The Online Library of Liberty

A project of Liberty Fund, Inc.

• Introduction
• Note On the Text
• Preface to the Former Editions
• Preface to the Present Edition
• Part I
• Essay I: Our Attachment to Objects of Distress
• Essay II: Foundation and Principles of Morality I
• Essay III: Liberty and Necessity
• Essay IV: Personal Identity I
• Appendix I Containing the Substance of a Pamphlet Wrote In Defence of the Third Essay 1
• Part Ii
• Essay I: Belief
• Essay II: External Senses
• Essay Iii I: Different Theories of Vision 1
• Essay Iv I: Matter and Spirit 1
• Essay V: Power, Cause and Effect
• Essay VI: Knowledge of Future Events
• Essay VII: Dread of Supernatural Powers In the Dark
• Essay VIII: Knowledge of the Deity
• Appendix
• Bibliography

essay iiiⁱ

Different Theories of Vision1

The sense of seeing is one of the most simple and distinct of all that belong to man. And yet by many philosophers it has been rendered so intricate, as to tempt plain people to a diffidence and distrust of it. The present Essay is intended to point out the errors of these philosophers, and to restore the sense of seeing to the authority it justly possesses in human nature, with respect to veracity. I have a further view, which is to put writers on their guard against attempting subjects beyond the sphere of human knowledge, of which I shall have occasion to give several mortifying instances, even in this narrow subject, the theory of vision,—scarce more excusable than the attempt made by the inhabitants of Shinar to erect a tower whose top should reach into heaven.2 Moderation is proper for man, no less in reasoning than in behaviour. Man, tho’ the chief of the terrestrial creation, is limited in the powers of his mind, as much as in those of his body. When he struggles to pass these limits, he acts and thinks in vain; and meets with nothing but disappointment and disgrace.

The connection between soul and body, and their manner of acting upon each other, are hid from us; and for ever will remain hid: we cannot form even the slightest conjecture how mind acts on matter, or matter on mind. And yet, writers talk familiarly on that mystery, as if they had been admitted to the councils of the Almighty in the formation of man.* A collection of all the strange and incoherent stuff that has been written on that subject, would fill a large volume. In the different theories of the sense of seeing adopted by writers of note, will be found, if I be not grossly mistaken, many rash attempts to build the tower of Babel; and to these I shall confine myself as being connected with the present work.

One capital error that all the writers on that subject have fallen into, is, to apply to mind axioms peculiar to matter. It holds true in matter, that one body cannot act upon another at a distance, nor be acted upon but by what is in contact with it. Extension in length, breadth, and thickness enters into every idea we can form of matter; and every thing that is so extended, is in our conception matter. Therefore, if mind or spirit be different from matter, which all admit, it cannot be so extended; consequently cannot occupy space, nor have any relation to place. Now, as local situation is implied in the axioms mentioned, there can be no foundation for applying them to mind, which has no local situation. The actions of the soul and body on each other, must be governed by laws intirely different from what govern the mutual actions of matter. As these laws are beyond the bounds of human knowledge, every attempt to explore them must prove abortive and be absurd. Writers however, by applying inadvertently to mind the axioms mentioned peculiar to matter, have been led into a labyrinth of metaphysical jargon, of the brain being the local situation of the soul, of phantasms or images carried along the nerves into the brain, where, being in contact with the soul, they make impressions upon it, &c. &c.

I begin with Aristotle’s account of vision. Taking it for granted that mind and matter cannot act upon each other at a distance, he is reduced to hold, that of every external object there is in the mind of the be holder a phantasm or species, having the form of the object without the matter, like the impression of a seal upon wax; and that by these, external objects are made visible.3 His followers add, that these phantasms or species, sent from external objects, make impressions on the passive intellect, which are perceived by the active intellect. This account of vision differs not from that of Epicurus, which is, that external objects send forth, constantly and in every direction, slender ghosts or films of themselves, which striking the mind are the means of perception.4 Had these philosophers instead of films and phantasms, stumbled on rays of light passing from the object to the eye of the spectator, they would have been nearer the truth. But they may well be excused, as they were groping in the dark and had no knowledge of pictures on the retinae. But they ought not so easily to be excused for stopping short at the first step: they could not expect to give satisfaction, but by explaining how it comes that these films and phantasms, of which the mind is not conscious, are however the means of seeing distant objects. Doctor Porterfield is the only writer who attempts to supply that defect. His words are,

What does one think of sensations having a local situation in the brain or sensorium, and of being traced back along imaginary perpendicular lines? Have these words any meaning?

