Because the premises from which all mankind are sup- posed to draw this conclusion, never entered into the minds of the far greater part, but are absolutely unknown to them. We have no feeling or perception of the pic- tures upon the retina, and as little surely of the position of them. In order to see objects erect, according to the principles of Kepler or Des Cartes, we must previously know, that the rays of light come from the object to the eye in straight lines; we must know, that the rays from different points of the object cross one another before they form the pictures upon the retina; and lastly, we must know, that these pictures are really inverted. Now, although all these things are true, and known to philoso-* phers, yet they are absolutely unknown to the far greatest part of mankind: nor is it possible that they who are absolutely ignorant of them should reason from them, and build conclusions upon them. Since therefore visible ob- jects appear erect to the ignorant as well as to the learned, this cannot be a conclusion drawn from premises which never entered into the minds of the ignorant. We have indeed had occasion to observe many instances of conclu- sions drawn, either by means of original principles or by habit, from premises which pass through the mind very quickly, and which are never made the objects of reflec- tion; but surely no man will conceive it possible to draw conclusions from premises which never entered into the mind at all.
Bishop Berkeley, having justly rejected this solution, iiives one founded upon his own principles; wherein he is followed by the judicious Dr. Smith in his Optics; and this we shall next explain and examine.
The ingenious writer conceives the ideas of sight to be altogether unlike those of touch. And since the notions we have of an object by these different senses have no similitude, we can learn only by experience how one sense will be affected by what, in a certain manner, affects the other. Figure, position, and even number, in tangible? objects, are ideas of touch; and although there is no -iuiilitude between these and the ideas of sight, yet we, 134 OF THE HUMAN MIND. [CHAP. VI.
learn by experience, that a triangle affects the sight in such a manner, and that a square affects it in such another manner: hence we judge that which affects it in the first manner to be a triangle, and that which affects it in the second to be a square. In the same way, finding from experience, that an object in an erect position affects the eye in one manner, and the same object in an inverted position affects it in another, we learn to judge, by the manner in which the eye is affected, whether the object is erect or inverted. In a word, visible ideas, according to this author, are signs of the tangible; and the mind passeth from the sign to the thing signified, not by means of any similitude between the one and the other, nor by any natu- ral principle; but by having found them constantly con- joined in experience, as the sounds of a language are with the things they signify. So that if the images upon the retina had been always erect, they would have shown the objects erect, in the manner that they do now that they are inverted: nay, if the visible idea which we now have from an inverted object had been associated from the beginning with the erect position of that object, it would have signified an erect position as readily as it now signifies an inverted one. And if the visible appearance of two shillings had been found connected from the beginning with the tangible idea of one shilling, that appearance would as naturally and readily have signified the unity of the object, as now it signifies its duplicity.
This opinion is undoubtedly very ingenious; and, if it is just, serves to resolve, not only the phenomenon now under consideration, but likewise that which we shall next con- sider, our seeing objects single with two eyes.
It is evident, that in this solution it is supposed, that we do not originally, and previous to acquired habits, see things either erect or inverted, of one figure or another, single or double, but learn from experience to judge of their tangible position, figure, and number, by certain visible signs.
Indeed, it must be acknowledged to be extremely diffi- cult to distinguish the immediate and natural objects of SECT. XI.] OF SEEING.
sight, from the conclusions which we have been accus- tomed from infancy to draw from them. Bishop Berkeley was the first that attempted to distinguish the one from the other, and to trace out the boundary that divides them. And if, in doing so, he hath gone a little to the right hand or to the left, this might be expected in a subject alto- gether new, and of the greatest subtilty. The nature of vision hath received great light from this distinction; and many phenomena in optics, which before appeared alto- gether unaccountable, have been clearly and distinctly resolved by it. It is natural and almost unavoidable to one who hath made an important discovery in philosophy, to carry it a little beyond its sphere, and to apply it to the resolution of phenomena which do not fall within its pro- vince. Even the great Newton, when he had discovered the universal law of gravitation, and observed how many of the phenomena of nature depend upon this, and other laws of attraction and repulsion, could not help expressing his conjecture, that all the phenomena of the material world depend upon attracting and repelling forces in the particles of matter. And I suspect that the ingenious Bishop of Cloyne, having found so many phenomena of vision reducible to the constant association of the ideas of sight and touch, carried this principle a little beyond its just limits.
In order to judge, as well as we can, whether it is so, let us suppose such a blind man as Dr. Saunderson, having all the knowledge and abilities which a blind man may have, suddenly made to see perfectly. Let us suppose him kept from all opportunities of associating his ideas of sight with those of touch, until the former become a little familiar; and the first surprise, occasioned by objects so new, being abated, he has time to canvass them, and to compare them, in his mind, with the notions which he for- merly had by touch, and in particular to compare in his mind that visible extension which his eyes present, with the extension in length and breadth with which he was before acquainted.
