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AN ESSAY TOWARDS

A NEW THEORY OF VISION

By

George Berkeley

(Based on the First Edition, Dublin, 1709)

Edited by David R. Wilkins

in section 23, 3rd sentence, a full stop has been changed to a comma before the word ‘but’;

in section 26, 2nd sentence, a full stop has been changed to a comma before the word ‘it’;

in section 27, 2nd sentence, a full stop has been changed to a semicolon before the words ‘it being’;

in section 60, a full stop has been changed to a colon before the words ‘The Visible Magnitude’;

in section 92, 2nd sentence, full stops have been changed to a comma and a semicolon respectively, before the words ‘so’ and ‘we’;

in section 107, 2nd sentence, a full stop has been changed to a colon before the word ‘Yet’;

in section 118, 1st sentence, a full stop has been changed to a colon before the word ‘In’;

in section 131, 4th sentence, a full stop has been changed to a comma before the word ‘so’;

in section 135, 2nd sentence, a full stop has been changed to a comma before the word ‘vid.’;

in section 150, 4th sentence, a full stop has been changed to a comma before the words ‘the very Ideas’.

David R. Wilkins Dublin, December 2002

iv

AN

ESSAY

Towards a

New Theory

OF

VISION.

By George Berkeley, M. A. Fellow of Trinity College, Dublin.

DUBLIN:

Printed by Aaron Rhames, at the Back of

Dick’s Coffee-House, for Jeremy Pepyat, Bookseller in Skinner-Row, MDCCIX.

To the Right Honourable

Sir John Percivale, Bart.

ONE OF

Her Majesty’s

Most Honourable

PRIVY COUNCIL

IN THE

Kingdom of Ireland.

Sir, I Cou’d not, without doing Violence to my Self, forbear upon this Occasion, to give some publick Testimony of the Great and Well-grounded Esteem I have conceiv’d for You, ever since I had the Honour and Happiness of Your Acquaintance. The outward Advantages of Fortune, and the early Honours with which You are Adorn’d, together with the Reputation You are known to have, amongst the Best and most Considerable Men, may well imprint Veneration and Esteem, on the Minds of those who behold You from a Distance. But these are not the chief Motives, that Inspire me with the Respect I bear You. A nearer Approach has given me the View of something in Your Person, infinitely beyond the External Ornaments of Honour and Estate. I mean, an Intrinsic Stock of Vertue and good Sense, a True Concern for Religion, and disinterested Love of Your Country. Add to these an uncommon proficiency in the best, and most useful Parts of Knowledge; together with (what in my Mind is a Perfection of the first Rank) a surpassing Goodness of Nature. All which I have Collected, not from the uncertain Reports of Fame but, from my own Experience. Within these few Months, that I have the Honour to be known unto You, the many delightful Hours I have pass’d in Your Agreeable and Improving Conversation, have afforded me the opportunity of Discovering in You many Excellent Qualities, which at once fill me with Admiration and Esteem. That one at those Years, and in those Circumstances of Wealth and Greatness, shou’d continue Proof against the Charms of Luxury, and those Criminal Pleasures, so fashionable and predominant in the Age we live in. That He shou’d preserve a sweet and modest Behaviour, free from that insolent and assuming Air, so familiar to those who are placed above the ordinary Rank of Men. That He shou’d manage a great Fortune with that Prudence and Inspection, and at the same time, expend it with that Generosity and Nobleness of Mind, as to shew Himself equally remote, from a sordid Parsimony, and a lavish, inconsiderate Profusion of the good Things He is intrusted with. This, surely, were Admirable and Praise-worthy. But that He shou’d moreover by an impartial Exercise of His Reason, and constant Perusal of the Sacred Scriptures, endeavour to attain a right Notion of the Principles of Natural and Revealed Religion. That He shou’d with the Concern of a true Patriot have the Interest of the Publick

