But hath any such discovery been made? No, not so much as in one subject, as far as I can learn. But in several subjects, the contrary seems to have been dis- covered. Dr. Porterfield hath given us two cases at length from Vesalius, and one from Caesalpinus, wherein the optic nerves, after touching one another as usual, ap- peared to be reflected back to the same side whence they came, without any mixture of their fibres. Each of these persons had lost an eye some time before his death, and the optic nerve belonging to that eye was shrunk, so that it could be distinguished from the other at the place where they met. Another case, which the same author gives from Vesalius, is still more remarkable; for in it the optic nerves did not touch at all; and yet, upon inquiry, those who were most familiar with the person in his lifetime declared, that he never complained of any defect of sight, or of his seeing objects double. Diemerbroeck tells us, that Aqua- K2 196 OF THE HUMAN MIND. [CHAP. VI.
pendens and Valverda likewise affirm, that they have met with subjects wherein the optic nerves did not touch.
As these observations were made before Sir Isaac New- ton put this query, it is uncertain whether he was ignorant of them, or whether he suspected some inaccuracy in them, and desired that the matter might be more carefully exa- mined. But from the following passage of the most accu- rate Winslow, it does not appear that later observations have been more favourable to his conjecture. " The union of these [optic] nerves, by the small curvatures of their cornua, is very difficult to be unfolded in human bodies. This union is commonly found to be very close, but in some subjects it seems to be no more than a strong adhe- sion, in others to be partly made by an intersection or crossing of fibres. They have been found quite separate; and in other subjects, one of them has been found to be very much altered both in size and colour, through its whole passage, the other remaining in its natural state."
Whe» we consider this conjecture of Sir Isaac Newton by itself, it appears more ingenious, and to have more verisimilitude, than any thing that has been offered upon the subject; and we admire the caution and modesty of the author, in proposing it only as a subject of inquiry; but when we compare it with the observations of anato- mists which contradict it, we are naturally led to this re- flection, That if we trust to the conjectures of men of the greatest genius in the operations of nature, we have only the chance of going wrong in an ingenious manner.
The second part of the query is, Whether the two spe- cies of objects from the two eyes are not, at the place where the optic nerves meet, united into one species or picture, half of which is carried thence to the sensoriiim in the right optic nerve, and the other half in the left? and whether these two halves are not so put together again at the sensoriiim as to make one species or picture?
Here it seems natural to put the previous question, What reason have we to believe that pictures of ob- jects are at all carried to the sensorium, either by the optic SECT. XIX.] OF SEEING. Wi nerves, or by any other nerves? Is it not possible, that this great philosopher, as well as many of a lower form, having been led into this opinion at first by education, may have continued in it, because he never thought of calling it in question? I confess this was my own case for a considerable part of my life. But since I was led by accident to think seriously what reason I had to believe it, I could find none at all. It seems to be a mere hypo- thesis, as much as the Indian philosopher's elephant. I am not conscious of any pictures of external objects in my sensorium, any more than in my stomach: the things which I perceive by my senses appear to be external, and not in any part of the brain; and my sensations, properly so called, have no resemblance of external objects.
The conclusion from all that hath been said, in no less than seven sections, upon our seeing objects single with two eyes, is this, That by an original property of human eyes, objects painted upon the centres of the two retlncc, or upon points similarly situate with regard to the'centres, appear in the same visible place; that the most plausible attempts to account for this property of the eyes have been unsuccessful; and therefore, that it must be either a primary law of our constitution, or the consequence of some more general law which is not yet discovered.
We have now finished what we intended to say, both of the visible appearances of things to the eye, and of the laws of our constitution by which those appearances are exhibited. But it was observed in the beginning of this chapter, that the visible appearances of objects serve only as signs of their distance, magnitude, figure, and other 1an«ible qualities. The visible appearance is that which is presented to the mind by nature, according to those laws of our constitution which have been explained. But the thing signified by that appearance is that which is pre- sented to the mind by custom.
When one speaks to us in a language that is familiar, we hear certain sounds, and this is all the effect that his discourse has upon us by nature: but by custom we under- stand the meaning of these sounds; and therefore we fix 198 OF THE HUMAN MIND. [CHAP. VI.
our attention not upon the sounds, but upon the thing sig- nified by them. In like manner, we see only the visible appearance of objects by nature; but we learn by custom to interpret these appearances, and to understand their meaning. And when this visual language is learned, and becomes familiar, we attend only to the things signified; and cannot, without great difficulty, attend to the signs by which they are presented. The mind passes from one to the other so rapidly and so familiarly that no trace of the sign is left in the memory, and we seem immediately, and without the intervention of any sign, to perceive the thing signified.