We have endeavoured to prove that a blind man may form a notion of the visible extension and figure of bodies, 136 OF THE HUMAN MIND. [CHAP. VI.
from the relation which it bears to their tangible extension and figure. Much more, when this visible extension and figure are presented to his eye, will he be able to com- pare them with tangible extension and figure, and to per- ceive, that the one has length and breadth as well as the other; that the one may be bounded by lines, either straight or curve, as well as the other. And therefore, he will perceive, that there may be visible as well as tangible circles, triangles, quadrilateral and multilateral figures. And although the visible figure is coloured, and the tan- gible is not, they may, notwithstanding, have the same figure; as two objects of touch may have the same figure, although one is hot and the other cold.
We have demonstrated, that the properties of visible figures differ from those of the plain figures which they represent: but it was observed at the same time, that when the object is so small as to be seen distinctly at one view, and is placed directly before the eye, the difference between the visible and the tangible figure is too small to be perceived by the senses. Thus, it is true, that of every visible triangle, the three angles are greater than two right angles; whereas, in a plain triangle, the three angles are equal to two right angles: but, when the visible triangle is small, its three angles will be so nearly equal to two right angles, that the sense cannot discern the difference. In like manner, the circumferences of unequal visible circles are not, but those of plain circles are, in the ratio of their diameters; yet in small visible circles the circum- ferences are very nearly in the ratio of their diameters ^ and the diameter bears the same ratio to the circumference, as in a plain circle, very nearly.
Hence it appears, that small visible figures (and such only can be seen distinctly at one view), have not only a resemblance to the plain tangible figures which have the same name, but are to all sense the same. So that if Dr. Saunderson had been made to see, and had attentively viewed the figures of the first book of Euclid, he might, by thought and consideration, without touching them, have found out that they were the very figures he was before so well acquainted with by touch.
SECT. XI.] OF SEEING. 13?
When plain figures are seen obliquely, their visible figure differs more than the tangible; and the representa- tion which is made to the eye, of solid figures, is still more imperfect; because visible extension hath not three, but two dimensions only. Yet, as it cannot be said that an exact picture of a man hath no resemblance of the man, or that a perspective view of a house hath no resemblance of the house; so it cannot be said, with any propriety, that the visible figure of a man, or of a house, hath no resem- blance of the objects which they represent.
Bishop Berkeley therefore proceeds upon a capital mistake, in supposing that there is no resemblance be- twixt the extension, figure, and position which we see, and that which we perceive by touch.
We may further observe, that Bishop Berkeley's system, with regard to material things, must have made him see this question, of the erect appearance of objects, in a very dif- ferent light from that in which it appears to those who do not adopt his system.
In his theory of vision, he seems indeed to allow, that there is an external material world: but he believed that this external world is tangible only, and not visible; and that the visible world, the proper object of sight, is not external but in the mind. If this is supposed, he that affirms that he sees things erect and not inverted, affirms that there is a top and a bottom, a right and a left in the mind. Now I confess I am not so well acquainted with the topography of the mind as to be able to affix a mean- ing to these words when applied to it.
We shall therefore allow, that if visible objects were not external, but existed only in the mind, they could have no figure, or position, or extension; and! that it would be absurd to affirm, that they are seen either erect or in- verted; or that there is any resemblance between them and the objects of touch. But when we propose the question, Why objects are seen erect and not inverted? we take it for granted, that we are not in Bishop Berkeley's ideal world, but in that world which men, who yield to the dictates of common sense, believe themselves to inhabit.
OF THE HUMAN MIND. [CHAP. VI.
We take it for granted, that the objects both of sight and touch, are external, and have a certain figure, and a cer- tain position with regard to one another, and with regard to our bodies, whether we perceive it or not.
When I hold my walking-cane upright in my hand, and look at it, I take it for granted that I see and handle the same individual object. — When I say that I feel it erect, my meaning is, that I feel the head directed from the ho- rizon, and the point directed towards it: and when I say that I see it erect, I mean that I see it with the head di- rected from the horizon, and the point towards it. I con- ceive the horizon as a fixed object both of sight and touch, with relation to which, objects are said to be high or low, erect or inverted: and when the question is asked, Why I see the object erect, and not inverted? it is the same as if you should ask, Why I see it in that position which it really hath? or, Why the eye shows the real position of objects, and doth not show them in an inverted position, as they are seen by a common astronomical telescope, or as their pictures are seen upon the retina of an eye when it is dissected?
SECT. XII.