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at Heart, and omit no means of Informing Himself what may be Prejudicial, or Advantageous to his Country, in order to prevent the one, and promote the other. In fine, that by a constant Application to the most severe and useful Studies, by a strict Observation of the Rules of Honour and Vertue, by frequent and serious Reflections on the mistaken Measures of the World, and the true End and Happiness of Mankind, He shou’d in all respects qualify Himself, bravely to run the Race that is set before Him, to deserve the Character of Great and Good in this Life, and be ever Happy hereafter. This were amazing, and almost incredible. Yet all this, and more than this, Sir, might I justly say of you; did either your Modesty permit, or your Character stand in Need of it. I know it might deservedly be thought a Vanity in me, to imagine that any thing coming from so obscure a Hand as mine, cou’d add a lustre to your Reputation. But I am withal sensible, How far I advance the Interest of my own, by laying hold on this Opportunity to make it known, that I am admitted into some degree of Intimacy, with a Person of Your Exquisite Judgment. And with that View, I have ventur’d to make You an Address of this Nature, which, the Goodness I have ever experienced in You inclines me to hope, will meet with a favourable Reception at Your Hands. Tho’ I must own I have Your Pardon to ask, for touching on what may, possibly, be Offensive to a Vertue You are possest of in a very distinguishing Degree. Excuse me, Sir, if it was out of my Power, to mention the Name of Sir John Percivale, without paying some Tribute to that Extraordinary and surprising Merit, whereof I have so lively and affecting an Idea, and which, I am sure, cannot be expos’d in too full a light for the Imitation of Others. Of late, I have been agreeably imploy’d in considering the most Noble, Pleasant, and Comprehensive of all the Senses. The fruit of that (Labour shall I call it or) Diversion is what I now Present You with, in Hopes it may give some Entertainment to one who, in the midst of Business and Vulgar Enjoyments, preserves a Relish for the more Refin’d Pleasures of Thought and Reflexion. My Thoughts concerning Vision have led me into some Notions, so far out of the common Road, that it had been improper to Address them to one of a narrow and contracted Genius. But You, Sir, being Master of a large and free Understanding, rais’d above the Power of those Prejudices that enslave the far greater Part of Mankind, may deservedly be thought a proper Patron for an Attempt of this Kind. Add to this, that You are no less dispos’d to Forgive, than qualify’d to discern, whatever Faults may occur in it. Nor do I think You defective in any one Point necessary to form an Exact Judgment on the most abstract and difficult Things, so much as in a just Confidence of Your own Abilities. And in this one Instance, give me leave to say, You shew a manifest weakness of Judgment. With Relation to the following Essay, I shall only add, that I beg your Pardon for laying a Trifle of that Nature in your Way, at a time when you are engag’d in the Important Affairs of the Nation, and desire you to think, that I am, with all Sincerity and Respect

Sir, Your most Faithful

And most Humble Servant

George Berkeley.

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XXXVI. How Converging, and Diverging Rays come to suggest the same Distance. XXXVII. A Person extream Purblind wou’d judge aright in the foremention’d Case. XXXVIII. Lines and Angles why useful in Optics. XXXIX. The not understanding this, a cause of Mistake. XL. A Query propos’d by Mr. Molyneux in his Dioptrics, consider’d. XLI. One born Blind wou’d not at first have any Idea of Distance by Sight. XLII. This not agreeable to the common Principles. XLIII. The proper Objects of Sight, not without the Mind, nor the Images of any thing without the Mind. XLIV. This more fully explain’d. XLV. In what Sense we must be understood to see Distance and external Things. XLVI. Distance and Things placed at a Distance, not otherwise perceiv’d by the Eye than by the Ear. XLVII. The Ideas of Sight more apt to be confounded with the Ideas of Touch than those of Hearing are. XLVIII. How this comes to pass. XLIX. Strictly speaking, we never see and feel the same thing. L. Objects of Sight twofold Mediate and Immediate. LI. These hard to separate in our Thoughts. LII. The received Accounts of our perceiving Magnitude by Sight, false. LIII. Magnitude perceiv’d as immediately, as Distance. LIV. Two kinds of sensible Extension, neither of which is infinitely Divisible. LV. The Tangible Magnitude of an Object Steddy, the Visible not. LVI. By what means, Tangible Magnitude is perceiv’d by Sight. LVII. This farther enlarged on. LVIII. No necessary Connexion between Confusion or Faintness of Appearance, and small or great Magnitude. LIX. The Tangible Magnitude of an Object, more heeded than the Visible; and why. LX. An Instance of this. LXI. Men do not Measure by Visible Feet or Inches. LXII. No necessary Connexion between Visible and Tangible Extension. LXIII. Greater Visible Magnitude might signifie Lesser Tangible Magnitude. LXIV. The Judgments we make of Magnitude depend altogether on Experience. LXV. Distance and Magnitude seen as Shame or Anger. LXVI. But we are prone to think otherwise, and why. LXVII. The Moon seems greater in the Horizon, than in the Meridian. LXVIII. The cause of this Phænomenon, assigned. LXIX. The Horizontal Moon, why greater at one time than another. LXX. The Account we have given, proved to be true. LXXI. And confirmed, by the Moon’s appearing greater in a Mist. LXXII. Objection answer’d. LXXIII. The way wherein Faintness suggests greater Magnitude Illustrated. LXXIV. Appearance of the Horizontal Moon, why thought difficult to explain. LXXV. Attempts towards the Solution of it made by several, but in vain. LXXVI. The Opinion of Dr. Wallis.