Here it will be observed, that neither Aristotle nor Epicurus make any doubt of seeing the external objects themselves: they only pretend to explain by what means these objects, though at a distance, are perceived. Des Cartes, adhering more rigidly to the axiom that objects at a distance cannot act upon us, denies that we have any perception of such objects, maintaining, that the objects we perceive are not external, but images or ideas in the mind. From which premises he concludes, that the existence of external objects cannot be known to us, otherwise than by a process of reasoning, inferring it from these images or ideas.5 Locke adopting this doctrine holds,

He accordingly employs a whole chapter to make out by reasoning the existence of external objects. Doctor Porterfield, adhering to this doctrine, expresses himself in very strong terms.

With respect to this theory, it cannot escape observation, that in two particulars it contradicts the testimony of our senses; first in denying that we see external objects; and next in affirming that we perceive images in the mind, which no man ever perceived. It may be further observed, that supposing these particulars to hold true, yet this account of vision remains wofully imperfect. We acknowledge pictures on the retinae; but how these pictures are conveyed to the brain, no man can justly say. Next, supposing them conveyed, no man can account how they should raise a perception in the mind. And admitting the perception, it ought naturally to be of the pictures; and yet we have not the slightest consciousness of these pictures.

But waving these observations, there occurs an argument founded on a stuborn fact directly inconsistent with this theory. The three philosophers last mentioned agree in maintaining, that as external objects are hid from our eye-sight, our belief of them must depend on a process of reasoning. Their reasonings have been found insufficient by two acute philosophers, Berkeley and Hume, as shall by and by be mentioned. But supposing them solid, what must be the condition of a great plurality, who are incapable of abstruse reasoning? they must remain utterly ignorant of external objects. Yet the direct contrary is vouched by the testimony of all men; these philosophers excepted, who renounce the evidence of their senses for the sake of a favourite opinion. Even children have as lively a conviction of external objects as the most acute reasoners. In fact, objects of sight are perceived so clearly, as that we cannot even conceive that the Author of our nature could have made them more clear, or have given us a more satisfactory conviction of them. The means by which this is done, are beyond the sphere of human knowledge: we do not therefore pretend to say how it is done: we only say that it is done.

Misled by the same error of applying to mind axioms that hold true of matter only, two philosophers, Berkeley and Hume, have given us theories still more wild. The former, taking it for granted that mind and matter cannot act upon each other at a distance, and perceiving the insufficiency of the arguments urged by Des Cartes and Locke for the existence of matter, has ventured bluntly to deny its existence. The latter, observing Berkeley’s reason for denying the existence of matter to be equally conclusive against the existence of mind, has with great intrepidity discarded both, giving quarter to nothing but to phantasms or ideas, floating in the great void without inhering in any subject or substratum; an absurdity farther distant from common sense than ever entered into the imagination of any other writer.

Upon our supposed inability to see objects at a distance, is grafted a difficulty that has puzzled many a philosopher, how it comes that with two eyes external objects appear single only. Supposing that external objects are not visible to us, and that we perceive nothing but the representative pictures in the retinae, it seems highly presumable, that the two perceptions raised by these two pictures should to every external object give the appearance of being double. Gassendus and Porta could not imagine any solution of the difficulty, but to contradict an evident fact, asserting that though both eyes are open, yet we only see with one at a time.7

The great Newton, sensible of the difficulty arising from the two pictures, endeavours to remove it in the following words.

The difficulty is attempted to be solved by uniting in the brain the two pictures, in order to produce a single perception. But whether this be fact or even probable, is what we can never know. One thing we know to be fact, that the external object appears single, even where the optic nerves happen not to be united. In a case reported by Vesalius, the optic nerves did not meet: yet the intimate companions of the man when alive, declared, that he never complained of any defect of sight, nor of objects appearing to him double. But what I chiefly remark here is, that Sir Isaac transgresses the bounds of human knowledge, in saying that the pictures in the retinae are carried along the optic nerves and united in the brain. Hypotheses may be thrown out at pleasure; but if they be of things surpassing our knowledge where we have no data either to verify or refute, they are no better than castles in the air. If the greatest philosopher ever existed be liable to this censure, it ought to be a most serious admonition to all others. Will the reader indulge me to observe further, that this hypothesis has not even a plausible appearance. I can well conceive a picture with the canvass, to be carried from place to place; but it is past my conception, how a painting can be detached from the retinae more than a painting from the canvass; or how in that detached state it can be carried to the brain, either entire or in halves. But supposing a picture formed in the brain, it must be different from those in the retinae; and how this is done is not said, nor how this new picture can raise a perception of the external object. Here we are left in utter darkness, where light is the most wanted. It may even be doubted, whether the pictures in the retinae contribute to vision. Their existence is no proof; because they are necessarily produced by rays of light acting on the eyes, precisely as on a camera obscura; and the same picture appears in an eye, even when separated from the body. This censure may be thought too severe; but where truth and reality are concerned, no partiality to any opinion ought to be admitted, not even to that of a Newton.*