The same Subject continued.
IT is impossible to give a satisfactory answer to this ques- tion, otherwise than by pointing out the laws of nature which take place in vision; for by these the phenomena of vision must be regulated.
Therefore I answer, First, That by a law of nature the rays of light proceed from every point of the object to the pupil of the eye in straight lines. Secondly, That by the laws of nature, the rays coming from any one point of the object to the various parts of the pupil are so refracted as to meet again in one point of the retina; and the rays from ditferent points of the object, first crossing each other, and then proceeding to as many different points of the retina, form an inverted picture of the object.
SECT. XII.] OF SEEING. 139 So far the principles of optics carry us; and experience further assures us, that if there is no such picture upon the retina, there is no vision; and that such as the picture on the retina is, such is the appearance of the object in colour and figure, distinctness or indistinctness, brightness or faintness.
It is evident, therefore, that the pictures upon the retina are, by the laws of nature, a mean of vision: but in what way they accomplish their end, we are totally ignorant. Philosophers conceive, that the impression made on the retina by the rays of light is communicated to the optic nerve, and by the optic nerve conveyed to some part of the brain, by them called the sensorium; and that the im- pression thus conveyed to the sensorium is immediately perceived by the mind, which is supposed to reside there. But we know nothing of the seat of the soul: and we are so far from perceiving immediately what is transacted in the brain, that of all parts of the human body we know least about it. It is indeed very probable that the optic nerve is an instrument of vision no less necessary than the retina; and that some impression is made upon it, by means of the pictures on the retina. But of what kind the impression is, we know nothing.
There is not the least probability that there is any pic- ture or image of the object either in the optic nerve or brain. The pictures on the retina are formed by the rays of light; and whether we suppose, with some, that their impulse upon the retina causes some vibration of the fibres of the optic nerve; or, with others, that it gives motion to some subtile fluid contained in the nerve; neither that vi- bration nor this motion can resemble the visible object which is presented to the mind. Nor is there any proba- bility that the mind perceives the pictures upon the retina. These pictures are no more objects of our perception than the brain is, or the optic nerve. No man ever saw the pictures in his own eye, nor indeed the pictures in the eye of another, until it was taken out of the head, and duly- prepared.
It is very strange, that philosophers of all ages should 140 OF THE HUMAN MIND. [CHAP. VI.
have agreed in this notion, That the images of external objects are conveyed by the organs of sense to the brain, and are there perceived by the mind. Nothing can be more unphilosophical. For, first, This notion hath no foundation in fact and observation. Of all the organs of sense, the eye only, as far as we can discover, forms any kind of image of its object; and the images formed by the eye are not in the brain, but only in the bottom of the eye; nor are they at all perceived or felt by the mind. Se- condly, It is as difficult to conceive how the mind per- ceives images on the brain, as how it perceives things more distant. If any man will show how the mind may perceive images in the brain, I will undertake to show how it may perceive the most distant objects: for if we give eyes to the mind, to perceive what is transacted at home in its dark chamber, why may we not make these eyes a little longer sighted? and then we shall have no occasion for that unphilosophical fiction of images in the brain. In a word, the manner and mechanism of the brain's percep- tion is quite beyond our comprehension: and this way of explaining it by images in the brain seems to be founded upon very gross notions of the mind and its operations; as if the supposed images in the brain, by a kind of contact, formed similar impressions or images of objects upon the mind, of which impressions it is supposed to be conscious.
We have endeavoured to show, throughout the course of this inquiry, that the impressions made upon the mind, by means of the five senses, have not the least resemblance to the objects of sense: and therefore, as we see no sha- dow of evidence that there are any such images in the brain, so we see no purpose, in philosophy, that the sup- position of them can answer. Since the picture upon the retina, therefore, is neither itself seen by the mind, nor produces any impression upon the brain or sensorium, which is seen by the mind, nor makes any impression upon the mind that resembles the object, it may still be asked, How this picture upon the retina causes vision?
Before we answer this question, it is proper to observe, that in the operations of the mind, as well as in those of SECT. XII.] OF SEEING. 141 bodies, we must often be satisfied with knowing that cer- tain things are connected, and invariably follow one an- other, without being able to discover the chain that goes between them. It is to such connexions that we give the name of latcs of nature; and when we say that one thing produces another by a law of nature, this signifies no more, but that one thing, which we call in popular language, the cause, is constantly and invariably followed by another, which we call the effect; and that we know not how they are connected. Thus, we see it is a fact, that bodies gra- vitate towards bodies; and that this gravitation is regu- lated by certain mathematical proportions, according to the distance of the bodies from each other, and their quan- tities of matter. Being unable to discover the cause of this gravitation, and presuming that it is the immediate opera- tion, either of the Author of nature, or of some subordi- nate cause which we have not hitherto been able to reach, we call it a law of nature. If any philosopher should hereafter be so happy as to discover the cause of gravita- tion, this can only be done by discovering some more ge- neral law of nature, of which the gravitation of bodies is a necessary consequence. In every chain of natural causes, the highest link is the primary law of nature, and the highest link which we can trace by just induction is either this primary law of nature, or a necessary consequence of it. To trace out the laws of nature, by induction from the phenomena of nature, is all that true philosophy aims at, and all that it can ever reach.