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LXXVII. It is shewn to be unsatisfactory. LXXVIII. How Lines and Angles may be of use in computing apparent Magnitudes. LXXIX. One born Blind, being made to See, what Judgment he’d make of Magnitude. LXXX. The Minimum Visibile the same to all Creatures. LXXXI. Objection Answered. LXXXII. The Eye at all times, perceives the same number of visible Points. LXXXIII. Two Imperfections in the Visive Faculty. LXXXIV. Answering to which, we may conceive two Perfections. LXXXV. In neither of these two Ways do Microscopes improve the Sight. LXXXVI. The Case of Microscopical Eyes, consider’d. LXXXVII. The Sight, admirably adapted to the ends of Seeing. LXXXVIII. Difficulty concerning Erect Vision. LXXXIX. The common way of Explaining it. XC. The same shewn to be false. XCI. Not distinguishing between Ideas of Sight and Touch, Cause of Mistake, in this matter. XCII. The Case of one Born Blind, proper to be consider’d. XCIII. Such a one might by Touch, attain to have Ideas of Upper and Lower. XCIV. Which Modes of Situation he’d attribute only to things Tangible. XCV. He’d not at first Sight think any thing he saw, High or Low, Erect or Inverted. XCVI. This Illustrated by an Example. XCVII. By what means he’d come to denominate Visible Objects, high or low, &c. XCVIII. Why he shou’d think those Objects highest, which are Painted on the lowest part of his Eye, and vice versˆa. XCIX. How he wou’d perceive by Sight, the Situation of External Objects. C. Our propension to think the contrary, no Argument against what hath been said. CI. Objection. CII. Answer. CIII. An Object cou’d not be known at first Sight by the Colour. CIV. Nor by the Magnitude thereof. CV. Nor by the Figure. CVI. In the first act of Vision, no Tangible Thing wou’d be suggested by Sight. CVII. Difficulty proposed concerning Number. CVIII. Number of things Visible, wou’d not at first Sight suggest the like number of things Tangible. CIX. Number, the Creature of the Mind. CX. One Born Blind wou’d not at first Sight, number Visible Things as others do. CXI. The Situation of any Object, determin’d with respect only to Objects of the same Sense. CXII. No Distance, great or small, between a Visible and Tangible Thing. CXIII. The not observing this, cause of Difficulty in Erect Vision. CXIV. Which otherwise includes nothing unaccountable. CXV. What is meant by the Pictures being inverted. CXVI. Cause of Mistake in this Matter. CXVII. Images in the Eye, not Pictures of external Objects.

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CLVI. The proper Objects of Sight incapable of being managed as Geometrical Figures. CLVII. The Opinion of those who hold plain Figures to be the Immediate Objects of Sight, considered. CLVIII. Plains no more the immediate Objects of Sight, than Solids. CLIX. Difficult to enter precisely into the Thoughts of the above-mentioned Intelligence. CLX. The Object of Geometry, its not being sufficiently understood, cause of Difficulty and useless Labour in that Science.

