It must, however, be admitted that Cullen wielded the method with great ability, especially in his application of it to the science of pathology, to which it was far better suited than to the art of therapeutics. For, we must always remember, that the science which investigates the laws of disease, is quite a different thing from the art which cures it. The science has a speculative interest, which is irrespective of all practical considerations, and which depends simply on the fact, that, when it is completed, it will explain the aberrations of the whole organic world. Pathology aims at ascertaining the causes which determine every departure from the natural type, whether of form or of function. Hence it is, that no one can take a comprehensive view of the actual state of knowledge, without studying the theoretic relations between pathology and other departments of inquiry. To do this, is the business, not of practical men, but of philosophers, properly so called. The philosophic pathologist is as different from the physician, as a jurist is different from an advocate, or as an agricultural chemist is different from a farmer, or as a political economist is different from a statesman, or as an astronomer, who generalizes the laws of the heavenly bodies, is different from a captain, who navigates his ship by a practical application of those laws. The two sets of functions may be united, and occasionally, though very rarely, they are, but there is no necessity for their being so. While, therefore, it would be absurdly presumptuous for an unprofessional person to pass judgment on the therapeutical system of Cullen, it is perfectly legitimate for any one, who has studied the theory of these matters, to examine his pathological system; because that, like all scientific systems, must be amenable to general considerations, which are to be taken, partly from the adjoining sciences, and partly from the universal logic of philosophic method.

It is from this latter, or logical, point of view, that Cullen's pathology is interesting for the purposes of the present chapter. The character of his investigations may be illustrated by saying, that his method in pathology is analogous to that which Adam Smith adopted at the same time, though in a very different field. Both were deductive; and both, before arguing deductively, suppressed some of the premisses from which they reasoned. That this suppression is the key to Adam Smith's method, and was an intentional part of his plan, I have already shown; as also that, in each of his two works, he supplied the premisses in which the other work was deficient. In this respect, he was far superior to Cullen. For, though Cullen, like Smith, began by mutilating his problem in order to solve it more readily, he, unlike Smith, did not see the necessity of instituting another and parallel inquiry, which should complete the scheme, by starting from the premisses that had been previously omitted.

What I have termed the mutilation of the problem, was effected by Cullen in the following manner. His object was, to generalize the phenomena of disease, as they are exhibited in the human frame; and it was obvious to him, as to every one else, that the human frame consists partly of solids and partly of fluids. The peculiarity of his pathology is, that he reasons almost entirely from the laws of the solids, and makes so little account of the fluids, that he will only allow them to be the indirect causes of disease, which, in a scientific view, are to be deemed strictly subordinate to the direct causes, as represented by the solid constituents of our body.[826] This assumption, though false, was perfectly justifiable, since, by curtailing the problem, he simplified its study; just as Adam Smith, in his Wealth of Nations, simplified the study of human nature, by curtailing it of all its sympathy. But this most comprehensive thinker was careful, in his Theory of Moral Sentiments, to restore to human nature the quality of which the Wealth of Nations had deprived it; and, by thus establishing two different lines of argument, he embraced the whole subject. In the same way, it was incumbent on Cullen, after having constructed a theory of disease by reasoning from the solids, to have constructed another theory by reasoning from the fluids; so that a coördination of the two theories might have raised a science of pathology, as complete as the then state of knowledge allowed.[827] But to this, his mind was unequal. Able though he was, he lacked the grasp of intellect which characterized Adam Smith, and which made that great man perceive, that every deductive argument which is founded on a suppression of premisses, must be compensated by a parallel argument, which takes those premisses into account.[828] So little was Cullen aware of this, that, having built up that system of pathology which is known to medical writers as Solidism, he never took the pains to accompany it by another system, which gave the first rank to the fluids. On the contrary, he believed that his plan was complete and exhaustive, and that what is termed Humoral Pathology was a fiction, which had too long usurped the place of truth.[829]

Several of the views advocated by Cullen were taken from Hoffmann, and several of the facts from Gaubius; but that his pathology, considered as a whole, is essentially original, is evident from a certain unity of design which is inconsistent with extensive plagiarism, and which proves that he had thoroughly thought out his subject for himself. Without, however, stopping to inquire how much he borrowed from others, I will briefly indicate a few of the salient points of his system, in order to enable the reader to understand its general character.

