This train of machinery the wisdom of God has made necessary to our perceiving objects. Various parts of the body concur to it, and each has its own function. First, The object, either immediately, or by some medium, must make an impression on the organ. The organ serves only as a medium by which an impression is made on the nerve; and the nerve serves as a medium to make an impression upon the brain. Here the material part ends; at least we can trace it no farther; the rest is all intellectual. clude in general-that, as the impressions on the organs, nerves, and brain, correspond exactly to the nature and conditions of the objects by which they are made, so our perceptions and sensations correspond to those impressions, and vary in kind, and in degree, as they vary. [82] Without this exact correspondence, the information we receive by our senses would not only be imperfect, as it undoubtedly is, but would be fallacious, which we have no reason to think it is. CHAPTER III. BRAIN. The proof of these impressions upon the nerves and brain in [81] perception is this, that, from many observations and experi- HYPOTHESES CONCERNING THE NERVES AND ments, it is found that, when the organ of any sense is perfectly sound, and has the impression made upon it by the object ever so strongly, yet, if the nerve which serves that organ be cut or tied hard, there is no perception; and it is well known that disorders in the brain deprive us of the power of perception when both the organ and its nerve are sound. There is, therefore, sufficient reason to conclude that, in perception, the object produces some change in the organ; that the organ produces some change upon the nerve; and that the nerve produces some change in the brain. And we give the name of an impression to those changes, because we have not a name more proper to express, in a general manner, any change produced in a body, by an external cause, without specifying the nature of that change. Whether it be pressure, or attraction, or repulsion, or vibration, or something unknown, for which we have name, still it may be called an impression. But, with regard to the particular kind of this change or impression, philosophers have never been able to discover anything at all. no But, whatever be the nature of those impressions upon the organs, nerves, and brain, we perceive nothing without them. Experience informs that it is so; but we cannot give a reason why it is so. In the constitution of man, perception, by fixed laws of nature, is connected with those impressions; but we can discover no necessary connection. The Supreme Being has seen fit to limit our power of perception; so that we perceive not without such impressions; and this is all we know of the matter. This, however, we have reason to con There can be no doubt that the whole organism of the sense, from periphery to centre, must co-operate simultaneously in perception; but there is no rea son to place the mind at the central extremity alone, and to hold that not only a certain series of organic changes, but a sensation, must precede the mental cognition. This is mere hypothesis, and opposed to the testimony of consciousness.-H. We are informed by anatomists, that, although the two coats which inclose a nerve, and which it derives from the coats of the brain, are tough and elastic, yet the nerve itself has a very small degree of consistence, being almost like marrow. It has, however, a fibrous texture, and may be divided and subdivided, till its fibres escape our senses; and, as we know so very little about the texture of the nerves, there is great room left for those who choose to indulge themselves in conjecture. The ancients conjectured that the nervous fibres are fine tubes, filled with a very subtile spirit, or vapour, which they called animal spirits; that the brain is a gland, by which the animal spirits are secreted from the finer part of the blood, and their continual waste repaired; and that it is by these animal spirits that the nerves perform their functions. Des Cartes has shewn how, by these animal spirits, going and returning in the nerves, muscular motion, perception, memory, and imagination, are effected. All this he has described as distinctly as if he had been an eye-witness of all those operations. But it happens that the tubular structure of the nerves was never perceived by the human eye, nor shewn by the nicest injections; and all that has been said about animal spirits, through more than fifteen centuries, is mere conjecture. Dr Briggs, who was Sir Isaac Newton's master in anatomy, was the first, as far as I know, who advanced a new system concerning [83] the nerves. He conceived them to be solid filaments of prodigious Briggs was not the first. The Jesuit, Hone. ratus Fabry, had before him denied the old hypothesis of spirits; and the new hypothesis of cerebral fibres, and fibrils, by which he explains the phænomena of sense, imagination and memory, is not only the first, but perhaps the most ingenious of the class that has been proposed. Yet the very name of Fabry is wholly unnoticed by those historians of philosophy who do not deem it superfluous to dwell on the tire. some reveries of Briggs, Hartley, and Bonnet.