Dr. Briggs taking it for granted with Sir Isaac, that two separate pictures in the brain must occasion the external object to appear double, endeavours to unite them in the following manner,

An inference from an object of sound to one of sight can never hold, as there is no resemblance between objects of different senses upon which to form any sort of comparison. I can readily conceive, that fibres having the same tone must produce similar sounds, or if you please the same sound; and that fibres having different tones must produce dissimilar sounds; but that fibres, whether having the same or different tones, should produce pictures, is to me utterly inconceivable. What else have we here but sounding words that have no meaning? I need scarce add, that the doctor’s comparison overturns his theory, instead of supporting it. Two sounds are perceived as different, whether concordant or discordant. Two sounds in unison make not an exception; for unisons produce harmony, and there is no harmony in a single sound.

Dr. Porterfield composed an ingenious treatise on vision, in which the present subject is handled at great length.9 He differs from both Sir Isaac and Dr. Briggs; for he admits that the two pictures on the retinae are by motion propagated along the fibres of the optic nerves to the brain, so as to raise two perceptions in the mind; and that the mind traces back these perceptions from the sensorium to the retinae, and from thence to the object perceived. Here the two perceptions are kept distinct through the whole process till the ultimate step; and he gives the following reason for the objects appearing single in place of double. “By an original law of our nature, we perceive visible objects in their true place; and consequently, an object seen with each eye in its true place at the same time, must appear single.”

Here it is taken for granted, that we see external objects, and that we see them with both eyes in the same place; inadvertently it must be acknowledged, as it flatly contradicts what he had been all along inculcating, that external objects are not visible otherwise than in imagination. It was incumbent on the Doctor to account for single vision upon his own theory; and yet he accounts for it on an opposite theory. It is true, that two bodies cannot occupy the same place at the same time; but they may occupy it in imagination, and ten thousand more. Had the Doctor adhered to his own theory, to wit, that we know nothing of external objects but by reasoning from ideas or images in the mind, every argument must have led him to conclude with Sir Isaac and Doctor Briggs, that the two pictures in the retinae ought to produce the appearance of a double external object. This of itself is a confutation of the Doctor’s theory, as in fact objects are never seen double when the eyes are in a sound state.

But it will afford a more satisfactory confutation, to examine what the result must be, from seeing external objects themselves and not their images. To pave the way, I shall premise an account of the other external senses that have double organs. I lay my two hands on a globe: an impression is made upon each hand, nay upon each finger, every one of which impressions must be felt by the mind. There is here no coincidence of place; and yet the object is not felt double. In hearing, an impression is made on the drum of each ear, which one would naturally think should raise in the mind two perceptions of sound; yet in fact we hear but one sound. The effect is similar with respect to smell from effluvia taken in at the two nostrils. There must be here some cause, that prevents a multiplication in appearance of the same object. Sir Isaac Newton and Doctor Briggs with respect to vision, explain this difficulty by uniting the two pictures into one, to produce a single perception only. With respect to the other senses, we are left in the dark; for it is not said that this explanation is applicable to any of them. Doctor Smith in his optics attributes single vision entirely to custom; which in effect is maintaining, that in childhood we see double, hear double, feel double, and smell double. This solution I cannot acquiesce in. If we commence life with double perceptions, they, instead of being altered by custom, will be confirmed by it. But perhaps the Doctor’s meaning is, that in time the perceiving the same object double being discovered to be an error, we learn to correct the error and to perceive the object single only as it is in reality. This supposed struggle between perception and reflection and the complete victory obtained by the latter, must be the work of time and ripe years; which could not escape remembrance. But as no man can say that he ever had such remembrance, it is a demonstration that there never existed such a struggle.