According to Cullen, all the solids in the human body are either simple or vital. The simple solids retain, after death, the properties which they possessed during life. But the vital solids, which form the fundamental part of the nervous system, are marked by properties, which disappear directly death occurs.[830] Hence, the simple solids, having fewer functions than the vital, have also fewer diseases; and the maladies to which they are liable admit of easy classification.[831] The real difficulty lies in the vital solids, because on their peculiarities the whole nervous system depends, and nearly all disorders are immediately due to changes in them. Cullen, therefore, made the nervous system the basis of his pathology; and, in speculating on its functions, he assigned the chief place to an occult principle, which he termed the Animal Power, or Energy, of the brain.[832] This principle acted on the vital solids. When the principle worked well, the body was healthy; when it worked ill, the body was unhealthy. Since, then, the state of the vital solids was the main cause of disorder, and since the Energy of the brain was the main cause of the state of the vital solids, it became important to know what the influences were which acted on the Energy, because in them we should find the beginning of the series. Those influences were divided by Cullen into physical and mental. The physical were, heat, cold, and effluvia, the three most potent of the material disturbers of the human frame.[833] The mental influences, which excited the brain to act on the solids, were comprised under six different heads, namely, the will, the emotions, the appetites, the propensities, and, finally, the two great principles of habit and of imitation, on which he, with good reason, laid considerable stress.[834] In arguing from these mental causes, and in generalizing the relations between them and the sensations of the body, he, faithful to his favourite method, proceeded deductively from the metaphysical principles then in vogue, without inquiring inductively into their validity, such an induction being, he thought, no part of his duty.[835] He was too anxious to get on with his dialectic, to be interrupted by so trifling a matter as the truth or falsehood of the premisses on which the reason rested. What he did in the metaphysical part of his pathology, he also did in its physical part. Although the blood and the nerves are the two leading features of the human economy, he did not search into them by a separate induction; he subjected them neither to chemical experiments in order to learn their composition, nor to microscopic observations in order to learn their structure.[836] This is the more observable, because though we must admit that animal chemistry was then generally neglected, and that its real meaning was scarcely understood until the wonderful labours of Berzelius revealed its importance, still the microscope was ready to Cullen's hands; it having been invented a hundred and fifty years before he completed his pathology, and having been in common scientific use for about a hundred years. But his love of synthesis overcame him. His system is constructed by reasoning from general principles; and of that process, he certainly was a consummate master. Between the premisses and the conclusion, he hardly ever lets error creep in. And, in reference to the results of his speculations, he had one immense merit, which will always secure to him a conspicuous place in the history of pathology. By insisting on the importance of the solids, he, one-sided though he was, corrected the equal one-sidedness of his predecessors; for, with extremely few exceptions, all the best pathologists, from Galen downwards, had erred in ascribing too much to the fluids, and had upheld a purely humoral pathology. Cullen turned the minds of men in the other direction; and though, in teaching them that the nervous system is the sole primary seat of disease, he committed a great mistake, it was a mistake of the most salutary kind. By leaning on that side, he restored the balance. Hence, I have no doubt, he indirectly encouraged those minute researches into the nerves, which he would not himself stop to make, but which, in the next generation, gave rise to the capital discoveries of Bell, Shaw, Mayo, and Marshall Hall. At the same time, the old humoral pathology, which had prevailed for many centuries, was practically pernicious, because, assuming that all diseases are in the blood, it produced that constant and indiscriminate venesection, which destroyed innumerable lives, besides the irreparable injury it often inflicted both on body and mind; weakening those whom it was unable to slay. Against this merciless onslaught, which made medicine the curse of mankind, the Solid Pathology was the first effective barrier.[837] Practically, therefore, as well as speculatively, we must hail Cullen as a great benefactor of his species; and we must regard his appearance as an epoch in the history of human comfort, as well as in the history of human thought.

It may, perhaps, facilitate the conceptions of unprofessional readers, if I give, in as few words as possible, a specimen of the way in which Cullen employed his method, in investigating the theory of some one class of diseases. For this purpose, I will select his doctrine of fever, which, though now generally abandoned, once exercised more influence than any other part of his pathology. Here, as elsewhere, he reasons from the solids.[838] Disregarding the state of the blood, he says, that the cause of all fever is a diminished energy of the brain.[839] Such diminution may be produced by various sedatives, the most common of which are effluvia, whether marsh or human, intemperance, fear, and cold.[840] Directly the energy of the brain is impaired, the disease begins. Rapidly passing through the nervous system, its first palpable effect is a chill, or cold fit, which is accompanied by a spasm on the extremities of the arteries, particularly where they touch the surface of the body.[841] This spasm on the extreme vessels, irritates the heart and arteries, and the irritation continues till the spasm is relaxed.[842] At the same time, the increased action of the heart restores the energy of the brain; the system rallies; the extreme vessels are relieved; while, as a consequence of the whole movement, sweat is excreted, and the fever abates.[843] Shutting out, therefore, all consideration of the fluids of the body, the successive stages of languor, cold fit, and hot fit, might, in Cullen's opinion, be generalized by reasoning merely from the solids, which, furthermore, produced his well-known distinction between fevers, the continuance of which is owing to an excess of spasm, and those, the continuance of which is owing to an excess of debility.[844]

A similar process of thought gave birth to his Nosology, or general classification of diseases, which some have regarded as the most valuable part of his labours;[845] though, for reasons already mentioned, we must, I think, reject all such attempts as premature, and as likely to work more harm than good, unless they are simply used as a contrivance to aid the memory. At all events, the Nosology of Cullen, though it exhibits clear traces of his powerful and organizing mind, is fast falling into disrepute, and we may be sure, that, for a long time yet, a similar fate will await its successors. Our pathological knowledge is still too young for so great an enterprise.[846] We have every reason to expect, that, with the aid of chemistry, and of the microscope, it will continue to grow more rapidly than it has hitherto done. Without venturing to predict the rate of its increase, we may form some idea of it, by considering what has been effected with resources very inferior to those we now possess. In a work of great authority, published in the year 1848, it is stated, that since the appearance of Cullen's Nosology, our mere enumeration of diseases has almost doubled, while our knowledge of the facts relating to disease has more than doubled.[847]

I have now only one more name to add to this splendid catalogue of the great Scotchmen of the eighteenth century.[848] But it is the name of a man, who, for comprehensive and original genius, comes immediately after Adam Smith, and must be placed far above any other philosopher whom Scotland has produced. I mean, of course, John Hunter, whose only fault was, an occasional obscurity, not merely of language, but also of thought. In this respect, and, perhaps, in this alone, Adam Smith had the advantage; for his mind was so flexible, and moved so freely, that even the vastest designs were unable to oppress it. With Hunter, on the contrary, it sometimes seemed as if the understanding was troubled by the grandeur of his own conceptions, and doubted what path it ought to take. He hesitated; the utterance of his intellect was indistinct.[849] Still, his powers were so extraordinary, that, among the great masters of organic science, he belongs, I apprehend, to the same rank as Aristotle, Harvey, and Bichat, and is somewhat superior either to Haller or Cuvier. As to this classification, men will differ, according to their different ideas of the nature of science, and, above all, according to the extent to which they appreciate the importance of philosophic method. It is from this latter point of view that I have, at present, to consider the character of John Hunter; and, in tracing the movements of his most remarkable mind, we shall find, that, in it, deduction and induction were more intimately united than in any other Scotch intellect, either of the seventeenth or eighteenth century. The causes of this unusual combination, I will now endeavour to ascertain. When they are understood, they will not only explain many peculiarities in his works, but will afford materials for speculation, to those who love to examine the development of ideas, and who are able to discern the way in which different schemes of national thought have given different shapes to national character, and have thereby modified the whole course of human affairs, to an extent of which the ordinary compilers of history have not the slightest suspicion.

