or percipient mind. It is then argued (sect. 44) that this mind-dependent visible outness has no resemblance to the tangible reality (sect. 45). This is the first passage in the Essay in which Touch and its data are formally brought into view. Tactual or [pg 100] locomotive experience, it is implied, is needed to infuse true reality into our conceptions of distance or outness. This cannot be got from seeing any more than from hearing, or tasting, or smelling. It is as impossible to see and touch the same object as it is to hear and touch the same object. Visible objects and ocular sensations can only be ideal signs of real things.
The sections in which Touch is thus introduced are among the most important in the Essay. They represent the outness given in hearing as wholly sensuous, ideal, or mind-dependent: they recognise as more truly real that got by contact and locomotion. But if this is all that man can see, it follows that his visible world, at any rate, becomes real only in and through percipient mind. The problem of an Essay on Vision is thus, to explain how the visible world of extended colour can inform us of tangible realities, which it does not in the least resemble, and with which it has no necessary connexion. That visible phenomena, or else certain organic sensations involved in seeing (sect. 3, 16, 21, 27), gradually suggest the real or tangible outness with which they are connected in the divinely constituted system of nature, is the explanation which now begins to dawn upon us.
Here an ambiguity in the Essay appears. It concludes that the visible world cannot be real without percipient realising mind, i.e. not otherwise than ideally: yet the argument seems to take for granted that we are percipient of a tangible world that is independent of percipient realising mind. The reader is apt to say that the tangible world must be as dependent on percipient mind for its reality as the visible world is concluded to be, and for the same reason. This difficulty was soon afterwards encountered in the book of Principles, where the worlds of sight and touch are put on the same level; and the possibility of unperceived reality in both cases is denied; on the ground that a material world cannot be realised in the total [pg 101] absence of Spirit—human and divine. The term “external” may still be applied to tactual and locomotive phenomena alone, if men choose; but this not because of the ideal character of what is seen, and the unideal reality of what is touched, but only because tactual perceptions are found to be more firm and steady than visual. Berkeley preferred in this way to insinuate his new conception of the material world by degrees, at the risk of exposing this juvenile and tentative Essay on Vision to a charge of incoherence.
The way in which visual ideas or phenomena “suggest” the outness or distance of things from the organ of sight having been thus explained, in what I call the First Part of the Essay, the Second and Third Parts (sect. 52-120) argue for the invisibility of real extension in two other relations, viz. magnitude and locality or situation. An induction of the visual signs of tangible size and situation is given in those sections. The result is applied to solve two problems then notable in optics, viz. (1) the reason for the greater visible size of the horizontal moon than of the moon in its meridian (sect. 67-87); and (2) the fact that objects are placed erect in vision only on condition that their images on the retina are inverted (sect. 88-120). Here the antithesis between the ideal world of coloured extension, and the real world of resistant extension is pressed with vigour. The “high” and “low” of the visible world is not the “high” and “low” of the tangible world (sect. 91-106). There is no resemblance and no necessary relation, between those two so-called extensions; not even when the number of visible objects happen to coincide with the number of tangible objects of which they are the visual signs, e.g. the visible and tangible fingers on the hand: for the born-blind, on first receiving sight, could not parcel out the visible phenomena in correspondence with the tangible.
The next Part of the Essay (sect. 121-45) argues for a specific as well as a numerical difference between the original data of sight and the data of touch and locomotion. Sight and touch perceive nothing in common. Extension in its various relations differs in sight from extension in touch. Coloured extension, which alone is visible, is found to be different in kind from resistant extension, which alone is tangible. And if actually perceived or concrete extensions differ thus, the question is determined. For all extension with which man can be concerned must be concrete (sect. 23). Extension in the abstract is meaningless (sect. 124-25). What remains is to marshal the scattered evidence, and to guard the foregoing conclusions against objections. This is attempted in sections 128-46.