We are so much accustomed to measure with our eye the ground which we travel, and to compare the judg- ments of distances formed by sight with our experience or information, that we learn by degrees, in this way, to form a more accurate judgment of the distance of terrestrial objects, than we could do by any of the means before mentioned. An object placed upon the top of a high building appears much less than when placed upon the ground at the same distance. When it stands upon the ground, the intervening tract of ground serves as a sign of its distance; and the distance, together with the visible magnitude, serves as a sign of its real magnitude. But when the object is placed on high, this sign of its distance is taken away: the remaining signs lead us to place it at a less distance; and this less distance, together with the visible magnitude, becomes a sign of a less real magni- tude.
The two first means we have mentioned would never of themselves make a visible object appear above a hun- dred and fifty or two hundred feet distant; because, be- yond that there is no sensible change, either of the con- formation of the eyes or the inclination of their axes. The third mean is but a vague and undeterminate sign when applied to distances above two or three hundred feet, unless we know the real colour and figure of the object; and the fifth mean, to be afterwards mentioned, can only be applied to objects which are familiar, or whose real magnitude is known. Hence it follows, that when un- known objects, upon or near the surface of the earth, are perceived to be at the distance of some miles, it is always by this fourth mean that we are led to that conclusion, 222 OF THE HUMAN MIND. [CHAP. VI.
Dr. Smith hath observed, very justly, that the known distance of the terrestrial objects, which terminate our view, makes that part of the sky which is towards the horizon appear more distant than that which is towards the zenith. Hence it comes to pass, that the apparent figure of the sky is not that of a hemisphere, but rather a less segment of a sphere. And hence likewise it comes to pass, that the diameter of the sun or moon, or the dis- tance between two fixed stars, seen contiguous to a hill, or to any distant terrestrial object, appears much greater than when no such object strikes the eye at the same time.
These observations have been sufficiently examined and confirmed by Dr. Smith. I beg leave to add, that when the visible horizon is terminated by very distant objects, the celestial vault seems to be enlarged in all its dimen- sions. When I view it from a confined street or lane, it bears some proportion to the buildings that surround me: but when I view it from a large plain, terminated on all hands by hills which rise one above another, to the dis- tance of twenty miles from the eye, methinks I see a new heaven, whose magnificence declares the greatness of its Author, and puts every human edifice out of countenance; for now the lofty spires and the gorgeous palaces shrink into nothing before it, and bear no more proportion to the celestial dome than their makers bear to its Maker.
5. There remains another mean by which we perceive the distance of visible objects, and that is, the diminution of their visible or apparent magnitude. By experience I know what figure a man, or any other known object makes to my eye, at the distance of ten feet: I perceive the gra- dual and proportional diminution of this visible figure, at the distance of twenty, forty, a hundred feet, and at greater distances, until it vanish altogether. Hence a certain visi- ble magnitude of a known object becomes the sign of a certain determinate distance, and carries along with it the conception and belief of that distance.
In this process of the mind the sign is not a sensation; it is an original perception. We perceive the visible figure and visible magnitude of the object by the original powers of vision; but the visible figure is used only as a sign of SECT. XXII.] OF SEEING. 223 the real figure, and the visible magnitude is used only as a sign either of the distance, or of the real magnitude of the object; and therefore these original perceptions, like other mere signs, pass through the mind without any attention or reflection.
This last mean of perceiving the distance of known ob- jects serves to explain some very remarkable phenomena in optics, which would otherwise appear very mysterious. When we view objects of known dimensions through opti- cal glasses, there is no other mean left of determining their distance but this fifth. Hence it follows, that known objects, seen through glasses, must seem to be brought nearer in proportion to the magnifying power of the glass, or to be removed to a greater distance, improportion to the diminish- ing power of the glass.
If a man who had never before seen objects through a telescope were told, that the telescope which he is about to use magnifies the diameter of the object ten times; when he looks through this telescope at a man six feet high, what would he expect to see? Surely he would very naturally expect to see a giant sixty feet high. But he sees no such thing. The man appears no more than six feet high, and consequently no bigger than he really is; but he appears ten times nearer than he is. The telescope indeed magnifies the image of this man upon the retina ten times in diameter, and must therefore magnify his visible figure in the same proportion; and as we have been ac- customed to see him of this visible magnitude, when he was ten times nearer than he is presently, and in no other case; this visible magnitude, therefore, suggests the conception and belief of that distance of the object with which it hath been always connected. We have been accustomed to conceive this amplification of the visible figure of a known object only as the effect or sign of its being brought nearer: and we have annexed a certain determinate distance to every degree of visible magnitude of the object; and therefore, any particular degree of visible magnitude, whether seen by the naked eye or by glasses, brings along with it the conception and belief of the distance which cor- 224 OF THE HUMAN MIND. [CHAP. VI.
responds to it. This is the reason why a telescope seems not to magnify known objects, but to bring them nearer to the eye.