When I look at the apple tree which stands before my window, I perceive at the first glance its distance and magnitude, the roughness of its trunk, the disposition of its branches, the figure of its leaves and fruit — I seem to perceive all these things immediately. The visible appear- ance which presented them all to the mind has entirely- escaped me; I cannot, without great difficulty and painful abstraction, attend to it even when it stands before me. Yet it is certain that this visible appearance only is pre- sented to my eye by nature, and that I learned by custom to collect all the rest from it. If I had never seen before now, I should not perceive either the distance or tangible figure of the tree, and it would have required the practice of seeing for many months, to change that original per- ception which nature gave me by my eyes into that which I now have by "custom.
The objects which we see naturally and originally, as hath been before observed, have length and breadth, but no thickness nor distance from the eye. Custom, by a kind of legerdemain, withdraws gradually these original and pro- per objects of sight, and substitutes in their place objects of touch, which have length, breadth, and thickness, and a determinate distance from the eye. By what means this change is brought about, and what principles of the human mind concur in it, we are next to inquire.
OF SEEING. 190 SECT. XX.
Of Perception in general.
SENSATION and the perception of external objects by the senses, though very different in their nature, have com- monly been considered as one and the same thing. The purposes of common life do not make it necessary to dis- tinguish them, and the received opinions of philosophers tend rather to confound them; but without attending care- fully to this distinction, it is impossible to have any just conception of the operations of our senses. The most simple operations of the mind admit not of a logical defini- tion: all we can do is to describe them, so as to lead those who are conscious of them in themselves to attend to them, and reflect upon them: and it is often very difficult to describe them so as to answer this intention.
The same mode of expression is used to denote sensa- tion and perception; and therefore we are apt to look upon them as things of the same nature. Thus, I feel a pain; I see a tree: the first denoteth a sensation, the last a percep- tion.— The grammatical analysis of both expressions is the same: for both consist of an active verb and an object. But if we attend to the things signified by these expres- sions, we shall find that in the first the distinction between the act and the object is not real but grammatical; in the second, the distinction is not only grammatical but real.
The form of the expression, I feel pain, might seem to imply that the feeling is something distinct from the pain felt; yet, in reality, there is no distinction. As thinking a thought is an expression which could signify no more than thinking, so feeling a pain signifies no more than being pained. What we have said of pain is applicable to every other mere sensation. It is difficult to give in- stances, very few of our sensations having names; and where they have, the name being common to the sensation and to something else which is associated with it. But 200 OF THE HUMAN MIND. [CHAP. VI.
when we attend to the sensation by itself, and separate it from other things which are conjoined with it in the imagi- nation, it appears to be something which can have no existence but in a sentient mind, no distinction from the act of the mind by which it is felt.
Perception, as we here understand it, hath always an object distinct from the act by which it is perceived; an object which may exist whether it be perceived or not. I perceive a tree that grows before my window; there is here an object which is perceived; and an act of the mind by which it is perceived; and these two are not only distin- guishable, but they are extremely unlike in their natures. The object is made up of a trunk, branches, and leaves; but the act of the mind by which it is perceived hath neither trunk, branches, nor leaves. I am conscious of this act of my mind, and I can reflect upon it; but it is too simple to admit of an analysis, and I cannot find proper words to describe it. I find nothing that resembles it so much as the remembrance of the tree, or the imagination of it. Yet both these differ essentially from perception; they differ likewise one from another. It is in vain that a philosopher assures me that the imagination of the tree, the remembrance of it, and the perception of it, are all one, and differ only in degree of vivacity. I know the con- trary; for I am as well acquainted with all the three as I am with the apartments of my own house. I know this also, that the perception of an object implies both a conception of its form and a belief of its present existence. I know, moreover, that this belief is not the effect of argu- mentation and reasoning; it is the immediate effect of my constitution.