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VI. There is another way, mention’d by the Optic Writers, whereby they will have us judge of those Distances, in respect of which, the breadth of the Pupil hath any sensible bigness. And that is the greater or lesser Divergency of the Rays, which issuing from the visible Point, do fall on the Pupil : That Point being judged nearest, which is seen by most diverging Rays; and that remoter, which is seen by less diverging Rays. And so on, the apparent Distance still increasing, as the Divergency of the Rays decreases, till at length it becomes infinite, when the Rays that fall on the Pupil are to Sense Parallel. And after this manner it is said we perceive Distances when we look only with one Eye.

VII. In this Case also, ’tis plain we are not beholding to Experience: It being a certain, necessary Truth, that the nearer the direct Rays falling on the Eye approach to a Parallelism, the farther off is the Point of their Intersection, or the visible Point from whence they flow.

VIII. I have here set down the common, current Accounts that are given of our perceiving near Distances by Sight, which, tho’ they are unquestionably receiv’d for true by Mathemati- cians, and accordingly made use of by them in determining the apparent Places of Objects, do nevertheless seem to me very unsatisfactory: And that for these following Reasons.

IX. First, It is evident that when the Mind perceives any Idea, not immediately and of it self, it must be by the means of some other Idea. Thus, for Instance, the Passions which are in the Mind of another, are of themselves, to me invisible. I may nevertheless perceive them by Sight, tho’ not immediately yet, by means of the Colours they produce in the Countenance. We do often see Shame or Fear in the Looks of a Man, by perceiving the Changes of his Countenance to Red or Pale.

X. Moreover it is evident, that no Idea which is not it self perceiv’d, can be to me the means of perceiving any other Idea. If I do not perceive the Redness or Paleness of a Man’s Face themselves, it is impossible I shou’d perceive by them the Passions which are in his Mind.

XI. Now from Sect. II. ’Tis plain that Distance is in it’s own nature imperceivable, and yet it is perceiv’d by Sight. It remains therefore, that it be brought into view by means of some other Idea, that is it self immediately perceiv’d in the Act of Vision.

XII. But those Lines and Angles, by means whereof Mathematicians pretend to explain the Perception of Distance, are themselves not at all perceiv’d, nor are they in Truth, ever thought of by those unskilful in Optics. I appeal to any ones Experience, whether upon Sight of an Object, he compute it’s Distance by the bigness of the Angle, made by the meeting of the two Optic Axes? Or whether he ever think of the greater or lesser Divergency of the Rays, which arrive from any Point to his Pupil? Nay, whether it be not perfectly impossible for him to perceive by Sense, the various Angles wherewith the Rays according to their greater, or lesser Divergence do fall on his Eye. Every one is himself the best Judge of what he perceives, and what not. In vain shall all the Mathematicians in the World tell me, that I perceive certain Lines and Angles which introduce into my Mind the various Ideas of Distance; so long as I my self am conscious of no such thing.

XIII. Since therefore those Angles and Lines are not themselves perceiv’d by Sight, it follows from Sect. X. that the Mind does not by them judge of the Distance of Objects.

XIV. Secondly, The Truth of this Assertion will be, yet, farther evident to any one that considers those Lines and Angles have no real Existence in Nature, being only an Hypothesis fram’d by Mathematicians, and by them introduc’d into Optics, that they might treat of that Science in a Geometrical way.

XV. The Third and Last Reason I shall give for my Rejecting that Doctrine, is, that tho’ we should grant the real Existence of those Optic Angles, &c. and that it was possible for the Mind to perceive them; yet these Principles wou’d not be found sufficient to explain the Phænomena of Distance. As shall be shewn hereafter.

XVI. Now, It being already shewn that Distance is suggested to the Mind, by the Mediation of some other Idea which is it self perceiv’d in the Act of Seeing; it remains that we enquire what Ideas, or Sensations there be that attend Vision, unto which we may suppose the Ideas of Distance are connected, and by which they are introduced into the Mind. And First, It is certain by Experience, that when we look at a near Object with both Eyes, according as it approaches, or recedes from us, we alter the Disposition of our Eyes, by lessening or widening the Interval between the Pupils. This Disposition or Turn of the Eyes is attended with a Sensation, which seems to me, to be that which in this Case brings the Idea of greater, or lesser Distance into the Mind.