When we look through a pin-hole, or a single micros- cope, at an object which is half an inch from the eye, the picture of the object upon the retina is not enlarged, but only rendered distinct; neither is the visible figure enlarged: yet the object appears to the eye twelve or fourteen times more distant, and as many times larger in diameter than it really is. Such a telescope as we have mentioned, amplifies the image on the retina, and the visible figure of the object, ten times in diameter, and yet it makes it seem no bigger, but only ten times nearer. These appearances had been long observed by the writers on optics; they tortured their invention to find the causes of them from optical principles; but in vain: they must be resolved into habits of perception, which are acquired by custom, but are apt to be mistaken for original percep- tions. The Bishop of Cloyne first furnished the world with the proper key for opening up these mysterious ap- pearances; but he made considerable mistakes in the application of it. Dr. Smith, in his elaborate and judici- ous treatise of Optics, hath applied it to the apparent dis- tance of objects seen with glasses, and to the apparent figure of the heavens, with such happy success, that there can be no more doubt about the cause of these phe- nomena.
SECT. XXIII.
Of the Signs used in other acquired Perceptions.
THE distance of objects from the eye is the most impor- tant lesson in vision. Many others are easily learned in consequence of it. The distance of the object, joined with its visible magnitude, is a sign of its real magnitude: and the distance of the several parts of an object, joined with its visible figure, becomes a sign of its real figure. Thus, SECT. XXIII.] OF SEEING. 225 when I look at a globe, which stands before me, by the original powers of sight, I perceive only something of a circular form, variously coloured. The visible figure hath uo distance from the eye, no convexity, nor hath it three dimensions; even its length and breadth are incapable of being measured by inches, feet, or other linear measures. But when I have learned to perceive the distance of every part of this object from the eye, this perception gives it convexity, and a spherical figure; and adds a third dimen- sion to that which had but two before. The distance of the whole object makes me likewise perceive the real mag- nitude: for being accustomed to observe how an inch or a foot of length affects the eye at that distance, I plainly perceive by my eye the linear dimensions of the globe, and can affirm with certainty that its diameter is about one foot and three inches.
It was shown in the seventh Section of this Chapter, that the visible figure of a body may, by mathematical reasoning, be inferred from its real figure, distance, and position, with regard to the eye: in like manner we may, by mathematical reasoning, from the visible figure, toge- ther with the distance of the several parts of it from the eye, infer the real figure and position. But this last infe- rence is not commonly made by mathematical reasoning-, nor indeed by reasoning of any kind, but by custom.
The original appearance which the colour of an object makes to the eye is a sensation for which we have no name, because it is used merely as a sign, and is never made an object of attention in common life: but this ap- pearance, according- to the different circumstances, signi- fies various things. If a piece of cloth of one uniform colour, is laid so that part of it is in the sun, and part in the shade; the appearance of colour in these different parts is very different: yet we perceive the colour to be the same; we interpret the variety of appearance as a sign of light and shade, and not as a sign of real difference in co- lour. But if the eye could be so far deceived, as not to perceive the difference of light in the two parts of the cloth, we should in that case interpret the variety of ap- L3 226 OF THE HUMAN MIND. [CHAP. VI.
pearance to signify a variety of colour in the parts of the cloth.
Again, if we suppose a piece of cloth placed as before, but having the shaded part so much brighter in the colour, that it gives the same appearance to the eye as the more enlightened part; the sameness of appearance will here be interpreted to signify a variety of colour, because we shall make allowance for the effect of light and shade.
When the real colour of an object is known, the ap- pearance of it indicates, in some circumstances, the degree of light or shade; in others, the colour of the circumam- bient bodies, whose rays are reflected by it; and in other circumstances, it indicates the distance or proximity of the object, as was observed in the last section; and by means of these many other things are suggested to the mind. Thus, an unusual appearance in the colour of familiar ob- jects may be the diagnostic of a disease in the spectator. The appearance of things in my room may indicate sun- shine or cloudy weather, the earth covered with snow, or blackened with rain. It hath been observed, that the colour of the sky, in a piece of painting, may indicate the country of the painter, because the Italian sky is really of a different colour from the Flemish.