-H. tenuity; and this opinion, as it accords better with observation, seems to have been more generally received since his time. As to the manner of performing their office, Dr Briggs thought that, like musical cords, they have vibrations differing according to their length and tension. They seem, however, very unfit for this purpose, on account of their want of tenacity, their moisture, and being through their whole length in contact with moist substances; so that, although Dr Briggs wrote a book upon this system, called Nova Visionis Theoria, it seems not to have been much followed. Sir Isaac Newton, in all his philosophical writings, took great care to distinguish his doctrines, which he pretended to prove by just induction, from his conjectures, which were to stand or fall according as future experiments and observations should establish or refute them. His conjectures he has put in the form of queries, that they might not be received as truths, but be inquired into, and determined according to the evidence to be found for or against them. Those who mistake his queries for a part of his doctrine, do him great injustice, and degrade him to the rank of the common herd of philosophers, who have in all ages adulterated philosophy, by mixing conjecture with truth, and their own fancies with the oracles of Nature. Among other queries, this truly great philosopher proposed this, Whether there may not be an elastic medium, or æther, immensely more rare than air, which pervades all bodies, and which is the cause of gravitation; of the refraction and reflection of the rays of light; of the transmission of heat, through spaces void of air; and of many other phaænomena ? In the 23d query subjoined to his “Optics,” he puts this question with regard to the impressions made on the nerves and brain in perception, Whether vision is effected chiefly by the vibrations of this medium, excited in the bottom of the eye by the rays of light, and propagated along the solid, pellucid, and uniform capillaments of the optic nerve? And whether hearing is effected [84] by the vibrations of this or some other medium, excited by the tremor of the air in the auditory nerves, and propagated along the solid, pellucid, and uniform capillaments of those nerves? And so with regard to the other senses. What Newton only proposed as a matter to be inquired into, Dr Hartley conceived to have such evidence, that, in his "Observations on Man," he has deduced, in a mathematical form, a very ample system concerning the faculties of the mind, from the doctrine of vibrations, joined with that of association. His notion of the vibrations excited in the nerves, is expressed in Propositions 4 and 5 of the first part of his "Observations on Man." "Prop. 4. External objects impressed on the senses occasion, first in the nerves on which they are impressed, and then in the brain, vibrations of the small, and, as one may say, infinitesimal medullary particles. Prop. 5. The vibrations mentioned in the last proposition are excited, propagated, and kept up, partly by the æther-that is, by a very subtile elastic fluid; partly by the uniformity, continuity, softness, and active powers of the medullary substance of the brain, spinal marrow, and nerves." The modesty and diffidence with which Dr Hartley offers his system to the world— by desiring his reader "to expect nothing but hints and conjectures in difficult and obscure matters, and a short detail of the principal reasons and evidences in those that are clear; by acknowledging, that he shall not be able to execute, with any accuracy, the proper method of philosophising, recommended and followed by Sir Isaac Newton; and that he will attempt a sketch only for the benefit of future enquirers"seem to forbid any criticism upon it. One cannot, without reluctance, criticise what is proposed in such a manner, and with so good intention; yet, as the tendency of this system of vibrations is to make all the operations of the mind mere mechanism, dependent [85] on the laws of matter and motion, and, as it has been held forth by its votaries, as in a manner demonstrated, I shall make some remarks on that part of the system which relates to the impressions made on the nerves and brain in perception. It may be observed, in general, that Dr Hartley's work consists of a chain of propositions, with their proofs and corollaries, digested in good order, and in a scientific form. A great part of them, however, are, as he candidly acknowledges, conjectures and hints only; yet these are mixed with the propositions legitimately proved, without any distinction. Corollaries are drawn from them, and other propositions grounded upon them, which, all taken together, make up a system. A system of this kind resembles a chain, of which some links are abundantly strong, others very weak. The strength of the chain is determined by that of the weakest links; for, if they give way, the whole falls to pieces, and the weight supported by it falls to the ground. Philosophy has been, in all ages, adulterated by hypotheses; that is, by systems built partly on facts, and much upon conjecture. It is pity that a man of Dr Hartley's knowledge and candour should have followed the multitude in this fallacious tract, after expressing his approbation of the proper method of philosophising, pointed out