The first question is, whether the fibres coming from corresponding points of the two retina, do not unite at the place. where the optic nerves meet, and continue united from thence to the brain; so that the right optic nerve, after the meeting of the two nerves, is composed of the fibres coming from the right side of both retina, and the left of the fibres coming from the left side of both retina ?

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This is undoubtedly a curious and rational question; because if we could find ground from anatomy to answer it in the affirmative, it would lead us a step forward in discovering the cause of the correspondence and sympathy which there is between certain points of the two retina. For although we know not what is the particular function of the optic nerves, yet it is probable, that some impression made upon them, and communicated along their fibres, is necessary to vision and whatever be the nature of this impression, if two fibres are united into one, an impression made upon one of them, or upon both, may probably produce the same effect. Anatomists think it a sufficient account of a sympathy between two parts of the body, when they are served by branches of the same nerve: we should therefore look upon it as an important discovery in anatomy, if it were found that the same nerve sent branches to the corresponding points of the retina.

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 discovered. Dr. Porterfield hath given us two cases at length from Vesalius, and one from Casalpinus, wherein the optic nerves, after touching one another as usual, appeared 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. erbroeck tells us, that Aquapendens and Valverda likewise affirm, that they have met with subjects wherein the optic nerves did not touch.

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As these observations were made before Sir Isaac Newton 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 examined. But from the following passage of the most accurate 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 adhesion, 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."

When 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 anatomists which contradict it, we are naturally led to this reflection, 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 species 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 sensorium 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 sensorium, 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 objects are at all carried to the sensorium, either by the optic 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 hypothesis, 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 retina, 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 appearance 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 tangible 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 presented 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 understand the meaning of these sounds; and therefore we fix our attention, not upon the sounds, but upon the things signified 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 appearance 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 presented 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 perception 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 proper 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.

SECTION XX.

OF PERCEPTION IN GENERAL.

SENSATION, and the perception of external objects by the senses, though very different in their nature, have commonly been considered as one and the same thing. The purposes of common life do not make it necessary to distinguish them, and the received opinions of philosophers tend rather to confound them; but, without attending carefully 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 definition: 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 sensation 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 perception. 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 expressions, 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 instances, 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 when we attend to the sensation by itself, and separate it from other things which are conjoined with it in the imagination, 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.

that

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 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 distinguishable, 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 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 contrary; 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 argumentation and reasoning; it is the immediate effect of my constitution.

I am aware, that this belief which I have in perception, stands exposed to the strongest batteries of skepticism. But they make no great impression upon it. The skeptic 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 suspicion. Reason, says the skeptic, 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 skeptic 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 deluded; and therefore you ought to resolve firmly to withhold assent, and to throw off all his 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 skeptic, 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 contented, and