It was already observed, that the original and acquired perceptions which we have by our senses are the language of nature to man, which, in many respects, hath a great affinity to human languages. The instances which we have given of acquired perceptions suggest this affinity, that as, in human languages, ambiguities are often found, so this language of nature in our acquired perceptions is not exempted from them. We have seen, in vision par- ticularly, that the same appearance to the eye may, in different circumstances, indicate different things. There- fore, when the circumstances are unknown upon which the interpretation of the signs depend, their meaning must be ambiguous; and when the circumstances are mistaken, the meaning of the signs must also be mistaken.
This is the case in all the phenomena which we call fallacies of the senses; and particularly m those which SECT. XXIII.] OF SEEING. 257 are called fallacies in vision. The appearance of things to the eye always corresponds to the fixed laws of nature; therefore, if we speak properly, there is no fallacy in the senses. Nature always speaketh the same language, and useth the same signs in the same circumstances: but we sometimes mistake the meaning of the signs, either through ignorance of the laws of nature, or through ignorance of the circumstances which attend the signs.
To a man unacquainted with the principles of optics, almost every experiment that is made with the prism, with the magic lantern, with the telescope, with the microscope, seems to produce some fallacy in vision. Even the ap- pearance of a common mirror, to one altogether unac- quainted with the effects of it, would seem most remark- ably fallacious. For how can a man be more imposed upon than in seeing that before him which is really behind him? How can he be more imposed upon than in being made to see himself several yards removed from himself? Yet children, even before they can speak their mother tongue, learn not to be deceived by these appearances. These, as well as all the other surprising appearances pro- duced by optical glasses, are a part of the visual lan- guage; and, to those who understand the laws of nature concerning light and colours, are in no wise fallacious, but have a distinct and true meaning.
SECT. XXIV.
Of the Analogy between Perception, and the Credit we give to Human Testimony.
THE objects of human knowledge are innumerable, but the channels by which it is conveyed to the mind are few. Among these, the perception of external things by our senses, and the informations which we receive upon human testimony, are not the least considerable: and so remark- able is the analogy between these two, and the analogy between the principles of the mind, which are subservient to the one, and those which are subservient to the other, 228 OF THE HUMAN MIND. [CHAP. VI.
that, without further apology, we shall consider them to- gether.
In the testimony of nature given by the senses, as well as in human testimony given by language, things are sig- nified to us by signs: and in one, as well as the other, the mind, either by original principles, or by custom, passes from the sign to the conception and belief of the things signified.
We have distinguished our perceptions into original and acquired; and language, into natural and artificial. Be- tween acquired perception and artificial language there is a great analogy; but still a greater between original per- ception and natural language.
The signs in original perception are sensations of which nature hath given us a great variety, suited to the variety of the things signified by them. Nature hath established a real connexion between the signs and the things signified; and Nature hath also taught us the interpretation of the signs; so that, previous to experience, the sign suggests the thing signified, and creates the belief of it.
The signs in natural language are features of the face, gestures of the body, and modulations of the voice; the variety of which is suited to the variety of the things sig- nified by them. Nature hath established a real connexion between these signs and the thoughts and dispositions of the mind which are signified by them; and nature hath taught us the interpretation of these signs; so that, pre- vious to experience, the signs suggest the thing signified, and create the belief of it.
A man in company, without doing good or evil, with- out uttering an articulate sound, may behave himself gracefully, civilly, politely; or, on the contrary, meanly, rudely, and impertinently. We see the dispositions of his mind by their natural signs in his countenance and beha- viour, in the same manner as we perceive the figure and other qualities of bodies by the sensations which naturt hath connected with them.
The signs in the natural language of the human counte nance and behaviour as well as the signs in our original SECT. XXIV.] OF SEEING. 229 < options, have the same signification in all climates, and in all nations; and the skill of interpreting them is not ac- quired, but innate.
In acquired perception, the signs are either sensations, or things which we perceive by means of sensations. The xion between the sign and the thing signified is •blished by nature: and we discover this connexion by perience; but not without the aid of our original per- ptions, or of those which we have 'already acquired. After this connexion is discovered, the sign, in like man- ner as in original perception, always suggests the thing :iiified, and creates the belief of it.
In artificial language, the signs are articulate sounds, whose connexion with the things signified by them is (^tablishcd by the will of men: and in learning our niocr tongue, we discover this connexion by experience; ;t not without the aid of natural language, or of what we (1 before attained of artificial language. And after this •nnexion is discovered, the sign, as in natural language, always suggests the thing signified, and creates the belief ol' it.
Our original perceptions are few, compared with the •tcquired; but without the former we could not possibly ain the latter. In like manner natural language is scanty, compared with artificial; but without the former we could not possibly attain the latter.
Our original perceptions, as well as the natural lanuage of human features and gestures, must be resolved 0 particular principles of the human constitution. Thus.
is by one particular principle of our constitution, that ritain features express anger; and by another particular principle, that certain features express benevolence. It is, i like manner, by one particular principle of our constituthat a certain sensation signifies hardness in the body ii I handle; and it is by another particular principle, that a certain sensation signifies motion in that body.
