But to proceed: what Locke had proved with regard to the sensations we have by smell, taste, and hearing, Bishop Berkeley proved no less unanswerably with regard to all our other sensations; to wit, that none of them can in the least resemble the qualities of a lifeless and insentient being, such as matter is conceived to be. Mr. Hume hath confirmed this by his authority and reasoning. This opinion surely looks with a very malian aspect upon the old hypothesis; yet that hypothesis hath still been retained, F3 106 OF THE HUMAN MIND. [CHAP. VI.
and conjoined with it. And what a brood of monsters hath this produced!
The firstborn of this union, and perhaps the most harm- less was, that the secondary qualities of body were mere sensations of the mind. To pass by Malebranche's notion of seeing all things in the ideas of the divine mind, as a foreigner never naturalized in this island; the next was Berkeley's system, that extension, and figure, and hard- ness, and motion; that land, and sea, and houses, and our own bodies, as well as those of our wives, and children, and friends, are nothing but ideas of the mind; and that there is nothing existing in nature but minds and ideas.
The progeny that followed is still more frightful; so that it is surprising that one could be found who had the courage to act the midwife, to rear it up, and to usher it into the world. No causes nor effects; no substances, material or spiritual; no evidence even in mathematical demonstration; no liberty nor active power; nothing ex- isting in nature, but impressions and ideas following each other, without time, place, or subject. Surely no age ever produced such a system of opinions, justly deduced with great acuteness, perspicuity, and elegance, from a principle universally received. The hypothesis we have mentioned is the father of them all. The dissimilitude of our sensations and feelings to external things is the inno- cent mother of most of them.
As it happens sometimes in an arithmetical operation, that two errors balance one another, so that the conclu- sion is little or nothing affected by them; but when one of them is corrected, and the other left, we are led farther from the truth than by both together; so it seems to have happened in the Peripatetic philosophy of sensation com- pared with the modern. The Peripatetics adopted two errors; but the last served as a corrective to the first, and rendered it mild and gentle; so that then: system had no tendency to scepticism. The moderns have retained the first of those errors, but have gradually detected and cor- rected the last. The consequence hath been, that the SECT. VI.] OF SEEING. 107 light we have struck out hath created darkness, and scep- ticism hath advanced hand in hand with knowledge, spread- ing its melancholy gloom, first over the material world, and at last over the whole face of nature. Such a phenomenon as this is apt to stagger even the lovers of light and know- ledge, while its cause is latent; but when that is detected it may give hopes that this darkness shall not be everlast- ing, but that it shall be succeeded by a more permanent light.
SECT. VII.
Of Visible Figure and Extension.
ALTHOUGH there is no resemblance, nor, as far as we know, any necessary connexion between that quality in a body which we call its colour, and the appearance which that colour makes to the eye; it is quite otherwise with regard to its figure and magnitude. There is certainly a resemblance, and a necessary connexion, between the visible figure and magnitude of a body, and its real figure and magnitude; no man can give a reason why a scarlet colour affects the eye in the manner it does; no man can be sure that it affects his eye in the same man- ner as it affects the eye of another, and that it has the same appearance to him as it has to another man; but we can assign a reason why a circle placed obliquely to the eye should appear in the form of an ellipse. The visible figure, magnitude, and position may, by mathematical rea- soning, be deduced from the real; and it may be demon- strated, that every eye that sees distinctly and perfectly, must, in the same situation, see it under this form and no other. Nay, we may venture to affirm, that a man born blind, if he were instnicted in mathematics, would be able to determine the visible figure of a body, when its reaJ figure, distance, and position, are given. Dr. Saunderson understood the projection of the sphere and perspective. Now, I require no more knowledge in a blind man, in order to his being able to determine the visible figure ot' 108 OF THE HUMAN MIND. [CHAP. VI.
bodies, than that he can project the outline of a given body, upon the surface of a hollow sphere, whose centre is in the eye. This projection is the visible figure he wants; for it is the same figure with that which is projected upon the tunica retina in vision.
A blind man can conceive lines drawn from every point of the object to the centre of the eye, making angles. He can conceive, that the length of the object will appear greater or less, in proportion to the angle which it sub- tends at the eye; and that, in like manner, the breadth, and in general the distance of any one point of the object from any other point, will appear greater or less, in proportion to the angles which those distances subtend. He can easily be made to conceive, that the visible appearance has no thickness, any more than a projection of the sphere, or a perspective draught. He may be informed that the eye, until it is aided by experience, does not represent one object as nearer or more remote than another. Indeed he would probably conjecture this of himself, and be made apt to think that the rays of light must make the same im- pression upon the eye, whether they come from a greater or a less distance.