Hunter remained in Scotland till the age of twenty, when he settled in London; and, though he was abroad for about three years, he abandoned his own country, and became, socially and intellectually, a native of England.[850] Hence, the early associations of his mind were formed in the midst of a deductive nation; the later associations, in the midst of an inductive one. For twenty years he lived among a people, who are, perhaps, the acutest reasoners in Europe, if you concede to them the principles from which they reason; but who, on the other hand, owing to their proneness to this method, are so greedy after general principles, that they will accept them on almost any evidence, and are, therefore, at once very credulous and very logical. In that school, and surrounded by those habits, the intellect of John Hunter was nurtured during the most impressible period of his life. Then the scene suddenly shifted. Coming to England, he passed forty years in the heart of the most empirical nation in Europe; a nation utterly abhorring all general principles, priding itself on its common sense, boasting, and with good reason too, of its practical sagacity, proclaiming aloud the superiority of facts over ideas, and despising every theory, unless some direct and immediate benefit could be expected to accrue from it. The young and ardent Scotchman found himself transplanted into a country totally different from that which he had just quitted; and such a difference could not fail to influence his mind. He saw, on every side, marks of prosperity, and of long and uninterrupted success, not only in practical, but also in speculative, life; and he was told that these things were effected by a system which made facts the first consideration. He was ambitious of fame, but he perceived that the road to fame was not the same in England as in Scotland. In Scotland, a great logician would be deemed a great man; in England, little account would be made of the beauty of his logic, unless he was careful that the premisses from which he argued, were trustworthy, and verified by experience. A new machine, a new experiment, the discovery of a salt, or of a bone, would, in England, receive a wider homage, than the most profound speculation from which no obvious results were apprehended. That this way of contemplating affairs has produced great good, is certain. But it is also certain, that it is a one-sided way, and satisfies only part of the human mind. Many of the noblest intellects crave for something which it cannot supply. In England, however, during the greater part of the eighteenth century, it was even more supreme than it is now, and was, indeed, so universal, that, from the year 1727 until nearly the close of the century, our country did not possess, in any branch of science, a speculator who had sufficient force to raise himself above those narrow views which were then deemed the perfection of wisdom.[851] Much was added to our knowledge, but its distant boundaries were not enlarged. Though there was an increase of curious and valuable details, and though several of the small and proximate laws of nature were generalized, it must be admitted, that those lofty generalizations, which we owe to the seventeenth century, remained stationary, and that no attempt was made to push beyond them. When John Hunter arrived in London, in 1748, Newton had been dead more than twenty years, and the English people, absorbed in practical pursuits, and now beginning, for the first time, to enter into political life, had become more averse than ever to inquiries which aimed at truth without regard to utility, and had accustomed themselves to value science chiefly for the sake of the direct and tangible benefit which they might hope to derive from it.

That Hunter must have been influenced by these circumstances, will be obvious to whoever considers how impossible it is for any single mind to escape from the pressure of contemporary opinion. But, inasmuch as all his early associations had inclined him in another direction, we perceive that, during his long residence in England, he was acted on by two conflicting forces. The country of his birth made him deductive; the country of his adoption made him inductive. As a Scotchman, he preferred reasoning from general principles to particular facts; as an inhabitant of England, he became inured to the opposite plan of reasoning from particular facts to general principles. In every country, men naturally give the first place to what is most valued. The English respect facts more than principles, and therefore begin with the facts. The Scotch consider principles as most important, and therefore begin with the principles. And, I make no doubt that one of the reasons why Hunter, in investigating a subject, is often obscure, is that, on such occasions, his mind was divided between these two hostile methods, and that, leaning sometimes to one and sometimes to the other, he was unable to determine which he should choose. The conflict darkened his understanding. Adam Smith, on the other hand, in common with all the great Scotchmen who remained in Scotland, was remarkably clear. He, like Hume, Black, and Cullen, never wavered in his method. These eminent men were not acted on by English influence. Of all the most illustrious Scotchmen of the eighteenth century, Hunter alone underwent that influence, and he alone displayed a certain hesitation and perplexity of thought, which seems unnatural to so great a mind, and which, as it appears to me, is best explained by the peculiar circumstances in which he was placed.