I am aware that this belief which I have in percep- tion stands exposed to the strongest batteries of scepticism. But they make no great impression upon it. The sceptic asks me, Why do you believe the existence of the external object which you perceive? This belief, sir, is none of my manufacture; it came from the mint of nature; it bears her image and superscription; and, if it is not right, the fault is not mine: I even took it upon trust, and without SECT. XX.] OF SEEING. 201 suspicion. Reason, says the sceptic, is the only judge of truth, and you ought to throw off every opinion and every belief that is not grounded on reason. Why, sir, should I believe the faculty of reason more than that of perception; they came both out of the same shop, and were made by the same artist; and if he puts one piece of false ware into my hands, what should hinder him from putting another?
Perhaps the sceptic will agree to distrust reason rather than give any credit to perception. For, says he, since, by your own concession, the object which you perceive and that act of your mind by which you perceive it are quite different things, the one may exist without the other; and as the object may exist without being perceived, so the perception may exist without an object. There is nothing so shameful in a philosopher as to be deceived and de- luded; and therefore you ought to resolve firmly to with- hold assent, and throw off this belief of external objects, which may be all delusion. For my part, I will never attempt to throw it off; and although the sober part of mankind will not be very anxious to know my reasons, yet if they can be of use to any sceptic, they are these.
First, Because it is not in my power: why then should I make a vain attempt? It would be agreeable to fly to the moon, and to make a visit to Jupiter and Saturn; but when I know that nature has bound me down by the law of gravitation to this planet which I inhabit, I rest con- tented and quietly suffer myself to be carried along in its orbit. My belief is carried along by perception as irre- sistibly as my body by the earth; and the greatest sceptic will find himself to be in the same condition. He may struggle hard to disbelieve the informations of his senses, as a man does to swim against a torrent; but ah! it is in vain. It is in vain that he strains every nerve, and wres- tles with nature and with every object that strikes upon his senses. For after all, when his strength is spent in the fruitless attempt, he will be carried down the torrent with the common herd of believers.
Secondly, I think it would not be prudent to throw off 202 OF THE HUMAN MIND. [CHAP. VI.
this belief, if it were in my power. If nature intended to deceive me and impose upon me by false appearances, and I, by my great cunning and profound logic have discovered the imposture; prudence would dictate to me in this case, even to put up with this indignity done me as quietly as I could, and not to call her an impostor to her face, lest she should be even with me in another way. For what do I gain by resenting this injury? You ought at least not to believe what she says. This indeed seems reasonable, if she intends to impose upon me. But what is the conse- quence? I resolve not to believe my senses. 1 break my nose against a post that comes in my way: I step into a dirty kennel; and, after twenty such wise and rational actions, am taken up and clapped into a madhouse. Now, I confess I would rather make one of the credulous fools whom nature imposes upon than of those wise and rational philosophers who resolve to withhold assent at all this ex- pense. If a man pretends to be a sceptic with regard to the informations of sense, and yet prudently keeps out of harm's way as other men do, he must excuse my suspicion that he either acts the hypocrite or imposes upon himself. For if the scale of his belief were so evenly poised as to lean no more on one side than to the contrary, it is impos- sible that his actions could be directed by any rules of common prudence.
Thirdly, Although the two reasons already mentioned are perhaps two more than enough, I shall offer a third. I gave implicit belief to the informations of nature by my senses, for a considerable part of my life, before I had learned so much logic as to be able to start a doubt con- cerning them. And now, when I reflect upon what is passed, I do not find that I have been imposed upon by this belief. I find that without it I must have perished by a thousand accidents. I find that without it I should have been no wiser now than when I was born. — I should not even have been able to acquire that logic which sug- gests these sceptical doubts with regard to my senses. Therefore I consider this instinctive belief as one of the best gifts of nature. I thank the Author of my being who SECT. XX.] OF SEEING. 203 bestowed it upon me before the eyes of my reason were opened, and still bestows it upon me to be my guide, where reason leaves me in the dark. And now I yield to the direction of my senses, not from instinct only, but from confidence and trust in a faithful and beneficent Monitor, grounded upon the experience of his paternal care ami goodness.
In all this I deal with the Author of my being no other- wise than I thought it reasonable to deal with my parents and tutors. I believed by instinct whatever they told me, long before I had the idea of a lie, or thought of the pos- sibility of their deceiving me. Afterwards, upon reflec- tion, I found they had acted like fair and honest people who wished me well. I found, that if I had not believed what they told me, before I could give a reason of my belief, I had to this day been little better than a change- ling. And although this natural credulity hath sometimes occasioned my being imposed upon by deceivers, yet it hath been of infinite advantage to me upon the whole; therefore I consider it as another good gift of nature. And I continue to give that credit, from reflection, to those of whose integrity and veracity I have had experience which before I gave from instinct.