All the writers on this subject take it for granted, that two perceptions must necessarily make the external object appear double; and they have reason to do so, supposing external objects not to be perceived but their ideas or images only. Sir Isaac Newton endeavours to reduce the two pictures in the eyes to a single picture in the brain, producing consequently but a single perception. Doctor Briggs attempts the same in a different way. Doctor Porterfield admits two perceptions; but in effect reduces them to one, when the object is seen with both eyes in the same place. But upon supposition of the real fact that the external objects themselves are perceived, the question is, why should two perceptions produce necessarily an appearance of two objects? Let us give attention to that question. If the external object could not be known but by a chain of reasoning, the conclusion from the double perception would naturally be that the external object should appear double. But the case differs widely where the external object is seen, and perhaps known. My little dog has a collar with my name inscribed: it has long been my companion; and I cannot mistake it for another. Viewing it with one eye, I know the creature: viewing it with the other, it is the same. What is there here but the seeing the same object at different times? Viewing now my dog with both eyes at once, it is still the same dog: the two perceptions are indeed varied as they now coincide in time; but what else can be the effect of this coincidence but a sight of the dog as formerly? I look to my dog, lay both hands upon him, and at the same time hear him bark. In this experiment, my perceptions are many and various; but as they are only different perceptions of the same object, they have no tendency to give it the appearance of more than one.

It is extremely true, that for ought we know of vision, our eyes might have been so framed, as to make an object appear double with one eye, instead of appearing single with two. But this would be a delusion, which cannot be imputed to the Author of our nature. He has provided us with the sense of seeing to perceive objects as they exist; and so effectually has he prevented delusion, that when by a distorted eye an object appears double, means are afforded to detect the error. Of the five senses, four have double organs, that if one be rendered useless, its office may be supplied by the other. These organs produce indeed two perceptions; but being perceptions of the same object, they cannot have the effect to make it appear double. No person thinks it necessary to explain, why an object repeatedly perceived in succession appears single: why not the same in simultaneous perceptions? In hearing, smelling, and touching, the object never appears double: why should it appear double in seeing? I am not satisfied with Dr. Porterfield’s explanation of single vision, in which it is taken for granted that the external object itself is seen by each eye separately; for though the two apparent objects must be blended when seen in the same place, it is however natural to think, that there should still remain an impression and conviction of two objects. But be this as it may, the Doctor undoubtedly errs in affirming that each eye has a separate object. Both eyes have but one object, evident from this, that the external object never appears double whether seen successively by one eye, or by both at once. And as the object appears single, his solution of blending two objects together does not hold. There can remain no doubt that the account given in this Essay of single vision is solid, when it is equally applicable to every one of the other senses that have double organs; whereas the Doctor’s explanation holds only in vision. In short, we are so constituted as to have a firm conviction of the reality of external objects from the perceptions of sight; and by the same constitution, we have a firm conviction of the Identity of an object, from a pair of organs as from a single organ. Is it above the power of the Almighty to make us perceive things as they really exist? In fact he has done so; and what better evidence can be required, than that when our eyes are found, an object is never seen double. Thus, a difficulty that has puzzled many a sage philosopher, turns out to be no difficulty at all.

Philosophers may exert their utmost powers to explain vision; but all in vain, for it is beyond the limits of human knowledge. There are difficulties in accounting for the other senses, no less puzzling. A sound, a smell, a taste, has not the slightest relation to the cause that produces it. This is set forth in the Essay immediately foregoing. Here is more work for a curious enquirer, attempting to transgress the limits of human knowledge. These things I do not pretend to explain; but humbly rest satisfied with the small portion of knowledge that is bestowed on me, because universal knowledge is not given to human beings.

I conclude with repeating what is observed above, that the connection between soul and body and their way of acting upon each other, are hid from us; and for ever will remain hid. Our senses not only guide us in acting, but are the means of manifold enjoyments. Their salutary effects are known to all; but by what means these effects are produced, is known to none. But we suffer not by our ignorance, as it gives no obstruction to the operation of our senses. We know from experience, that in vision, representations of the external object are painted on the retinae of our two eyes; but in what manner these pictures contribute to vision or whether they at all contribute, we know not. One thing only is certain, that our perception is not of the pictures but of the object itself. And after all, why are these great philosophers, in explaining vision, more bold than in explaining other actions of the mind on the body. It is not pretended that the circulation of the blood can be explained from any power in matter; and as little walking, or breathing, or moving the hand on a musical instrument. In these instances, and in many others that might be mentioned, the soul is the first mover; and writers venture not to say how the body is moved by the soul. Why then such intricate and obscure theories concerning vision? The seeing external bodies as they are, is an operation as simple as any of these now mentioned.

[1. ]This essay is new to the third edition.

[2. ]Shinar refers to a region of Babylonia (in modern-day southern Iraq), where the Tower of Babel was built (Genesis 11:1–9).