These are all the principles which we suppose our blind mathematician to have; and these he may certainly acquire by information and reflection. It is no less certain, that from these principles, having given the real figure and magnitude of a body, and its position and distance with regard to the eye, he can find out its visible figure and magnitude. He can demonstrate in general, from these principles, that the visible figure of all bodies will be the same with that of their projection upon the surface of a hollow sphere, when the eye is placed in the centre. And he can demonstrate, that their visible magnitude will be greater or less, according as their projection occupies a greater or less part of the surface of this sphere.
To set this matter in another light, let us distinguish betwixt the position of objects with regard to the eye, and their distance from it. Objects that lie in the same right line drawn from the centre of the eye, have the same posi- SECT. VII.] OF SEEING. 100 tion, however different their distances from the eye may be: but objects which lie in different right lines drawn from the eye's centre, have a different position; and this difference of position is greater or less, in proportion to the angle made at the eye by the right lines mentioned. Hav- ing thus defined what we mean by the position of objects with regard to the eye, it is evident, that as the real figure ' a body consists in the situation of its several parts with regard to one another, so its visible figure consists in the position of its several parts with regard to the eye; and as he that hath a distinct conception of the sitoation of the parts of the body with regard to one another must have a distinct conception of its real figure; so he that conceives distinctly the position of its several parts with regard to the eye, must have a distinct conception of its visible figure. Xow, there is nothing surely to hinder a blind man from Conceiving the position of the several parts of a body with regard to the eye, any more than from conceiving their (.nation with regard to one another; and therefore I connde, that a blind man may attain a distinct conception of 'lie visible figure of bodies.
Although we think the arguments that have been offered are sufficient to prove that a blind man may conceive the visible extension and figure of bodies; yet, in order to remove some prejudices against this truth, it will be of use • compare the notion which a blind mathematician might form to himself of visible figure, with that which is pre- sented to the eye in vision, and to observe wherein they differ.
First, Visible figure is never presented to the eye but in ' onjunction with colour; and although there be no con- nexion between them from the nature of the things, yet, :iving so invariably kept company together, we are hardly ble to disjoin them even in our imagination. What mightily increases this difficulty is, that we have never been accustomed to make visible figure an object of thought. It is only used as a sign, and, having served this purpose, passes away without leaving a trace behind. The drawer or designer, whose business it is to hunt this 110 OF THE HUMAN MIND. [CHAP. VI.
fugitive form, and to take a copy of it, finds how difficult his task is, after many years labour and practice. Happy! if at last he can acquire the art of arresting it in his imagi- nation, until he can delineate it. For then it is evident, that he must be able to draw as accurately from the life as from a copy. But how few of the professed masters of designing are ever able to arrive at this degree of perfec- tion? it is no wonder, then, that we should find so great difficulty in conceiving this form apart from its constant associate, when it is so difficult to conceive it at all. But our blind man's notion of visible figure will not be asso- ciated with colour, of which he hath no conception; but it will perhaps be associated with hardness or smoothness, with which he is acquainted by touch. These different associations are apt to impose upon us, and to make things seem different, which in reality are the same.
Secondly, The blind man forms the notion of visible figure to himself by thought, and by mathematical rea- soning from principles; whereas the man that sees has it presented to his eye at once, without any labour, without any reasoning, by a kind of inspiration. A man may form to himself the notion of a parabola, or a cycloid, from the mathematical definition of those figures/although he had never seen them drawn or delineated. Another, who knows nothing of the mathematical definition of the figures may see them delineated on paper, or feel them cut out in wood. Each may have a distinct conception of the figures, one by mathematical reasoning, the other by sense. Now, the blind man forms his notion of visible figure in the same manner as the first of these formed his notion of a parabola or a cycloid, which he never saw.
Third, Visible figure leads the man that sees, directly to the conception of the real figure, of which it is a sign. But the blind man's thoughts move in a contrary direction. For he must first know the real figure, distance, and situa- tion of the body, and from thence he slowly traces out the visible figure by mathematical reasoning. Nor does his nature lead him to conceive this visible figure as a sign; it is a creature of his own reason and imagination.
SECT. VIII.] OF SEEING. Ill SECT. VIII.
Some Queries concerning Visible Figure answered.
