CHAPTER III.

HYPOTHESES CONCERNING THE NERVES AND BRAIN.

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 a 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 subtle 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 shown 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 shown 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 the nerves. He conceived them to be solid filaments of prodigious 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 doctrine, 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 ether, 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 phenomena? 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 ex

cited 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 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 enquired 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. "Proposition 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 ether, that is, by a very subtle 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 inquirers," 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, dependant 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 considered 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 1. 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 ether, or of the infinitesimal particles of the brain and nerves, there 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 phenomena, and to build a system upon them, is what, I conceive, we call building a castle in the air.

When men pretend to account for any of the operations of nature, the causes assigned by them ought, as Sir Isaac Newton 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 seeing 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, that we know no other way in which the phenomena 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 ether 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 ether 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 ether 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 established." If, therefore, we regard the authority of Sir Isaac Newton, we ought to hold the existence of such an ether 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.

But, says Dr. Hartley, "Supposing the existence of the ether, and of its properties, to be destitute of all direct evidence, still, if it serves to account for a great variety of phenomena, it will have an indirect evidence in its favor by this means." There never was an hypothesis invented by an ingenious man which has not this evidence in its favor. The vortices of Des Cartes, the Sylphs and Gnomes of Mr. Pope, serve to account for a great variety of phenomena.

When a man has, with labour and ingenuity, wrought up an hypothesis into a system, he contracts a fondness for it, which is apt 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 show 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 phenomena of nature by hypothesis 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 experimenta 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 probability of being led with precision, and certainty, to a 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 phenomena 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 phenomena which fall under our observation. And we can then only allow some weight to this argument in favor of hypotheses, when it can be shown, that the cause of any one phenomenon 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 foretel," when future generations shall put all kinds of evidences

and inquiries into mathematical forms; and, as it were, reduce Aristotle's ten Categories, and Bishop Wilkins' 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 hypotheses.

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 phenomena. Vibrations and vibratiuncles of the medullary substances 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 sensations, 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 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 connexion between vibrations in the medullary substance 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 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 connexion between vibrations and thought, when the existence of such vibrations was never proved? The proof of their connexion 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