There is a much greater similitude than is commonly imagined between the testimony of nature given by our senses, and the testimony of men given by language. The credit we give to both is at first the effect of instinct only. \Vhen we grow up and begin to reason about them, the credit given to human testimony is restrained and weakened by the experience we have of deceit. But the credit given to the testimony of our senses is established.and confirmed by the uniformity and constancy of the laws of nature.
Our perceptions are of two kinds; some are natural and original, others acquired, and the fruit of experience. When 1 perceive that this is the taste of cider, that of brandy; and this is the smell of an apple, that of an orange; that this is the noise of thunder, that the ringing of bells; this the sound of a coach passing, that the voice of such a friend; these perceptions, and others of the same kind are not original, they are acquired. But the percep- - 204 OF THE HUMAN MIND. [CHAP. VI.
tion which I have by touch, of the hardness and softness of bodies, of their extension, figure, and motion, is not ac- quired; it is original.
In all our senses the acquired perceptions are many more than the original, especially in sight. By this sense \ve perceive originally the visible figure and colour of bodies only, and their visible place: but we learn to perceive by the eye almost every thing which we can perceive by touch. The original perceptions of this sense serve only as signs to introduce the acquired.
The signs by which objects are presented to us in per- ception are the language of nature to man; and as, in many respects, it hath great affinity with the language of man to man; so particularly in this, that both are partly natural and original, partly acquired by custom. Our original or natural perceptions are analogous to the na- tural language of man to man, of which we took notice in the 4th Chapter; and our acquired perceptions are analo- gous to artificial language, which, in our mother tongue, is got very much in the same manner with our acquired per- ceptions, as we shall afterwards more fully explain.
Not only men, but children, idiots, and brutes, acquire by habit many perceptions which they had not originally. Almost every employment in life hath perceptions of this kind that are peculiar to it. The shepherd knows every sheep of his flock, as we do our acquaintance, and can pick them out of another flock one by one. The butcher knows by sight the weight and quality of his beeves and sheep before they are killed. The farmer perceives by his eye, very nearly, the quantity of hay in a rick, or of corn in a heap. The sailor sees the burden, the build, and the distance of a ship at sea, while she is a great way oft. Every man accustomed to writing distinguishes his ac- quaintance by their hand-writing as he does by their faces. And the painter distinguishes, in the works of his art, the style of all the great masters. In a word, acquired per- ception is very different in different persons according to the diversity of objects about which they are employed, and the application they bestow in observing them.
Perception ought not only to be distinguished from sen- SECT. XX.] OF SEEING. 205 sation, but likewise from that knowledge of the objects of sense which is got by reasoning. There is no reasoning in perception, as hath been observed. The belief which is implied in it is the effect of instinct. But there are many things, with regard to sensible objects, which we can infer from what we perceive; and such conclusions of reason ought to be distinguished from what is merely perceived. When I look at the moon, I perceive her to be sometimes circular, sometimes horned, and sometimes gibbous. This is simple perception, and is the same in the philosopher and in the clown: but from these various appearances of her enlightened part, I infer that she is really of a spheri- cal figure. This conclusion is not obtained by simple per- ception, but by reasoning. Simple perception has the same relation to the conclusions of reason drawn from our perceptions, as the axioms in mathematics have to the pro- positions. I cannot demonstrate that two quantities which are equal to the same quantity are equal to each other; neither can I demonstrate, that the tree which I perceive exists. But, by the constitution of my nature, my belief is irresistibly carried along by my apprehension of the axiom; and, by the constitution of my nature, my belief is no less irresistibly carried along by my perception of the tree. All reasoning is from principles. The first prin- ciples of mathematical reasoning are mathematical axioms and definitions; and the first principles of all our reasoning about existences are our perceptions. The first principles of every kind of reasoning are given us by nature, and are of equal authority with the faculty of reason itself, which is also the gift of nature. The conclusions of reason are all built upon first principles, and can have no other foun- dation. Most justly, therefore, do such principles disdain to be tried by reason, and laugh at all the artillery of the logician, when it is directed against them.