But our acquired perceptions, and the information we eceive by means of artificial language, must be resolved • general principles of the human constitution. When OF THE HUMAN MIND. [CHAP. VI.
a painter perceives that this picture is the work of Ra- phael, that the work of Titian; a jeweller, that this is a true diamond, that a counterfeit; a sailor, that this is a ship of five hundred tons, that of four hundred; these different acquired perceptions are produced by the same general principles of the human mind which have a dif- ferent operation in the same person, according as they are variously applied, and in different persons according to the diversity of their education and manner of life. In like manner, when certain articulate sounds convey to my mind the knowledge of the battle of Pharsalia; and others, the knowledge of the battle of Pultowa; when a French- man and an Englishman receive the same information by different articulate sounds; the signs used in these differ- ent cases produce the knowledge and belief of the thing signified, by means of the same general principles of the human constitution.
Now, if we compare the general principles of our con- stitution, which fit us for receiving information from our fellow creatures by language, with the general principles which fit us for acquiring the perception of things by our senses, we shall find them to be very similar in their na- ture and manner of operation.
When we begin to learn our mother tongue we perceive, by the help of natural language, that they who speak to us use certain sounds to express certain things: we imi- tate the same sounds when we would express the same things, and find that we are understood.
But here a difficulty occurs which merits our attention, because the solution of it leads to some original principles of the human mind, which are of great importance, and of very extensive influence. We know by experience that men have used such words to express such things. But all experience is of the past, and can of itself give no notion or belief of what is future. How come we then to believe and to rely upon it with assurance, that men who have it in their power to do otherwise will continue to use the same words when they think the same things? Whence comes this knowledge and belief, this foresight we ought SECT. XXIV.] OF SEEING. 231 rather to call it, of the future and voluntary actions of our fellow creatures? Have they promised that they will never impose upon us by equivocation or falsehood? No, they have not. And, if they had, this would not solve the dif- ficulty: for such promise must be expressed by words, or by other signs; and, before we can rely upon it, we must be assured that they put the usual meaning upon the signs which express that promise. No man of common sense ever thought of taking a man's own word for his honesty; and it is evident that we take his veracity for granted, when we lay any stress upon his word or promise. I might add, that this reliance upon the declarations and tes- timony of men is found in children long before they know what a promise is.
There is, therefore, in the human mind an early anticipa- tion, neither derived from experience, nor from reason, nor from any compact or promise, that our fellow creatures will use the same signs in language when they have the same sentiments.
This is, in reality, a kind of prescience of human ac- tions; and it seems to me to be an original principle of the human constitution, without which it would be incapable of language, and consequently incapable of instruction.
The wise and beneficent Author of nature, who intend- ed that we should be social creatures, and that we should receive the greatest and most important part of our know- ledge by the information of others, hath, for these pur- poses, implanted in our natures two principles that tally with each other.
The first of these principles is, a propensity to speak truth, and to use the signs of language, so as to convey our real sentiments. This principle has a powerful opera- tion even in the greatest liars; for, where they lie once they speak truth a hundred times. Truth is always upper- most, and is the natural issue of the mind. It requires no art or training, no inducement or temptation, but only that we yield to a natural impulse. Lying, on the contrary, is doing violence to our nature; and is never practised, even by the worst men, without some temptation. Speaking truth is like using our natural food, which we would do 232 OF THE HUMAN MIND. [CHAP. VI.
from appetite, although it answered no end; but lying is like taking physic, which is nauseous to the taste, and which no man takes but for some end wliich he cannot otherwise attain.
If it should be objected, That men may be influenced by moral or political considerations to speak truth, and therefore, that their doing so is no proof of such an origi- nal principle as we have mentioned: I answer, first, That moral or political considerations can have no influence until we arrive at years of understanding and reflection: and it is certain from experience that children keep to truth invariably, before they are capable of being influ- enced by such considerations. Secondly, When we are influenced by moral or political considerations, we must be conscious of that influence, and capable of perceiving it upon reflection. Now, when I reflect upon my actions most attentively, I am not conscious that in speaking truth I am influenced on ordinary occasions by any motive moral or political. I find that truth is always at the door of my lips, and goes forth spontaneously, if not held back. It requires neither good nor bad intention to bring it forth, but only that I be artless and undesigning. There may, indeed, be temptations to falsehood, which would be too strong for the natural principle of veracity, unaided by principles of honour or virtue; but where there is no such temptation, we speak truth by instinct; and this instinct is the principle I have been explaining.