XVII. Not, that there is any natural or necessary Connexion between the Sensation we perceive by the Turn of the Eyes, and greater or lesser Distance. But because the Mind has by constant Experience, found the different Sensations corresponding to the different Dispositions of the Eyes, to be attended each, with a Different Degree of Distance in the Object: There has grown an Habitual or Customary Connexion, between those two sorts of Ideas. So that the Mind no sooner perceives the Sensation arising from the different Turn it gives the Eyes, in order to bring the Pupils nearer, or farther asunder; but it withal perceives the different Idea of Distance which was wont to be connected with that Sensation. Just as upon hearing a certain Sound, the Idea is immediately suggested to the Understanding, which Custom had united with it.

XVIII. Nor do I see, how I can easily be mistaken in this Matter. I know evidently that Distance is not perceived of it self. That by consequence, it must be perceived by means of some other Idea which is immediately perceiv’d, and varies with the different Degrees of Distance. I know also that the Sensation arising from the Turn of the Eyes is of it self, immediately perceiv’d, and various Degrees thereof are connected with different Distances; which never fail to accompany them into my Mind, when I view an Object distinctly with both Eyes, whose Distance is so small that in respect of it, the Interval between the Eyes has any considerable Magnitude.

XIX. I know it is a receiv’d Opinion, that by altering the disposition of the Eyes, the Mind perceives whether the Angle of the Optic Axes is made greater or lesser. And that accordingly by a kind of Natural Geometry, it judges the Point of their Intersection to be nearer, or farther off. But that this is not true, I am convinc’d by my own Experience. Since I am not conscious, that I make any such use of the Perception I have by the Turn of my Eyes. And for me to make those Judgments, and draw those Conclusions from it, without knowing that I do so, seems altogether incomprehensible.

XXVII. Thirdly, an Object being placed at the Distance above specified, and brought nearer to the Eye, we may nevertheless prevent, at least for some time, the Appearance’s growing more confus’d, by straining the Eye. In which Case, that Sensation supplys the place of confused Vision, in aiding the Mind to judge of the Distance of the Object; it being esteemed so much the nearer, by how much the effort, or straining of the Eye in order to distinct Vision, is greater.

XXVIII. I have here set down those Sensations or Ideas, that seem to me to be the constant and general Occasions of introducing into the Mind, the different Ideas of near Distance. ’Tis true in most Cases, that divers other Circumstances contribute to frame our Idea of Distance, viz. the particular Number, Size, Kind, &c. of the things seen. Concerning which as well as all other the forementioned Occasions which suggest Distance, I shall only observe, they have none of them, in their own Nature, any Relation or Connexion with it. Nor is it possible, they shou’d ever signifie the various Degrees thereof, otherwise than as by Experience they have been found to be connected with them.

XXIX. I shall proceed upon these Principles to account for a Phænomenon, which has hitherto strangely puzzled the Writers of Optics, and is so far from being accounted for by any of their Theories of Vision, that it is, by their own Confession, plainly repugnant to them. And of Consequence, if nothing else cou’d be objected, were alone sufficient to bring their Credit in Question. The whole Difficulty I shall lay before you in the Words of the Learned Doctor Barrow, with which he concludes his Optic Lectures.

O

P

Q

Z

B

A

E F

O

P

Q

Z

B

A

E F

Hæc sunt, quæ circa partem Opticæ præcipue Mathematicam dicenda mihi suggessit meditatio. Circa reliquas, (quæ φυσικωτ ραι´ sunt, ade´oque sæpiuscule pro certis principiis plausibiles conjecturas venditare necessum habent) nihil fere quicquam admodum verisimile succurrit, a pervulgatis (ab iis, inquam, quæ Keplerus, Scheinerus, Cartesius, & post illos alii tradiderunt) alienum aut diversum. Atqui tacere malo, quam toties oblatam cramben reponere. Proinde receptui cano; nec ita tamen ut prors´us discedam, anteaquam improbam