One of the ablest of his commentators has justly observed, that his natural inclination was, to conjecture what the laws of nature were, and then reason from them, instead of reasoning to them by slow and gradual induction.[852] This process of deduction was, as I have shown, the favourite method of all Scotchmen, and, therefore, was precisely the course which we should have expected him to adopt. But, inasmuch as he was surrounded by the followers of Bacon,[853] this natural bias was warped, and a large part of his marvellous activity was employed in observations and experiments, such as no Scotch thinker, living in Scotland, would ever have engaged in. He himself declared, that thinking was his delight;[854] and there can be no doubt that, had he been differently situated, thinking would have been his principal pursuit. As it was, the industry with which he collected facts, is one of the most conspicuous features in his career. His researches covered the whole range of the animal kingdom, and were conducted with such untiring zeal, that he dissected upwards of five hundred different species, exclusive of dissections of different individuals, and exclusive, too, of dissections of a large number of plants.[855] The results were carefully arranged and stored up in that noble collection which he formed, and of the magnitude of which we may gain some idea from the statement, that, at his death, it contained upwards of ten thousand preparations illustrative of the phenomena of nature.[856] By this means, he became so intimately acquainted with the animal kingdom, that he made a vast number of discoveries, which, considered singly, are curious, but which, when put together, constitute an invaluable body of new truths. Of these, the most important are, the true nature of the circulation in crustacea and insects;[857] the organ of hearing in cephalopods;[858] the power possessed by mollusks of absorbing their shells;[859] the fact that bees do not collect wax, but secrete it;[860] the semicircular canals of the cetacea;[861] the lymphatics of birds;[862] and the air-cells in the bones of birds.[863] We are also assured, that he anticipated the recent discoveries respecting the embryo of the kangaroo;[864] and his published works prove, that, in the human subject, he discovered the muscularity of the arteries,[865] the muscularity of the iris,[866] and the digestion of the stomach after death by its own juice.[867] Although, in his time, animal chemistry was not yet raised to a system, and was consequently little heeded by physiologists, Hunter endeavoured, by its aid, to search out the qualities of the blood, so as to ascertain the properties of its constituents.[868] He also examined it in different stages of embryonic life, and by minutely tracking it through its periods of development, he made the capital discovery, that the red globules of the blood are formed later than its other components. His contemporaries, however, were so little alive to the importance of this great physiological truth, that it fell dead upon them, and, being forgotten, it was, about fifty years afterwards, rediscovered, and was announced, in 1832, as a law of nature which had just been brought to light.[869] This is one of many instances in the history of our knowledge, which proves how useless it is for a man to advance too far beyond the age in which he lives.[870] But Hunter, besides making the discovery, also saw its meaning. From it, he inferred that the function of the red globules is to minister to the strength of the system, rather than to its repair.[871] This is now universally admitted; but it was not admitted till long after his death. Its recognition is chiefly owing to the rapid advance of animal chemistry, and to improvements in the microscope. For, by the employment of these resources, it has become manifest, that the red globules, the respiratory process, the production of animal heat, and the energy of the locomotive organs, are but different parts of a single scheme.[872] Their connexion with each other is established, not only by a comparison of different species, but also by a comparison of different members of the same species. In human beings, for example, the locomotive and other animal functions are more active in persons of a sanguine temperament than in those of a lymphatic temperament; while, in sanguine temperaments, the globules are more numerous than in lymphatic ones. The knowledge of this fact we owe to Lecanu;[873] and to him we are also indebted for an analogous fact, corroborating the same view. He has shown, that the blood of women contains more water and fewer red globules than the blood of men;[874] so that here again we discern the relation between these globules and the energy of animal life. Inasmuch, however, as these researches were not made until many years after the death of Hunter, the coincidence between them and his speculative conclusions is a striking instance of his power of generalization, and of that unrivalled knowledge of comparative anatomy, which supplied him with materials from which, in spite of the backwardness of animal chemistry, he was able to draw an inference, which later and minuter researches have decisively verified.[875]

Having thus, by a comprehensive survey of the animal world, associated its remarkable faculty of movement with the state of its blood, Hunter turned his attention to another aspect of the question, and took into consideration the movements of the vegetable world, in the hope that, by comparing these two divisions of nature, he might detect some law, which, being common to both, should unite into one study all the principles of organic motion. Though he failed in this great undertaking, some of his generalizations are very suggestive, and well illustrate the power and grasp of his mind. Looking at the organic kingdom as a whole, he supposed that its capacity of action, both in animals and in vegetables, was of three kinds. The first kind, was the action of the individual upon the materials it already possessed; and this gave rise to growth, secretion, and other functions, in which the juice of the plant was equivalent to the blood of the animal.[876] The second kind of action had for its object to increase these materials; it was always excited by want, and its result was, to nourish and preserve the individual.[877] The third kind was entirely due to external causes, including the whole material world, all the phenomena of which were a stimulus to some kind of action.[878] By combining, in different ways, these different sources of motion, and by studying every incitement to action, first, in reference to one of the three great divisions just indicated, and, secondly, in reference to the power of action, as distinguished from the quantity of action,[879] Hunter believed that some fundamental truths might be obtained, if not by himself, at all events by his successors. For, he thought that, though animals can do many things which plants cannot, still, the immediate cause of action is in both cases the same.[880] In animals, there is more variety of motion, but in plants there is more real power. A horse is certainly far stronger than a man. Yet a small vine cannot only support, but can raise, a column of fluid five times higher than a horse can. Indeed, the power which a plant exercises of holding a leaf erect during an entire day, without pause and without fatigue, is an effort of astonishing vigour, and is one of many proofs, that a principle of compensation is at work, so that the same energy which, in the animal world, is weakened by being directed to many objects, is, in the vegetable world, strengthened by being concentrated on a few.[881]

In pursuing these speculations, which, amid much that is uncertain, contain, I firmly believe, a large amount of important, though neglected, truth, Hunter was led to consider how motion is produced by various forces, such as magnetism, electricity, gravitation, and chemical attraction.[882] This carried him into inorganic science, where, as he clearly saw, the foundation of all organic science must be laid. Just as, on the one hand, the human frame could never be successfully studied, except by the aid of principles which had been collected from an investigation of animals below man,[883] so, on the other hand, the laws of those very animals must, he said, be approached through the laws of common or inorganic matter.[884] He, therefore, aimed at nothing less than to unite all the branches of physical science, taking them in the order of their relative complexity, and proceeding from the simplest to the most intricate. With this view, he examined the structure of the mineral kingdom, and, by an extensive comparison of crystals, he sought to generalize the principles of form, in the same way as, by a comparison of animals, he sought to generalize the principles of function. And, in doing this, he took into account, not only regular crystals, but also irregular ones.[885] For, he knew that, in nature, nothing is really irregular or disorderly; though our imperfect apprehension, or rather the backwardness of our knowledge, prevents us from discerning the symmetry of the universal scheme. The beauty of the plan, and the necessity of the sequence, are not always perceptible. Hence, we are too apt to fancy that the chain is broken, because we cannot see every link in it. From this serious error, Hunter was saved by his genius, even more than by his knowledge. Being satisfied that every thing which happens in the material world, is so connected and bound up with its antecedents, as to be the inevitable result of what had previously occurred, he looked with a true philosophical eye at the strangest and most capricious shapes, because to him they had a meaning and a necessary purpose. To him, they were neither strange nor capricious. They were deviations from the natural course; but it was a fundamental tenet of his philosophy, that nature, even in the midst of her deviations, still retains her regularity.[886] Or, as he elsewhere expresses it, deviation is, under certain circumstances, part of the law of nature.[887]