There are laws of nature by which the operations of the mind are regulated; there are also laws of nature that govern the material system: and as the latter are the ulti- mate conclusions which the human faculties can reach in the philosophy of bodies, so the former are the ultimate conclusions we can reach in the philosophy of minds.
To return, therefore, to the question above proposed, we may see, from what hath been just now observed, that it amounts to this: By what law of nature is a picture upon the retina the mean or occasion of my seeing an external object of the same figure and colour, in a contrary position, and in a certain direction from the eye?
142 OF THE HUMAN MIND. [CHAP. VI.
It will, without doubt, be allowed, that I see the whole object in the same manner and by the same law by which I see any one point of it. Now, I know it to be a fact, that, in direct vision, I see every point of the object in the direction of the right line that passeth from the centre of the eye to that point of the object: and I know likewise, from optics, that the ray of light that comes to the centre of my eye passes on to the retina in the same direction. Hence it appears to be a fact, that every point of the ob- ject is seen in the direction of a right line passing from the picture of that point on the retina through the centre of the eye. As this is a fact that holds universally and inva- riably, it must either be a law of nature, or the necessary consequence of some more general law of nature. And according to the just rules of philosophizing, we may hold it for a law of nature, until some more general law be dis- covered, whereof it is a necessary consequence, which I suspect can never be done.
Thus we see, that the phenomena of vision lead us by the hand to a law of nature, or a law of our constitution, of which law our seeing objects erect by inverted images is a necessary consequence. For it necessarily follows, from the law we have mentioned, that the object whose picture is lowest on the retina must be seen in the highest direction from the eye; and that the object whose picture is on the right of the retina must be seen on the left; so that if the pictures had been erect in the retina, we should have seen the object inverted. My chief intention in handling this question was to point out this law of nature; which, as it is a part of the constitution of the human mind, belongs pro- perly to the subject of this inquiry. For this reason I shall make some further remarks upon it, after doing justice to the ingenious Dr. Porterfield, who, long ago, in the Medical Essays, or more lately, in his Treatise of the Eye, pointed out as a primary law of our nature, That a visible object appears in the direction of a right line perpendicular to the retina at that point where its image is painted. If lines drawn from the centre of the eye to all parts of the retina be per- pendicular to it, as they must be very nearly, this coincides with the law we have mentioned, and is the same in other SECT. XII.] OF SEEING. 14:} words. In order, therefore, that we may have a more distinct notion of this law of our constitution, we may observe, 1. That we can give no reason why the retina is, of all parts of the body, the only one on which pictures made by the rays of light cause vision; and therefore we must re- solve this solely into a law of our constitution. We may form such pictures by means of optical glasses upon the hand, or upon any other part of the body; but they are not felt, nor do they produce any thing like vision. A picture upon the retina is as little felt as one upon the hand; but it produces vision; for no other reason that we know, but because it is destined by the wisdom of nature to this purpose. The vibrations of the air strike upon the eye, the palate, and the olfactory membrane, with the same force as upon the membrana tympani of the ear: the impression they make upon the last produces the sen- sation of sound; but their impression upon any of the former produces no sensation at all. This may be ex- tended to all the senses, whereof each hath its peculiar laws, according to which, the impressions made upon the organ of that sense produce sensations or perceptions in the mind that cannot be produced by impressions made upon any other organ.
2. We may observe, that the laws of perception, by the different senses, are very different, not only in respect of the nature of the objects perceived by them, but likewise in respect of the notices they give us of the distance and situation of the object. In all of them the object is con- ceived to be external, and to have real existence, inde- pendent of our perception: but in one, the distance, figure, and situation of the object, are all presented to the mind; in another, the figure and situation, but not the distance; and in others, neither figure, situation, nor distance. In vain do we attempt to account for these varieties in the manner of perception by the different senses, from prin- ciples of anatomy or natural philosophy. They must at last be resolved into the will of our Maker, who intended that our powers of perception should have certain limits, and adapted the organs of perception, and the laws of na- ture by which they operate, to his wise purposes.
144 OF THE HUMAN MIND. [CHAP. Vl.