IT may be asked, What kind of thing is this visible figure? Is it a sensation or an idea? If it is an idea, from what sensation is it copied? These questions may seem trivial or impertinent to one who does not know that there is a tribunal of inquisition erected by certain modern philoso- phers, before which every thing in nature must answer. The articles of inquisition are few indeed, but very dreadful in their consequences. They are only these: Is the pri- soner an impression or an idea? If an idea, from what impression copied? Now, if it appears that the prisoner is neither an impression nor an idea copied from some im- pression, immediately, without being allowed to offer any thing in arrest of judgment, he is sentenced to pass out of existence, and to be, in all time to come, an empty un- meaning sound, or the ghost of a departed entity.
Before this dreadful tribunal, cause and effect, time and place, matter and spirit, have been tried and cast: how then shall such a poor flimsy form as visible figure stand before it? It must even plead guilty, and confess that it is neither an impression nor an idea. For alas! It is no- torious, that it is extended in length and breadth; it may be long or short, broad or narrow, triangular, quadrangular, or circular: and therefore, unless ideas and impressions are extended and figured it cannot belong to that category.
If it should still be asked, To what category of beings does visible figure then belong? I can only, in. answer, give some token, by which those who are better acquainted with the categories may chance to find its place. It is, as we have said, the position of the several parts of a figured body, with regard to the eye. The different posi- tions of the several parts of the body with regard to the eye, when put together, make a real figure, which is truly extended in length and breadth, and which represents a 112 OF THE HUMAN MIND. [CHAP. VI.
figure that is extended in length, breadth, and thickness. In like manner the projection of the sphere is a real figure, and hath length and breadth, but represents the sphere which hath three dimensions. A projection of the sphere, or a perspective view of a palace, is a representative in the very same sense as visible figure is, and wherever they have their lodgings in the categories, this will be found to dwell next door to them.
It may farther be asked, Whether there be any sensa- tion proper to visible figure, by which it is suggested in vision? Or by what means is it presented to the mind? This is a question of some importance, in order to our having a distinct notion of the faculty of seeing: and to give all the light to it we can, it is necessaiy to compare this sense with other senses, and to make some supposi- tions, by which we may be enabled to distinguish things that are apt to be confounded, although they are totally different.
There are three of our senses which give us intelligence of things at a distance: smell, hearing, and sight. In smelling and in hearing we have a sensation or impression upon the mind, which, by our constitution, we conceive to be a sign of something external: but the position of this external thing, with regard to the organ of sense, is not presented to the mind along with the sensation. When I hear the sound of a coach, I could not, previous to expe- rience, determine whether the sounding body was above or below, to the right hand or to the left. So that the sensation suggests to me some external object as the cause or occasion of it; but it suggests not the position of that object, whether it lies in this direction or in that. The same thing may be said with regard to smelling. But the case is quite different with regard to seeing. When I see an object, the appearance which the colour of it makes, may be called the sensation, which suggests to me some external thing as its cause; but it suggests likewise the individual direction and position of this cause with regard to the eye. I know it is precisely in such a direction, and in no other. At the same time, I am not conscious of SECT. VIII.] OF SEEING. 113 any thing that can be called sensation, but the sensation of colour. The position of the coloured thing is no sen- sation, but it is by the laws of iny constitution presented to the mind along with the colour, without any additional sensation.
Let us suppose, that the eye were so constituted that the rays coming from any one point of the object were not, as they are in our eyes, collected in one point of the retina, but diffused over the whole: it is evident to those who understand the structure of the eye, that such an eye as we have supposed, would show the colour of a body as our eyes do, but that it would neither show figure nor position. The operation of such an eye would be precisely similar to that of hearing and smell; it would give no per- ception of figure or extension, but merely of colour. Xor is the supposition we have made altogether imaginary: for it is nearly the case of most people who have cataracts, whose crystalline, as Mr. Cheselden observes, does not altogether exclude the rays of light, but diffuses them over the retina, so that such persons see things as one does through a glass of broken jelly: they perceive the colour, but nothing of the figure or magnitude of objects.
Again, if we should suppose, that smell and sound were conveyed in right lines from the objects, and that every sensation of hearing and smell suggested the precise direc- tion or position of its object; in this case, the operations of hearing and smelling would be similar to that of seeing; we should smell and hear the figure of objects, in the same sense as we now see it; and every smell and sound would be associated with some figure in the imagination, as colour is in our present state.