To generalize such irregularities, or, in other words, to show that they are not irregularities at all, was the main object of Hunter's life, and was the noblest part of his mission. Hence, notwithstanding his vast achievements in physiology, his favourite pursuit was pathology,[888] where, the phenomena being more complex, the intellect has more play. In this great field, he studied the aberrations of structure and of function, in the vegetable, as well as in the animal, world;[889] while, for the aberrations of form, which are the external manifestations of disturbed structure, he took into consideration the appearances presented by the mineral kingdom. There, the power of crystallization is the leading feature, and there, violations of symmetry constitute the essential disorder, whether the deformity of the crystal is subsequent to its production, or whether, being the result of what happened before its production, it is an original, and, if we may so say, congenital, defect. In either case, it is a deviation from the normal type, and, as such, is analogous to the monstrosities, both of animals and of vegetables.[890] The mind of Hunter, by sweeping through this immense range of thought, attained to such commanding views of the philosophy of disease, that, in that department, he is certainly without a rival. As a physiologist, he was equalled, or perhaps excelled, by Aristotle; but as a pathologist, he stands alone, if we consider what pathology was when he found it, and what it was when he left it.[891] Since his death, the rapid advance of morbid anatomy and of chemistry has caused some of his doctrines to be modified, and some of them to be overturned. This has been the work of inferior men, wielding superior chemical and microscopical resources. To say that the successors of John Hunter are inferior to him, is no disparagement to their abilities, since he was one of those extremely rare characters who only appear at very long intervals, and who, when they do appear, remodel the fabric of knowledge. They revolutionize our modes of thought; they stir up the intellect to insurrection; they are the rebels and demagogues of science. And though the pathologists of the nineteenth century have chosen a humbler path, this must not blind us to their merits, or prevent us from being grateful for what they have done. We cannot, however, be too often reminded, that the really great men, and those who are the sole permanent benefactors of their species, are not the great experimenters, nor the great observers, nor the great readers, nor the great scholars, but the great thinkers. Thought is the creator and vivifier of all human affairs. Actions, facts, and external manifestations of every kind, often triumph for a while; but it is the progress of ideas which ultimately determines the progress of the world. Unless these are changed, every other change is superficial, and every improvement is precarious. It is, however, evident that, in the present state of our knowledge, all ideas respecting nature must refer either to the normal or to the abnormal; that is to say, they must be concerned either with what is regular, uniform, and obedient to recognized principles, or else with what is irregular, perturbed, and disobedient. Of these two divisions, the first belongs to science; the second, to superstition. John Hunter formed the superb conception of merging both classes of ideas into one, by showing that nothing is irregular, that nothing is perturbed, that nothing is disobedient. Centuries, perhaps, may elapse before that conception will be consummated. But what Hunter effected towards it, places him at the head of all pathologists, ancient or modern. For, with him, the science of pathology did not mean the laws of disease in man alone, or even in all animals, or even in the whole organic kingdom; but it meant the laws of disease and of malformation in the entire material world, organic and inorganic. His great object was, to raise a science of the abnormal. He determined to contemplate nature as a vast and united whole, exhibiting, indeed, at different times different appearances, but preserving, amidst every change, a principle of uniform and uninterrupted order, admitting of no deviation, undergoing no disturbance, and presenting no real irregularity, albeit to the common eye, irregularities abound on every side.

As pathology was the science to which Hunter was most devoted, so also was it that in which his natural love of deduction was most apparent. Here, far more than in his physiological inquiries, do we find a desire to multiply original principles from which he could reason; in opposition to the inductive method, which always aims at diminishing these principles by gradual and successive analysis. Thus, for instance, in his animal pathology, he attempted to introduce, as an ultimate principle from which he could argue, the idea that all diseases move more rapidly towards the skin than towards internal parts, by virtue of some hidden force, which also obliges vegetables to approach the surface of the earth.[892] Another favourite proposition, which he often used as a major premiss, and by its aid constructed deductively a pathological argument, was, that in no substance, be it what it may, can two processes go on in the same part at the same time.[893] By applying this universal proposition to the more limited phenomena of animal life, he inferred that two general diseases cannot co-exist in the same individual; and he relied so much on this ratiocination, that he refused to credit any testimony by which it was impugned.[894] There is reason to believe that his conclusion is erroneous, and that different diseases can so accompany each other, as to be united in the same individual, at the same time, and in the same part.[895] Whether or not this be the case, it is equally interesting to notice the process of thought which led Hunter to bestow infinitely more pains in arguing from the general theory, than in arguing to it. Indeed, he can hardly be said to have argued to it at all, since he obtained it by a rough and hasty generalization from what seemed to be the obvious properties of inorganic matter. Having thus obtained it, he applied it to the pathological phenomena of the organic world, and especially of the animal world. That he should have adopted this course, is a curious proof of the energy of his deductive habits, and of the force of mind which enabled him so to set at naught the traditions of his English contemporaries, as to follow a method which, in the opinion of every one who surrounded him, was not only full of danger, but could never lead to truth.