by Bacon and Newton. The last con sidered it as a reproach when his system was called his hypothesis; and says, with disdain of such imputation, Hypotheses non fingo. And it is very strange that Dr Hartley should not only follow such a method of philosophising himself, but that he should direct others in their inquiries to follow it. So he does in Proposition 87, Part I., where he deduces rules for the ascertainment of truth, from the rule of false, in arithmetic, and from the art of decyphering; and in other places. As to the vibrations and vibratiuncles, whether of an elastic æther, or of the infinitesimal particles of the brain and nerves, there [86] may be such things for what we know; and men may rationally inquire whether they can find any evidence of their existence; but, while we have no proof of their existence, to apply them to the solution of phænomena, and to build a system upon them, is what I conceive we call building a castle in the air. no other way in which the phænomena may be produced, and, therefore, they must be produced in this way. There is, therefore, no proof of vibrations in the infinitesimal particles of the brain and nerves. It may be thought that the existence of an elastic vibrating æther stands on a firmer foundation, having the authority of Sir Isaac Newton. But it ought to be observed that, although this great man had formed conjectures about this æther near fifty years before he died, and had it in his eye during that long space as a subject of inquiry, yet it does not appear that he ever found any convincing proof of its existence, but considered it to the last as a question whether there be such an æther or not. In the premonition to the reader, prefixed to the second edition of his "Optics," anno 1717, he expresses himself thus with regard to it :-"Lest any one should think that I place gravity among the essential properties of bodies, I have subjoined one question concerning its cause; a question, I say, for I do not hold it as a thing estab When men pretend to account for any of the operations of Nature, the causes assigned by them ought, as Sir Isaac New-lished." ton has taught us, to have two conditions, otherwise they are good for nothing. First, They ought to be true, to have a real existence, and not to be barely conjectured to exist, without proof. Secondly, They ought to be sufficient to produce the effect. As to the existence of vibratory motions in the medullary substance of the nerves and brain, the evidence produced is this: First, It is observed that the sensations of sceing and hearing, and some sensations of touch, have some short duration and continuance. Secondly, Though there be no direct evidence that the sensations of taste and smell, and the greater part of these of touch, have the like continuance, yet, says the author, analogy would incline one to believe that they must resemble the sensations of sight and hearing in this particular. Thirdly, The continuance of all our sensations being thus established, it follows, that external objects impress vibratory motions on the medullary substance of the nerves and brain; because no motion, besides a vibratory one, can reside in any part for a moment of time. This is the chain of proof, in which the first link is strong, being confirmed by experience; the second is very weak; and the third still weaker. For other kinds of motion, besides that of vibration, may have some continuance-such as rotation, bending or unbending of a spring, and perhaps others which we are unacquainted with; nor do we know whether it is motion that is produced in the nerves-it may be pressure, attraction, repulsion, or something we do not know. This, indeed, is the common refuge of all hypotheses, [87] that we know If, therefore, we regard the authority of Sir Isaac Newton, we ought to hold the existence of such an æther as a matter not established by proof, but to be examined into by experiments; and I have never heard that, since his time, any new evidence has been found of its existence. 66 66 But," says Dr Hartley, supposing the existence of the æther, and of its properties, to be destitute of all direct evidence, still, if it serves to account for a great variety of phænomena, it will have an indirect evidence in its favour by this means." There never was an hypothesis invented by an ingenious man which has not this evidence in its favour. The vortices of Des Cartes, the sylphs and gnomes of Mr Pope, serve to account for a great variety of phænomena. When a man has, with labour and ingenuity, wrought up an hypothesis into a system, he contracts a fondness for it, which is apt [88] to warp the best judgment. This, I humbly think, appears remarkably in Dr Hartley. In his preface, he declares his approbation of the method of philosophising recommended and followed by Sir Isaac Newton; but, having first deviated from this method in his practice, he is brought at last to justify this deviation in theory, and to bring arguments in defence of a method diametrically opposite to it. "We admit," says he, "the key of a cypher to be a true one when it explains the cypher completely." I answer, To find the key requires an understanding equal or superior to that which made the cypher. This instance, therefore, will then be in