distinction of Proper and Common Sensibles appears at first to contradict Berkeley's doctrine of the absolute heterogeneity of the visible and the tangible. Aristotle, however, seems to question the perceptibility in mere sense of the Common Sensibles, and to regard them rather as produced by the activity of intelligence, in conjunction with sense.*

Some writers in Optics, in medieval times, and in the early period of modern philosophy, advanced beyond Aristotle, in explaining the relation of our matured notion of distance to what we are originally conscious of 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 distinctive principles of the New Theory in any of these speculations.

Excepting some hints by Des Cartes, Malebranche was among the first dimly to anticipate Berkeley, in resolving our supposed power of seeing distances into an interpretation of arbitrary signs which we learn by experience gradually to understand; and also in founding his explanation upon consciousness. The most important part of Malebranche's account of vision is contained in his Recherche de la Verite (Liv. I. ch. 9), in one of those chapters in which he discusses the general fallaciousness of the senses, and in particular of sight, in its judgments about extension. He accounts for the inevitable uncertainty of what we take to be immediate visual perceptions of the distances of things, by an analysis of the manner in which these supposed perceptions are formed-in short, by assigning them not to sense proper but to the misinterpretation of our sense-experience in thought. He enumerates as signs of distance :(1) The angle made by the rays of our eyes, and the change in the position of our eyes corresponding to the changes of this angle -which he illustrates, like Descartes and others, by the blind man and cross sticks; (2) the figure of the organ of vision, dependent on the tension of its muscles; (3) the size of the images painted at the bottom of the eye; (4) the power of the object upon the eye;

Sir A. Grant (Ethics of Aristotle, vol. II. p. 172), remarks as to the doctrine that the Common Sensibles are apprehended concomitantly by the senses

This is surely the true view; we see in the apprehension of number, figure, and the like, not an operation of sense, but the mind putting its own forms and categories, i. e. itself, on the external object. It would follow then that the senses cannot really be separated from the mind; the senses and the mind each contribute an element to every knowledge. Aristotle's doctrine of κown

ato@nois would go far, if carried out, to modify his doctrine of the simple and innate character of the senses, e. g. sight, (cf. Eth. II. 1, 4), and would prevent its absolute collision with Berkeley's Theory of Vision.' -See Sir W. Hamilton, Reid's Works, pp. 828-830.

Mr. Stewart (Collected Works, vol. I. p. 341, note) quotes Aristotle's Ethics II. I as evidence that Berkeley's doctrine, with respect to the acquired perceptions of sight, was quite unknown to the best metaphysicians of antiquity.'

(5) the clearness and distinctness of the image formed in the eye; (6) the relation of the object to which the visual judgment refers to other intervening objects.

That the Recherche of Malebranche, published more than thirty years before the Essay, was familiar to Berkeley, and that this with other chapters was much under his eye, before the publication of his New Theory, is proved by internal evidence, and by his commonplace book. I am not able to discover signs of a similar connection between Berkeley and the chapter on the mystery of sensation, in Glanvill's Scepsis Scientifica (ch. 5), published some years before the Recherche of Malebranche, where Glanvill refers to 'a secret deduction,' through which— from motions, &c., of which we are immediately percipient-we 'spell out' figures, distances, magnitudes, and colours, which have no resemblance to them.

The nearest approach to the New Theory is to be found in a passage which first appeared in the fourth edition of Locke's Essay, published in 1694, to which Berkeley refers in his own Essay (sect. 132-35), and which, on account of its importance in the history of the Theory of Vision, and of our notion of Extension, I shall here transcribe at length :

'We are further to consider concerning Perception that the ideas we receive by sensation are often, in grown people, altered by the judgment, without our taking notice of it. When we set before our eyes a round globe of any uniform colour, e. g. gold, alabaster, or jet, it is certain that the idea thereby imprinted in our mind is of a flat circle, variously shadowed, with several degrees of light and brightness coming to our eyes. But, we having by use been accustomed to perceive what kind of appearance convex bodies are wont to make in us, what alterations are made in the reflection of light by the difference in the sensible figures of bodies-the judgment presently, by an habitual custom, alters the appearances into their causes; so that, from that which is truly variety of shadow or colour, collecting the figure, it makes it pass for a mark of figure, and frames to itself the perception of a convex figure and an uniform colour, when the idea we receive from them is only a plane variously coloured, as is evident in painting.

To which purpose I shall here insert a problem of that very ingenious and studious promoter of real knowledge, the learned and worthy Mr. Molyneux, which he was pleased to send me in a letter some months since, and it is this:-Suppose a man born blind, and now adult, and taught by his touch to distinguish between a cube and a sphere of the same metal, and nighly of the same bigness, so as to tell, when he felt the one and the other, which is the cube and which the sphere.

Suppose then the cube and the sphere placed on a table, and the blind man be made to see: quere, whether, by his sight, before he touched them, he could not distinguish and tell, which is the globe and which the cube? To which the acute and judicious proposer answers: 'Not.' For, though he has obtained the experience of how a globe, how a cube affects his touch; yet he has not obtained the experience that what affects his touch so and so, must affect his sight so and so; so that a protuberant angle in the cube, that pressed his hand unequally, shall appear to his eye as it does in the cube.-I agree with this thinking gentleman, whom I am proud to call my friend, in his answer to this his problem, and am of opinion that the blind man, at first sight, would not be able to say with certainty which was the globe and which the cube, whilst he only saw them; though he would unerringly name them by his touch, and certainly distinguish them by the difference in their figures felt.