We have reason to believe, that the rays of light make some impression upon the retina; but we are not consci- ous of this impression; nor have anatomists or philoso- phers been able to discover the nature and effects of it; whether it produces a vibration in the nerve, or the motion of some subtile fluid contained in the nerve, or something different from either, to which we cannot give a name. Whatever it is, we shall call it the material impression; 114 OF THE HUMAN MIND. [CHAP. VI.
remembering carefully, that it is not an impression upon the mind, but upon the body; and that it is no sensation, nor can resemble sensation any more than figure or motion can resemble thought. Now, this material impression, made upon a particular point of the retina, by the laws of our constitution, suggests two things to the mind, namely, the colour and the position of some external object. No man can give a reason why the same material impression might not have suggested sound or smell, or either of these, along with the position of the object. That it should sug- gest colour and position, and nothing else, we can resolve only into our constitution, or the will of our Maker. And since there is no necessary connexion between these two things suggested by this material impression, it might, if it had so pleased our Creator, have suggested one of them without the other. Let us suppose, therefore, since it plainly appears to be possible that our eyes had been so framed as to suggest to us the position of the object, without suggesting colour or any other quality: what is the con- sequence of this supposition? It is evidently this, that the person endued with such an eye, would perceive the visible figure of bodies, without having any sensation or impre&- sion made upon his mind. The figure he perceives is alto- gether external; and therefore cannot be called an impres- sion upon the mind, without the grossest abuse of language. If it should be said, that it is impossible to perceive a figure, unless there be some impression of it upon the mind; I beg leave not to admit the impossibility of this, without some proof: and I can find none. Neither can I conceive what is meant by an impression of figure upon the mind. I can conceive an impression of figure upon wax, or upon any body that is fit to receive it; but an impression of it upon the mind is to me quite unintelligible; and although I form the most distinct conception of figure, I cannot, upon the strictest examination, find any impression of it upon my mind.
If we suppose, last of all, that the eye hath the power restored of perceiving colour, I apprehend that it will be allowed, that now it perceives figure in the very same SECT. VIII.] OF SEEING. 115 manner as- before, with this difference only, that colour is always joined with it.
In answer therefore to the question proposed, there seems to be no sensation that is appropriated to visible figure, or whose office it is to suggest it. It seems to be suggested immediately by the material impression upon the organ of Avhich we are not conscious: and why may not a material impression upon the retina suggest visible figure, as well as the material impression made upon the hand, when we grasp a ball, suggests real figure? In the one case, one and the same material impression suggests both colour and visible figure; and in the other case, one and the same material impression suggests hardness, heat, or cold, and real figure, all at the same time. • We shall conclude this section with another question upon this subject. Since the visible figure of bodies is a real and external object to the eye, as their tangible figure is to the touch; it may be asked, whence arises the diffi- culty of attending to the first and the facility of attending to the last? It is certain, that the first is more frequently presented to the eye than the last is to the touch; the first is as distinct and determinate an object as the last, and seems in its own nature as proper for speculation. Yet so little hath it been attended to that it never had a name in any language, until Bishop Berkeley gave it that which we have used after his example, to distinguish it from the figure which is the object of touch.
The difficulty of attending to the visible figure of bodies, and making it an object of thought, appears so similar to that which we find in attending to our sensations that both have probably like causes. Nature intended the visible figure as a sign of the tangible figure and situation of bodies, and hath taught us by a kind of instinct to put it always to this use. Hence it happens, that the mind passes over it with a rapid motion, to attend to the things signified by it. It is as unnatural to the mind to stop at the visible figure, and attend to it, as it is to a spherical body to stop upon an inclined plane. There is an inward 116 OF THE HUMAN MIND. [CHAP. VI.
principle which constantly carries it forward, and which cannot be overcome but by a contrary force.
There are other external things which nature intended for signs; and we find this common to them all, that the mind is disposed to overlook them, and to attend only to the things signified by them. Thus, there are certain mo- difications of the human face, which are natural signs of the present disposition of the mind. Every man under- stands the meaning of these signs, but not one of a hundred ever attended to the signs themselves, or knows any thing about them. Hence you may find many an excellent practical physiognomist, who knows nothing of the propor- tions of a face, nor can delineate or describe the expres- sion of any one passion.
An excellent painter or statuary can tell, not only what are the proportions of a good face, but what changes every passion makes in it. This, however, is one of the chief mysteries of his art, to the acquisition of which infinite labour and attention, as well as a happy genius, are required. But when he puts his art in practice, and happily expresses a passion by its proper signs, every one understands the meaning of these signs without art and without reflection.