point, when he who attempts to decypher the works of Nature by an hypothesis, has an understanding equal or superior to that which made them. The votaries of hypotheses have often been challenged to shew one useful discovery in the works of Nature that was ever made in that way. If instances of this kind could be produced, we ought to conclude that Lord Bacon and Sir Isaac Newton have done great disservice to philosophy by what they have said against hypotheses. But, if no such instance can be produced, we must conclude, with those great men, that every system which pretends to account for the phænomena of Nature by hypotheses or conjecture, is spurious and illegitimate, and serves only to flatter the pride of man with a vain conceit of knowledge which he has not attained. The author tells us, "that any hypothesis that has so much plausibility as to explain a considerable number of facts, helps us to digest these facts in proper order, to bring new ones to light, and to make erperimenta crucis for the sake of future inquirers." Let hypotheses be put to any of these uses as far as they can serve. Let them suggest experiments, or direct our inquiries; but let just induction alone govern our belief. "The rule of false affords an obvious and strong instance of the possibility of being led, with precision and certainty, to a [89] true conclusion from a false position. And it is of the very essence of algebra to proceed in the way of supposition." This is true; but, when brought to justify the accounting for natural phænomena by hypotheses, is foreign to the purpose. When an unknown number, or any unknown quantity, is sought, which must have certain conditions, it may be found in a scientific manner by the rule of false, or by an algebraical analysis; and, when found, may be synthetically demonstrated to be the number or the quantity sought, by its answering all the conditions required. But it is one thing to find a quantity which shall have certain conditions; it is a very different thing to find out the laws by which it pleases God to govern the world and produce the phænomena which fall under our observation. And we can then only allow some weight to this argument in favour of hypotheses, when it can be shewn that the cause of any one phænomenon in nature has been, or can be found, as an unknown quantity is, by the rule of false, or by algebraical analysis. This, I apprehend, will never be, till the æra arrives, which Dr Hartley seems to foretell, "When future generations shall put all kinds of evidences and enquiries into mathematical forms; and, as it were, reduce Aristotle's ten Categories, and Bishop Wilkin's forty Summa Genera to the head of quantity alone, so as to make mathematics and logic, natural history and civil history, natural philosophy and philosophy of all other kinds, coincide omni ex parte." Since Sir Isaac Newton laid down the rules of philosophising in our inquiries into the works of Nature, many philosophers have deviated from them in practice; perhaps few have paid that regard to them which they deserve. But they have met with very general approbation, as being founded in reason, and pointing out the only path to the knowledge of Nature's works. Dr Hartley is the only author I have met with who reasons against them, and has taken pains to find out arguments in defence of the exploded method of hypothesis. [90] Another condition which Sir Isaac Newton requires in the causes of natural things assigned by philosophers, is, that they be sufficient to account for the phænomena. Vibrations, and vibratiuncles of the medullary substance of the nerves and brain, are assigned by Dr Hartley to account for all our sensations and ideas, and, in a word, for all the operations of our minds. Let us consider very briefly how far they are sufficient for that purpose. It would be injustice to this author to conceive him a materialist. He proposes his sentiments with great candour, and they ought not to be carried beyond what his words express. He thinks it a consequence of his theory, that matter, if it can be endued with the most simple kinds of sensation, might arrive at all that intelligence of which the human mind is possessed. He thinks that his theory overturns all the arguments that are usually brought for the immateriality of the soul, from the subtilty of the internal senses, and of the rational faculty; but he does not take upon him to determine whether matter can be endued with sensation or no. He even acknowledges that matter and motion, however subtilly divided and reasoned upon, yield nothing more than matter and motion still; and therefore he would not be any way interpreted so as to oppose the immateriality of the soul. It would, therefore, be unreasonable to require that his theory of vibrations should, in the proper sense, account for our sensations. It would, indeed, be ridiculous in any man to pretend that thought of any kind must necessarily result from motion, or that vibrations in the nerves must necessarily produce thought, any more than the vibrations of a pendulum. Dr Hartley disclaims this way of thinking, and therefore it ought not to be imputed to him. All that he pretends is, that, in the human constitution, there is a certain connection between vibrations in the