[* ]Quam bellum est velle confiteri potius nescire quod nescias, quam ista effutientem nauseare, atque ipsum displicere I Cic. de Natur. Deor. l. 1. [“And tell me this: are we also to assume that the gods bear the names which we allot to them?” Cicero, On the Nature of the Gods (De Natura Deorum), trans. P. G. Walsh (Oxford: Clarendon Press, 1997), I.84, p. 32.]

[3. ]See Aristotle, On the Soul, trans. J. A. Smith, The Complete Works of Aristotle, ed. Jonathan Barnes (Princeton: Princeton University Press, 1984), 2.7.

[4. ]As described by Lucretius in his De rerum natura (“On the Nature of Things”), Epicurus (341–270 ) put forth an atomistic theory of vision, according to which objects emit tiny particles (eidola) which retain the shape of the bodies from which they emanate, and which enter the eye to cause visual sensation.

[* ]Medical Essays, vol. 3. p. 228 [William Porterfield, “An essay concerning the motions of our eyes. Part I. Of their external motions” (1737) in Medical Essays and Observations, Published by a Society in Edinburgh, 5 vols., 5th ed. (London and Edinburgh, 1771). The Medical Essays were published by the Philosophical Society of Edinburgh, to which both Kames and Porterfield belonged. William Porterfield (1695–1771), Professor of Medicine at the University of Edinburgh, was a leading authority on the senses and the author of A Treatise on the Eye (1759).]

[5. ]Descartes argued that while the senses provide us with useful information concerning the material world, they cannot give us reliable knowledge about the real nature of things.

[6. ]Not verbatim, but Kames’s paraphrase of Locke’s argument that the senses furnish the soul “with ideas to think on.” By “compounding those Ideas, and reflecting on its own Operations,” writes Locke, “it increases its Stock, as well as Facility, in remembring, imagining, reasoning, and other modes of thinking” (Essay, II.i.20, p. 116).

[* ]Medical Essays, vol. 3. p. 220.

[7. ]Pierre Gassendi (1592–1655), French Catholic priest, philosopher, astronomer, and mathematician, best known for his attempt to reconcile Epicurus with Christianity(Syntagma Philosophicum, 1658) and for his experiments in astronomy (he was the first to observe the planetary transit of Mercury). Giambattista della Porta (c. 1535–1615), an Italian natural philosopher with interests both in magic and in optics, described his experiments with the camera obscura in his Magia Naturalis (1558; English trans., Natural Magick, 1658) and discussed binocular vision in De refractione, optices parte (“On Refraction, the Division of Light”), which was published in 1593.

[* ]15th Query subjoined to his optics. [Sir Isaac Newton (1642–1727), Opticks: Or, a Treatise of the Reflexions, Refractions, Inflexions and Colours of Light (London, 1704), pp. 320–1.]

[* ]The framing systems upon conjectures beyond the bounds of human knowledge, is far from being rare among philosophers. Take the following notable instance from Avicen an Arabian philosopher. His opinion is, that man may be formed out of the earth without father or mother, in the following manner.

A piece of matter being in a fermentation by the mixture of four qualities; hot, cold, dry, moist, there arise some bubbles; in the midst of which there is a little bubble full of a spirituous and aërial substance; into which by the command of God a spirit is infused. Opposite to this bubble there arises another bubble divided into three receptacles by thin membranes, filled with an aërial sub-stance; and in these are placed certain faculties subject to the governing spirit, and appointed to communicate every thing to that spirit. The first mentioned bubble by its flaming heat is formed into a conical figure like fire; by which means the thick body about it becomes of the same figure, being solid flesh covered with a thick membrane, which is what we call the heart. Now considering the great waste of moisture from so much heat, some part must be formed to supply that waste. This spirit is endued with a sense both of what is convenient for it and what is hurtful, so as to attract the one and repel the other. For these services there are two parts formed, namely the brain and the liver. The first presides over all things relating to sense; the latter over all things relating to nutrition. This is an epitome of much indigested stuff imagined by Arabian doctors on this subject. [The physician and philosopher Avicenna or Ibn Sina (980–1037), a central figure in medieval Islamic philosophy whose influence extended to the West, most notably to Thomas Aquinas, who commented extensively on Avicenna’s al-Shifa (“On the Soul”).]

[8. ]William Briggs (1642–1704), who discovered the optic papilla (or optic disk, also known as the “blind spot”), was the author of Ophthalmographia (Cambridge, 1676; London, 1685) and Nova visionis theoria (1685).

[9. ]William Porterfield, A treatise on the eye, the manner and phaenomena of vision, 2 vols. (London: A. Miller, and Edinburgh: G. Hamilton and J. Balfour, 1759).