What has been said of painting might easily be applied to all the fine arts. The difficulty in them all consists in knowing and attending to those natural signs, whereof every man understands the meaning.
We pass from the sign to the thing signified with ease and by natural impulse; but to go backward from the thing signified to the sign is a work of labour and difficulty. Visible figure, therefore, being intended by nature to be a sign, we pass on immediately to the thing signified, and cannot easily return to give any attention to the sign.
Nothing shows more clearly our indisposition to attend to visible figure and visible extension than this, that although mathematical reasoning is no less applicable to them than to tangible figure and extension, yet they have entirely escaped the notice of mathematicians. While that figure and that extension which are objects of touch have SECT. VIII.] OF SEEING. 117 been tortured ten thousand ways for twenty centuries, and a very noble system of science has been drawn out of them; not a single proposition do we find with regard to the figure and extension which are the immediate objects of sight!
When the geometrician draws a diagram with the most perfect accuracy; when he keeps his eye fixed upon it, while he goes through a long process of reasoning, and demonstrates the relations of the several parts of his figure; he does not consider that the visible figure presented to his eye is only the representative of a tangible figure, upon which all his attention is fixed; he does not consider that these two figures have really different properties; and that what he demonstrates to be true of the one is not true of the other.
This perhaps will seem so great a paradox, even to mathematicians, as to require demonstration before it can be believed. Nor is the demonstration at all difficult, if the reader will have patience to enter but a little into the mathematical consideration of visible figure, which we shall call the geometry ofvisibles.
SECT. IX.
Of the Geometry of Visibles.
1 N this geometry, the definitions of a point of a line, whe- ther straight or curve; of an angle, whether acute, or right, or obtuse; and of a circle, are the same as in common geometry. The mathematical reader will easily enter into the whole mystery of this geometry, if he attends duly to these few evident principles.
1. Supposing the eye placed in the centre of a sphere, every great circle of the sphere will have the same appear- ance to the eye as if it was a straight line. For the cur- viture of the circle being turned directly toward the eye, is not perceived by it. And for the same reason, any line which is drawn in the plane of a great circle of the sphere, 118 OF THE HUMAN MIND. [CHAP. VI.
whether it be in reality straight or curve, \vill appear straight to the eye.
2. Every visible right line will appear to coincide with some great circle of the sphere; and the circumference of that great circle, even when it is produced until it returns into itself, will appear to be a continuation of the same visible right line, all the parts of it being visibly in direc- turn. For the eye, perceiving only the position of objects with regard to itself, and not their distance, will see those points in the same visible place which have the same po- sition with regard to the eye, how different soever their distances from it may be. Now, since a plane passing through the eye and a given visible right line will be the plane of some great circle of the sphere, every point of the visible right line will have the same position as some point of the great circle; therefore they will both have the same visible place, and coincide to the eye: and the whole cir- cumference of the great circle, continued even until it re- turns into itself, will appear to be a continuation of the same visible right line.
Hence it follows: Hence it follows: 3. That every visible right line, when it is continued in directum, as far as it may be continued, will be represented by a great circle of a sphere, in whose centre the eye is placed. It follows, 4. That the visible angle comprehended under two visible right lines is equal to the spherical angle compre- hended under the two great circles which are the represen- tatives of these visible lines. For, since the visible lines appear to coincide with the great circles, the visible angle comprehended under the former must be equal to the visi- ble angle comprehended under the latter. But the visible angle comprehended under the two great circles, when seen from the centre, is of the same magnitude with the spherical angle which they really comprehend, as mathe- maticians know; therefore the visible angle made by any two visible lines is equal to the spherical angle made by the two great circles of the sphere which are their repre- sentatives.
SECT. IX.] OF SEEING. 119 5. Hence it is evident, that every visible right-lined triangle will coincide in all its parts with some spherical triangle. The sides of the one will appear equal to the sides of the other, and the angles of the one to the angles of the other, each to each; and therefore the whole of the one triangle will appear equal to the whole of the other. In a word, to the eye they will be one and the same, and have the same mathematical properties. The properties, therefore, of visible right-lined triangles, are not the same with the properties of plain triangles, but are the same with those of spherical triangles.
0. Every lesser circle of the sphere will appear a circle to the eye, placed, as we have supposed all along, in the centre of the sphere. And, on the other hand, every visi- ble circle will appear to coincide with some lesser circle of the sphere.