medullary sub stance of the nerves and brain, and the thoughts of the mind; so that the last depend entirely upon the first, and every kind of thought [91] in the mind arises in consequence of a corresponding vibration, or vibratiuncle in the nerves and brain. Our sensations arise from vibrations, and our ideas from vibratiuncles, or miniature vibrations; and he comprehends, under these two words of sensations and ideas, all the operations of the mind. But how can we expect any proof of the connection between vibrations and thought, when the existence of such vibrations was never proved? The proof of their connection cannot be stronger than the proof of their existence; for, as the author acknowledges that we cannot infer the existence of the thoughts from the existence of the vibrations, it is no less evident that we cannot infer the existence of vibrations from the existence of our thoughts. The existence of both must be known before we can know their connection. As to the existence of our thoughts, we have the evidence of consciousness, a kind of evidence that never was called in question. But as to the existence of vibrations in the medullary substance of the nerves and brain, no proof has yet been brought. All, therefore, we have to expect from this hypothesis, is, that in vibrations, considered abstractly, there should be a variety in kind and degree, which tallies so exactly with the varieties of the thoughts they are to account for, as may lead us to suspect some connection between the one and the other. If the divisions and subdivisions of thought be found to run parallel with the divisions and subdivisions of vibrations, this would give that kind of plausibility to the hypothesis of their connection, which we commonly expect even in a mere hypothesis; but we do not find even this. For, to omit all those thoughts and operations which the author comprehends under the name of ideas, and which he thinks are connected with vibratiuncles; to omit the perception of external objects, which he comprehends under the name of sensatims; to omit the sensations, properly so called, which accompany our passions [92] and affections, and to confine ourselves to the sensations which we have by means of our external senses, we can perceive no correspondence between the variety we find in their kinds and degrees, and that which may be supposed in vibrations. We have five senses, whose sensations differ totally in kind. By each of these, excepting perhaps that of hearing, we have a variety of sensations, which differ specifically, and not in degree only. How many tastes and smells are there which are spefically different, each of them capable of all degrees of strength and weakness? Heat and cold, roughness and smoothness, hardness and softness, pain and pleasure, are sensations of touch that differ in kind, and each has an endless variety of degrees. Sounds have the qualities of acute and grave, loud and low, with all different degrees of each. The varieties of colour are many more than we have names to express. How shall we find varieties in vibrations corresponding to all this variety of sensations which we have by our five senses only? I know two qualities of vibrations in an uniform elastic medium, and I know no more. They may be quick or slow in various degrees, and they may be strong or weak in various degrees; but I cannot find any division of our sensations that will make them tally with those divisions of vibra tions. If we had no other sensations but those of hearing, the theory would answer well; for sounds are either acute or grave, which may answer to quick or slow vibrations; or they are loud or low, which answer to strong or weak vibrations. But then we have no variety of vibrations corresponding to the immense variety of sens ations which we have by sight, smell, taste, and touch. Dr Hartley has endeavoured to find out other two qualities of vibrations; to wit, that they may primarily affect one part of the brain or another, and that they may vary in their direction according as they enter by different external nerves; but these [93] seem to be added to make a number; for, as far as we know, vibrations in an uniform elastic substance spread over the whole, and in all directions. However, that we may be liberal, we shall grant him four different kinds of vibrations, each of them having as many degrees as he pleases. Can he, or any man, reduce all our sensations to four kinds? We have five senses, and by each of them a variety of sensations, more than sufficient to exhaust all the varieties we are able to conceive in vibrations. Dr Hartley, indeed, was sensible of the difficulty of finding vibrations to suit all the variety of our sensations. His extensive knowledge of physiology and pathology could yield him but a feeble aid; and, there fore, he is often reduced to the necessity of heaping supposition upon supposition, conjecture upon conjecture, to give some credibility to his hypothesis; and, in seeking out vibrations which may correspond with the sensations of one sense, he seems to forget that those must be omitted which have been appropriated to another. Philosophers have accounted in some degree for our various sensations of sound by the vibrations of elastic air; but it is to be |