Other parts of his pathology abound with similar instances, which show how anxious he was to assume principles on which he could build arguments. Of this kind were his ideas respecting sympathy, as connected with action. He suggested, that the simplest forms of sympathy would probably be found in the vegetable world, because there, the general arrangements are less intricate than in the animal world.[896] On this supposition, he constructed a series of curious and refined speculations, of which, however, I must confine myself to giving a very short summary. As animals sympathize more than vegetables, this helps us to understand why it is that their movements are more numerous. For, sympathy, being a susceptibility to impression, is also a principle of action.[897] Like other principles of action, it may be either natural or diseased.[898] But, whichever it be, it can, in plants, have only one mode of development, because, in them, it can only be influenced by stimulus; while in animals, which have sensation, it has necessarily three modes, one from stimulus, one from sensation, and a third compounded of the other two.[899] These are the largest divisions of sympathy, if we consider the organic world as a whole. In single cases, however, sympathy admits of still further subdivision. We may reason from it, in reference to the age of the individual;[900] we may also reason from it in reference to temperament, since, in point of fact, temperament is nothing but susceptibility to action.[901] And when sympathy is in action, we may, by analyzing our idea of it, reduce it to five different heads, and may classify it as continued, or contiguous, or remote, or similar, or dissimilar.[902] All these supplied Hunter with principles from which, by reasoning deductively, he attempted to explain the facts of disease; for, according to him, disease merely consists in a want of combination of actions.[903] By this process of thought, he was induced to neglect those predisposing causes, to which inductive pathologists pay great attention, and with which the works of his English contemporaries were much occupied. Such causes could only be generalized from observation, and Hunter made no account of them. Indeed, he even denies their real existence, and asserts that a predisposing cause is simply an increased susceptibility to form disposition to action.[904]

By reasoning from the twofold ideas of action and of sympathy, Hunter constructed the deductive or synthetic part of his pathology. This he did as a Scotchman, and to this, had he always lived in Scotland, he would probably have confined himself. But being for forty years surrounded by Englishmen, and having his mind impregnated by English habits, he contracted something of their mode of thought. We, accordingly, find that a considerable portion of his pathology is as inductive as the most eager disciple of Bacon could desire; forming, in this respect, a striking contrast to the purely synthetic method of Cullen, the other great pathologist of Scotland. In the attempt, however, which Hunter made to mix these two methods, he perplexed both himself and his readers. Hence that obscurity, which even his warmest admirers have noticed, though they have not perceived its cause. Vast as his powers were, he was unable to effect a complete union between induction and deduction. That this should have happened, will not surprise any one, who considers how some of the greatest thinkers have failed in this, the most difficult of all enterprises. Among the ancients, Plato failed in induction, and all his followers failed with him; since none of them have placed sufficient confidence in facts, and in the process of reasoning from particulars to generals. Among the moderns, Bacon was deficient in deduction, and every Baconian has been similarly deficient; it being the essential vice of that school to despise reasoning from general propositions, and to underrate the value of the syllogism. It may, indeed, be doubted if the history of the world supplies more than two instances of physical philosophers being as great in one form of investigation as in the other. They are Aristotle and Newton, who wielded each method with equal ease, combining the skill and boldness of deduction with the caution and perseverance of induction, masters alike of synthesis and of analysis, as capable of proceeding from generals to particulars, as from particulars to generals, sometimes making ideas precede facts, and sometimes making facts precede ideas, but never faltering, never doubting which course to take, and never allowing either scheme unduly to encroach on its opposite. That Hunter should be unable to perform this, merely proves that he was inferior to these two men, whose almost incredible achievements entitle them to be termed the prodigies of the human race. But what he did was wonderful, and, in his own department, has never been rivalled. Of the character and extent of his inquiries, I have given a sketch, which, notwithstanding its imperfections, may serve to illustrate the antagonism of the Scotch and English intellects, by showing how the methods peculiar to each nation struggled for mastery in that great mind, which was exposed to the action of both. Which method predominated in Hunter, it would be hard to say. But it is certain, that his understanding was troubled by their conflict. It is also certain, that, owing to his love of deduction, or of reasoning from general ideas, he exercised much less sway over his English contemporaries, than he would have done if he had exclusively followed their favourite method of reasoning from particular facts. Hence, the disproportion between his influence and his merits. As to his merits, it is now admitted that, in addition to his physiological discoveries, and the great pathological views which he propounded, we may trace to him nearly all the surgical improvements which were introduced within about forty years after his death.[905] He was the first who explained, and, indeed, the first who recognized, the disease of inflammation of the veins, which is of frequent occurrence, and, under the name of phlebitis, has latterly been much studied, but which, before his time, had been ascribed to the most erroneous causes.[906] On general inflammation, he threw so much light, that the doctrines which he advocated, and which were then ridiculed as whimsical novelties, are now taught in the schools, and have become part of the common traditions of the medical profession.[907] He, moreover, introduced what is probably the most capital improvement in surgery ever effected by a single man; namely, the practice in aneurism of tying the artery at a distance from the seat of disease. This one suggestion has saved thousands of lives; and both the suggestion, and the first successful execution of it, are entirely owing to John Hunter, who, if he had done nothing else, would, on this account alone, have a right to be classed among the principal benefactors of mankind.[908]

But, so far as his own immediate reputation was concerned, all was in vain. He was in the midst of a people who had no sympathy with that mode of thought which was most natural to him. They cared nothing for ideas, except with a few to direct and tangible results; he valued ideas for themselves, and for the sake of their truth, independently of all other considerations. His English contemporaries, prudent, sagacious, but short-sighted, seeing few things at a time, but seeing those things with admirable clearness, were unable to appreciate his comprehensive speculations. Hence, in their opinion, he was little else than an innovator and an enthusiast.[909] Hence, too, even the practical improvements which he introduced were coldly received, because they proceeded from so suspicious a source. The great Scotchman, thrown among a nation whose habits of mind were uncongenial to his own, stood, says one of the most celebrated of his disciples, in a position of solitary and comfortless superiority.[910] Indeed, so little was he regarded by that very profession of which he was the chiefest ornament, that, during the many years in which he delivered lectures in London on anatomy and on surgery, his audience never amounted to twenty persons.[911]