quandam difficultatem (pro sinceritate quam & vobis & veritati debeo minime dissumulan- dam) in medium protulero, quæ doctrinæ nostræ, hactenus inculcatæ, se objicit adversam, ab eˆa saltem nullam admittit solutionem. Illa, breviter, talis est: Lenti vel Speculo cavo E B F exponatur punctum visibile A, ita Distans ut Radii ex A manantes ex inflectione ver- sus axem A B cogantur. Sitque radiationis Limes (seu puncti A imago, qualem supra passim statuimus) punctum Z. Inter hoc autem & inflectentis verticem B uspiam positus concip- iatur Oculus. Quæri jam potest ubi loci debeat punctum A apparere? Retrorsum ad punc- tum Z videri non fert Natura (cum omnis impressio sensum afficiens proveniat a partibus A) ac experientia reclamat. Nostris auteme placitis consequi videtur, ipsum ad partes anticas apparens, ab intervallo longissime dissito, (quod & maximum sensibile quodvis Intervallum quodammodo exsuperet) apparere. Cum enim quo Radiis minus divergentibus attingitur Ob- jectum, eo (seclusis utique prænotionibus & præjudiciis) longius abesse sentiatur; et quod Parallelos ad Oculum Radios projicit, remotissime positum æstimetur. Exigere Ratio videtur, ut quod convergentibus radiis apprehenditur, adhuc magis, si fieri posset, quoad apparentiam elongetur. Quin & circa Casum hunc generatim inquiri possit, quidnam omnino sit, quod apparentem puncti A locum determinet, faciatque quod constanti ratione nunc propius, nunc remotius appareat? Cui itidem dubio, nihil quicquam ex hactenus dictorum Analogiˆa, re- sponderi posse videtur, nisi debere punctum A perpetuo longissime semotum videri. Verum experientia secus attestatur, illud pro diversˆa Oculi inter puncta B, Z, positione varie distans; nunquam fere (si unquam) longinquius ipso A libere spectato, subinde vero multo propinquius adparere; quinimo, quo oculum appellentes radii magıs convergunt eo speciem Objecti propius accedere. Nempe, si puncto B admoveatur Oculus, suo (ad lentem) fere nativo in loco con- spicitur punctum A (vel æque distans, ad Speculum;) ad O reductus oculus ejusce speciem appropinquantem cernit; ad P adhuc vicinius ipsum existimat; ac ita sensim, donec alicubi tandem, velut ad Q, constituto oculo objectum summe propinquum apparens, in meram con- fusionem incipiat evanescere. Quæ sane cuncta rationibus atque decretis nostris repugnare videntur, aut cum iis saltem parum amice conspirant. Neque nostram tantum sententiam pul- sat hoc experimentum; at ex æquo cæteras quas nˆorim omnes; veterem imprimıs ac vulgatam, nostræ præ reliquis affinem ita convellere videtur, ut ejus vi coactus doctissimus A. Tacquetus isti principio (cui pene soli totam inædificaverat Catoptricam suam) ceu infido ac inconstanti renunciˆarit, adeoque suam ipse doctrinam labefactˆarit; id tamen, opinor, minime facturus, si rem totam inspexisset penitius, atque difficultatis fundum attigisset. Apud me vero non ita pollet hæc, nec eousque præpollebit ulla difficultas, ut ab iis quæ manifeste rationi con- sentanea video, discedam; præsertim quum ut hˆıc accidit, ejusmodi difficultas in singularis cujuspiam casˆus disparitate fundetur. Nimirum in præsente casu peculiare quiddam, naturæ subtilitati involutum, delitescit, ægre fortassıs, nisi perfectius explorato videndi modo, dete- gendum. Circa quod nil, fateor, hactenus excogitare potui, quod adblandiretur animo meo, nedum plan`e satisfaceret. Vobis itaque nodum hunc, utinam feliciore conatu, resolvendum committo.

In English as follows. ‘I have here delivered what my Thoughts have suggested to me, concerning that part of Optics which is more properly Mathematical. As for the other parts of that Science (which being rather Physical, do consequently abound with plausible Conjectures, instead of certain Principles) there has in them scarce any thing occurr’d to my Observation, different from