When a long train of reaso?iing is necessary in demon- strating a mathematical proposition, it is easily distinguished from an axiom, and they seem to be things of a very dif- ferent nature. But there are some propositions which lie so near to axioms that it is difficult to say, whether they 206 OF THE HUMAN MIND. [CHAP. VI.
ought to be held as axioms, or demonstrated as proposi- tions. The same thing holds with regard to perception, and the conclusions drawn from it. Some of these conclu- sions follow our perceptions so easily, and are so imme- diately connected with them, that it is difficult to fix the limit which divides the one from the other.
Perception, whether original or acquired, implies no ex- ercise of reason; and is common to men, children, idiots, and brutes. The more obvious conclusions drawn from our perceptions, by reason, make what we call common understanding; by which men conduct themselves in the common affairs of life, and by which they are distinguished from idiots. The more remote conclusions which are drawn from our perceptions, by reason, make what wt commonly call science in the various parts of nature, whe- ther in agriculture, medicine, mechanics, or in any part of natural philosophy. When I see a garden in good order, containing a great variety of things of the best kinds, and in the most flourishing condition, I immediately conclude from these signs the skill and industry of the gardener. A farmer, when he rises in the morning, and perceives that the neighbouring brook overflows his field, concludes that a great deal of rain hath fallen in the night. Perceiving his fence broken and his corn trodden down, he concludes that some of his own or his neighbours cattle have broken loose. Perceiving that his stable-door is broken open, and some of his horses gone, he concludes that a thief has carried them off. He traces the prints of his horses' feet in the soft ground, and by them discovers which road the thief hath taken. These are instances of common under- standing, which dwells so near to perception that it is dif- ficult to trace the line which divides the one from the other. In like manner the science of nature dwells so near to common understanding that we cannot discern where the latter ends and the former begins. I perceive that bodies lighter' than water swim in water, and that those which are heavier sink. Hence I conclude, that it a body remains wherever it is put under water, whether at the top or bottom, it is precisely of the same weight with SECT. XX.] OF SEEING. 207 water. If it will rest only when part of it is above water, it is lighter than water. And the greater the part above water is, compared with the whole, the lighter is the body. If it had no gravity at all, it would make no impression upon the water, but stand wholly above it. Thus, every man, by common understanding, has a rule by which he judges of the specific gravity of bodies which swim in water: and a step or two more leads him into the science of hydrostatics.
All that we know of nature or of existences may be compared to a tree which hath its root, trunk, and branches. In this tree of knowledge perception is the root, common understanding is the trunk, and the sciences are the branches.
SECT. XXI.
Of the Process of Nature in Perception.
ALTHOUGH there is no reasoning in perception, yet there are certain means and instruments, which, by the appoint- ment of nature, must intervene between the object and our perception of it; and by these our perceptions are limited and regulated. First, if the object is not in contact with the organ of sense, there must be some medium which passes between them. Thus, in vision, the rays of light; in bearing, the vibrations of elastic air; in smelling, the elHuvia of the body smelled must pass from the object to the organ; otherwise we have no perception. Secondly, There must be some action or impression upon the organ of sense, either by the immediate application of the object, or by the medium that goes between them. Tliirdly, Th< nerves which go from the brain to the organ must receiv* some impression by means of that which was made upon the organ; and probably, by mean:-; of the nerves, some impression must be made upon the brain. Fourthly, Thi- impression made upon the organ, nerves, and brain, is fol- lowed by a sensation. And, last of all. This sensation is followed by the perception of the object.
208 OF THE HUMAN MIND. [CHAP. VI.
Thus our perception of objects is the result of a train of operations; some of which affect the body only, others affect the mind. We know very little of the nature of some of these operations; we know not at all how the y are connected together, or in what way they contribute to that perception which is the result of the whole: but, by the laws of our constitution, we perceive objects in this and in no other way.
There may be other beings who can perceive external objects without rays of light, or vibrations of air, or effluvia of bodies, without impressions on bodily organs, or even without sensations: but we are so framed by the Author of nature, that even when we are surrounded by external objects, we may perceive none of them. Our faculty of perceiving an object lies dormant until it is roused and sti- mulated by a certain corresponding sensation. Nor is this sensation always at hand to perform its office; for it enters into the mind only in consequence of a certain cor- responding impression made on the organ of sense by the object.