I have now completed my examination of the Scotch intellect as it unfolded itself in the seventeenth and eighteenth centuries. The difference between those two periods must strike every reader. In the seventeenth century, the ablest Scotchmen wasted their energies on theological subjects, respecting which we have no trustworthy information, and no means of obtaining any. On these topics, different persons and different nations, equally honest, equally enlightened, and equally competent, have entertained, and still entertain, the most different opinions, which they advocate with the greatest confidence, and support by arguments, perfectly satisfactory to themselves, but contemptuously rejected by their opponents. Each side deeming itself in possession of the truth, the impartial inquirer, that is, he who really loves truth, and knows how difficult it is to obtain it, seeks for some means by which he may fairly adjudicate between these conflicting pretensions, and determine which is right and which is wrong. The further he searches, the more he becomes convinced that no such means are to be found, and that these questions, if they do not transcend the limits of the human understanding, do, certainly, transcend its present resources, and have no chance of being answered, while other and much simpler problems are still unsolved. It would be strange, indeed, if we, ignorant of so many lower and subordinate matters, should be able to reach and penetrate these remote and complicated mysteries. It would be strange if we, who, notwithstanding the advances we have made, are still in the infancy of our career, and who, like infants, can only walk with unsteady gait, and are scarce able to move without stumbling, even on plain and level ground, should, nathless, succeed in scaling those dizzy heights, which, overhanging our path, lure us on where we are sure to fall. Unfortunately, however, men are, in every age, so little conscious of their deficiencies, that they not only attempt this impossible task, but believe they have achieved it. Of those who are a prey to this delusion, there are always a certain number, who, seated on their imaginary eminence, are so inflated by the fancied superiority, as to undertake to instruct, to warn, and to rebuke the rest of mankind. Giving themselves out as spiritual advisers, and professing to teach what they have not yet learned, they exhibit in their own persons that most consistent of all combinations, a combination of great ignorance with great arrogance. From this, other evils inevitably follow. The ignorance produces superstition; the arrogance produces tyranny. Hence it is, that, in a country like Scotland, where the pressure of long-continued and adverse circumstances has consolidated the power of these pretenders to wisdom, such sad results become conspicuous in every direction. Not only the national character, but also the national literature, feel their influence, and are coloured by them. It was, therefore, natural that, in Scotland, in the seventeenth century, when the authority of the clergy was most uncontrolled, the consequences of that authority should be most apparent. It was natural that a literature should be created such as that of which I have given some account; a literature which encouraged superstition, intolerance, and bigotry; a literature full of dark misgivings, and of still darker threats; a literature which taught men that it was wrong to enjoy the present, and that it was right to tremble at the future; a literature, in a word, which, spreading gloom on every side, soured the temper, corrupted the affections, numbed the intellect, and brought into complete discredit those bold and original inquiries, without which there can be no advance in human knowledge, and consequently no increase of human happiness.

To this, the literature of the eighteenth century offered a striking and most exhilarating contrast. It seemed as if, in a moment, all was changed. The Baillies, the Binnings, the Dicksons, the Durhams, the Flemings, the Frasers, the Gillespies, the Guthries, the Halyburtons, the Hendersons, the Rutherfords, and the rest of that monkish rabble, were succeeded by eminent and enterprising thinkers, whose genius lighted up every department of knowledge, and whose minds, fresh and vigorous as the morning, opened for themselves a new career, and secured for their country a high place in the annals of European intellect. Something of what they effected, I have endeavoured to narrate; much, however, has been left untold. But I have brought forward sufficient evidence to convince even the most sceptical reader of the splendour of their achievements, and of the difference between the noble literature which they produced, and those wretched compositions which disfigured the preceding century.

Still, great as the difference was, the two literatures had, as I have shown, one important point in common. Both were essentially deductive; and the proof of this I have given at considerable length, because, though it has, so far as I am aware, escaped the attention of all previous inquirers, its consequences were of the utmost moment to the fortunes of Scotland, and are, moreover, full of interest to those who, in their investigations of human affairs, desire to penetrate below the mere surface and symptoms of things.

If we take a general view of those countries where science has been cultivated, we shall find that, wherever the deductive method of inquiry has predominated, knowledge, though often increased and accumulated, has never been widely diffused. On the other hand, we shall find that, when the inductive method has predominated, the diffusion of knowledge has always been considerable, or, at all events, has been beyond comparison greater than when deduction was prevalent. This holds good, not only of different countries, but also of different periods in the same country. It even holds good of different individuals in the same period, and in the same country. If, in any civilized nation, two men, equally gifted, were to propound some new and startling conclusion, and one of these men were to defend his conclusion by reasoning from ideas or general principles, while the other man were to defend his by reasoning from particular and visible facts, there can be no doubt that, supposing all other things the same, the latter man would gain most adherents. His conclusion would be more easily diffused, simply because a direct appeal, in the first instance, to palpable facts, strikes the vulgar with immediate effect; while an appeal to principles is beyond their ken, and as they do not sympathize with it, they are apt to ridicule it. Facts seem to come home to every one, and are undeniable. Principles are not so obvious, and, being often disputed, they have, to those who do not grasp them, an unreal and illusory appearance, which weakens their influence. Hence it is that inductive science, which always gives the first place to facts, is essentially popular, and has on its side those innumerable persons, who will not listen to the more refined and subtle teachings of deductive science. Hence, too, we find historically that the establishment of the modern inductive philosophy, with its varied and attractive experiments, its material appliances, and its constant appeal to the senses, has been intimately connected with the awakening of the public mind, and coincides with that spirit of inquiry, and with that love of liberty, which have been constantly advancing since the sixteenth century. We may assuredly say, that scepticism and democracy are the two leading features of this great scientific movement. The seventeenth century, which ushered in the Baconian philosophy, was remarkable for its insubordinate spirit, especially in the country where that philosophy originated, and where it most flourished. In the next age, it was transplanted into France, and there, too, it worked upon the popular mind, and, was, as I have already pointed out, one of the principal causes of the French Revolution.