'This I have set down, and leave with my reader, as an occasion for him to consider how much he may be beholden to experience, improvement, and acquired notions, where he thinks he had not the least use of, or help from them: and the rather because this observing gentleman further adds that, having, upon the occasion of my book, proposed this problem to divers very ingenious men, he hardly ever met with one that at first gave the answer to it which he thinks true, till by hearing his reasons they were convinced.

'But this is not I think usual in any of our ideas but those received by sight because sight, the most comprehensive of the senses, conveying to our minds the ideas of light and colours, which are peculiar only to that sense; and also the far different ideas of space, figure, and motion, the several varieties of which change the appearance of its proper object, i. e. light and colours; we bring ourselves by use to judge of the one by the other. This, in many cases, by a settled habit, in things whereof we have frequent experience, is performed so constantly and so quick, that we take that for the perception of our sensation, which is an idea formed by our judgment; so that one, i. e. that of sensation, serves only to excite the other, and is scarce taken notice of itself; as a man who reads or hears with attention and understanding takes little notice of the character or sounds, but of the ideas that are excited in him by them.

'Nor need we wonder that this is done with so little notice, if we consider how very quick the actions of the mind are performed; for, as itself is thought to take up no space, to have no extension, so its actions seem to require no time, but many of them seem to be crowded

into an instant. I speak this in comparison of the actions of the body.... Secondly, we shall not be much surprised that this is done with us in so little notice, if we consider how the facility we get of doing things, by a custom of doing, makes them often pass in us without notice. Habits, especially such as are begun very early, come at last to produce actions in us which often escape our observation. . . . And therefore it is not so strange that our mind should often change the idea of its sensation into that of its judgment, and make the one serve only to excite the other, without our taking notice of it.' (Essay concerning Human Understanding, Book II. ch. 9, § 8.)

...

This remarkable passage anticipates by implication the conception of the synthetic operation of imagination and habit, upon the materials originally given in sense, which, under the name of 'suggestion,' is the constructive principle of the New Theory.

The following sentences, relative to the invisibility of distances, contained in the Treatise of Dioptrics (published in 1690) of Locke's friend and correspondent William Molyneux-who was tutor in Trinity College, Dublin, when Berkeley entered the University, and whose son was Berkeley's pupil-illustrate Locke's statements, while they may be compared with the opening sections of the following Essay :—

'In plain vision the estimate we make of the distance of objects (especially when so far removed that the interval between our two eyes bears no sensible proportion thereto, or when looked upon with one eye only) is rather the act of our judgment than of sense; and acquired by exercise, and a faculty of comparing, rather than natural. For, distance of itself is not to be perceived; for, 'tis a line (or a length) presented to our eye with its end toward us, which must therefore be only a point, and that is invisible. Wherefore distance is chiefly perceived by means of interjacent bodies, as by the earth, mountains, hills, fields, trees, houses, &c. Or by the estimate we make of the comparative magnitude of bodies, or of their faint colours, &c. These I say are the chief means of apprehending the distance of objects that are considerably remote. But as to nigh objects-to whose distance the interval of the eyes bears a sensible proportion-their distance is perceived by the turn of the eyes, or by the angle of the optic axes (Gregorii Opt. Promot. prop. 28). This was the opinion of the ancients, Alhazen, Vitellio, &c. And though the ingenious Jesuit Tacquet (Opt. Lib. I. prop. 2) disapprove thereof, and objects against it a new notion of Gassendus (of a man's seeing only with one eye at a time one and the same object), yet this notion of Gassendus being absolutely false (as I could demonstrate were it not beside my present purpose), it makes nothing against this opinion.

C

'Wherefore, distance being only a line and not of itself perceivable, if an object were conveyed to the eye by one single ray only, there were no other means of judging of its distance but by some of those hinted before. Therefore when we estimate the distance of nigh objects, either we take the help of both eyes, or else we consider the pupil of one eye as having breadth, and receiving a parcel of rays from each radiating point. And, according to the various inclinations of the rays from one point on the various parts of the pupil, we make our estimate of the distance of the object. And therefore (as is said before), by one single eye we can only judge of the distance of such objects to whose distance the breadth of the pupil has a sensible proportion. . . . For, it is observed before (prop. 29, sec. 2, see also Gregorii Opt. Promot. prop. 29) that for viewing objects remote and nigh, there are requisite various conformations of the eye-the rays from nigh objects that fall on the eye diverging more than those from more remote objects.' (Treatise of Dioptrics, Part I. prop. 31.)

The preceding passages indicate the state of opinion regarding the mental fact of vision about the time Berkeley's Essay appeared, especially among those with whose works he was familiar. I shall now refer to one or two illustrations of the change in scientific belief which his treatise produced. *

The New Theory has occasioned much interesting criticism since 1709. At first indeed it drew little attention. For twenty years after its publication the allusions to it were few. The account of Cheselden's famous experiment, published in 1728, in the Philosophical Transactions, which seemed to bring the Theory to the test of a palpable physical experiment, drew some scientific interest to Berkeley's reasonings. The state of theological controversy about the same time confirmed the tendency to discuss a doctrine which represented ordinary vision as the interpretation of divinely constituted signs.

Occasional discussions of some of the principles involved in the New Theory may be found in the Gentleman's Magazine, from 1732 till Berkeley's death in 1753. Some adverse criticism is also contained in Dr. Smith's Optics, published in 1738.

The essential parts of Berkeley's analysis are very well explained by Voltaire, in his Elemens de la Philosophie de Newton. The following passage from that work is here given on its own account, and also

* A work resembling Berkeley's in its title, but in little else, appeared more than twenty years before the Essay-the Nova

Visionis Theoria of Dr. Briggs, published in 1685.