If we look still closer into this interesting question, we shall find further corroboration of the view, that the inferences of an inductive philosophy are more likely to be diffused than those of a deductive one. Inductive science rests immediately upon experience, or, at all events, upon experiment, which is merely experience artificially modified. Now, an immense majority of mankind, even in the most advanced countries, are, by the constitution of their minds, incapable of seizing general principles and applying them to daily affairs, without doing serious mischief, either to themselves, or to others. Such an application requires not only great dexterity, but also a knowledge of those disturbing causes which affect the operation of all general theorems. The task, being so difficult to perform, is rarely attempted; and average men, possessed of a tolerably sound judgment, do, with good reason, rely mainly on experience, which is to them a safer and more useful guide than any principle, however accurate and scientific it might be. This begets in their minds a prejudice on behalf of experimental inquiries, and a corresponding dislike of the opposite and more speculative method. And it can, I think, hardly be doubted, that one of the causes of the triumph of the Baconian philosophy, is the growth of the industrious classes, whose business-like and methodical habits are eminently favourable to empirical observations of the uniformities of sequence, since, indeed, on the accuracy of such observations the success of all practical affairs depends. Certainly, we find that the overthrow of the purely deductive scholasticism of the Middle Ages has been everywhere accompanied by the spread of trade; and whoever will carefully study the history of Europe, will discern many traces of a connexion between the two movements, both of which are marked by an increasing respect for material and empirical interests, and a disregard of ideal and speculative pursuits.

The relation between all this and the popular tendency of induction is obvious. For one person who can think, there are at least a hundred persons who can observe. An accurate observer is, no doubt, rare; but an accurate thinker is far rarer. Of this the proofs are too abundant to be disputed. Indeed, no one can mix with his fellow-creatures, without seeing how much more natural it is for them to notice than to reflect; and how extremely unusual it is to meet with any one whose conversation, or whose writings, bear marks of patient and original thought. And, inasmuch as thinkers are more prone to accumulate ideas, while observers are more prone to accumulate facts, the overwhelming predominance of the observing class is a decisive reason why induction, which begins with facts, is always more popular than deduction, which begins with ideas. It is often said, and probably with truth, that all deduction is preceded by induction; so that, in every syllogism, the major premiss, however obvious and necessary it may appear, is merely a generalization of facts, or record of what the senses had already observed. But this opinion, whether true or false, does not affect what I have just stated, because it concerns the origin of our knowledge, and not its subsequent treatment; that is to say, it is a metaphysical opinion, rather than a logical one. For, even supposing that all deduction rests ultimately on induction, it is, nevertheless, certain that there are innumerable cases in which the induction takes place at so early a period of life that we are unconscious of it, and can by no effort recall the process. The axioms of geometry afford a good specimen of this. No one can tell when or how he first believed that the whole is greater than its part, or that things which are equal to the same thing are equal to one another. All these preliminary steps are concealed from us, and the strength and dexterity of deduction are displayed in the subsequent steps by which the major premiss is adjusted, and, as it were, fitted to the minor. This often requires great subtlety of thought, and, in every instance, the external world is put aside, and lost sight of. The process, being ideal, has no concern, either with observations or experiments. The suggestions of the senses are shut out, while the mind passes through a long train of successive syllogisms, in which each conclusion is turned into the premiss of a new argument, until, at length, an inference is deductively obtained, which, to those who merely hear it enunciated, seems to have no connexion with the first premisses, though, in reality, it is the necessary consequence of them.

A method, so recondite, and so hidden from the public gaze, can never command the public sympathy. Unless, therefore, the human mind should undergo some remarkable change in its nature as well as in its resources, the sensuous process of working upwards from particular facts to general principles, will always be more attractive than the ideal process of working downwards from principles to facts. In both cases, there is, no doubt, a line of argument essentially ideal; just as, in both cases, there is an assemblage of facts essentially sensuous. No method is pure, or stands entirely by itself. But, inasmuch as, in induction, the facts are more prominent than the ideas, while, in deduction, the ideas are more prominent than the facts, it is evident that conclusions arrived at by the former plan will, as a general rule, obtain a wider assent than conclusions arrived at by the latter plan. Obtaining a wider assent, they will produce more decisive results, and will be more likely to shape the national character and influence the course of national affairs.

The only exception to this, is theology. There, the inductive method, as I have already observed, is inapplicable, and nothing remains but deduction, which is quite sufficient for the purposes of the theologian. For, he has a peculiar resource which supplies him with general principles, from which he can argue; and the possession of this resource forms the fundamental difference between him and the man of science. Science is the result of inquiry; theology is the result of faith. In the one, the spirit of doubt; in the other, the spirit of belief. In science, originality is the parent of discovery, and is, therefore, a merit; in theology, it is the parent of heresy, and is, therefore, a crime. Every system of religion the world has yet seen, recognizes faith as an indispensable duty; but to every system of science it is a hindrance, instead of a duty, inasmuch as it discourages those inquisitive and innovating habits on which all intellectual progress depends. The theologian, thus turning credulity into an honour, and valuing men in proportion as they are simple-minded and easy of belief, has little need to trouble himself with facts, which, indeed, he sets at open defiance, in his eagerness to narrate portentous, and often miraculous, events. To the inductive philosopher such a license is forbidden. He is obliged to ground his inferences on facts which no one disputes, or which, at all events, any one can either verify for himself, or see verified by others. And if he does not adopt this course, his inferences, be they ever so true, will have the greatest difficulty in working themselves into the popular mind, because they will savour of a subtlety and refinement of thought, which, more than any thing else, predisposes common understandings to reject the conclusions at which philosophers arrive.