scientific observation, indeed, (as has been remarked,) a certain skill and practice beyond that possessed by ordinary persons is requisite; but when this has been acquired, no extraordinary qualifications are needed. The qualities which render a person a competent authority in matters of opinion are of rarer occurrence. Many of them are, separately, not frequent; and the combination of them in the same individual is necessarily more uncommon; a person whose testimony to a fact would be unimpeachable, might be utterly devoid of authority as a guide of opinion. Besides, a person who is a credible witness for one matter of fact, is equally credible for all others which may fall under his observation; whereas (as we shall see hereafter) no man is equally competent as a guide of opinion in all subjects, and, in general, the authority of each person is, as compared with the sum total of human affairs, confined within a narrow range. § 6. The first qualification is, that a person should have devoted much study and thought to the subject-matter, if it be merely speculative; and that if it be practical, he should also have had adequate experience respecting it. Secondly, his mental powers must be equal to the task of comprehending the subject, and they must be of the sort fitted to it. Thirdly, he ought to be exempt, as far as possible, from personal interest in the matter; or, if he be not exempt, his honesty and integrity ought to be such as to afford a reasonable security against the perversion of his opinions by views of his individual advantage. § 7. The three qualifications just stated may now be examined with somewhat more of detail. I. If the subject be extensive-if it be one of the great departments into which human knowledge is divided—a careful study of it, continued for several years, or even for a large part of a life, combined with frequent meditation, and, if possible, personal observation, is requisite in order to enable a man to understand it inflexible legal rules on this subject. Thus, in some cases, the concurrent testimony of two witnesses to the same fact has been rendered necessary; the evidence of persons standing in a near degree of affinity to one of the parties, or having a pecuniary interest in the cause, has been excluded. But experience has shown that, however useful maxims of this sort may be as guiding the discretion of the court in weighing the credibility of witnesses, the entire exclusion of evidence on such grounds obstructs the administration of justice. thoroughly and to treat it with a sound and comprehensive judgment. All the great luminaries of science, whether mathematical, physical, metaphysical, ethical, or political, have fulfilled this condition. None of them would have acquired the authority, which their opinions, as such, independently of their reasons, possess, if they had not applied all their mental faculties during a large part of their lives to the subjects on which they wrote. The extent and complexity of the more important departments of science are so great, that no person, however penetrating his intellect, can master any of them without years of patient study and reflection. Each science involves the consideration of a vast number of particular facts and phenomena, which the mind can only apprehend by passing over them in succession, and examining them separately. In order that the foundations of a science should be duly laid, and its superstructure raised and enlarged, this process must be frequently repeated, either for parts or for the whole. The institution of proper experiments, and the verification of the experiments of others, are likewise often slow and delicate processes; though they are powerful aids, of which the physical sciences enjoy the exclusive use. In the moral sciences, new facts, and combinations of facts, are constantly presenting themselves; but the observation of these, and their verification, together with the subsequent elimination of such concomitant phenomena as do not properly enter into the problem, require patient and sustained attention on the part of the scientific inquirer. If the subject be one, not of mere speculation, but of knowledge to be applied in practice, a further condition is requisite. Not only must the guide of opinion have studied the subject theoretically, not only must he, if a lawyer, for example, have rendered himself familiar with the system of law, if a physician, have learned the science of medicine, but he must have practised it; he must have acquired the ability of applying this theoretical knowledge to actual cases, which experience alone can confer. In order that a person should be eminent in a learned profession, it is necessary that he should combine a knowledge of its principles, with that judgment, tact, dexterity, and promptitude of applying them to actual cases, which are derived from habits of practice. The like may be said of persons conversant in the constructive arts, as architects and engineers, of the military and naval services, of agriculturists, gardeners, manufacturers of different sorts, &c. In order that they may give sound advice with respect to any practical question belonging to their own department, it is necessary that they should combine actual experience with abstract knowledge. In some cases, that experience implies even manual skill, which can only be acquired by practice. For example, a surgeon would not be a competent judge on a question of practical surgery, unless his judgment were assisted and corrected by actual manipulation of his instruments. In like manner, a person cannot be a competent judge of works of art, such as statues, pictures, coins, and engravings; or of articles of trade, as horses, wines, plate, &c., without practical observation and experience. In these cases, a certain training of the sight is necessary, analogous to the training of the hands and limbs in a mechanical employment or trade requiring bodily dexterity. The senses can be educated so as to discern marks which are invisible to the unpractised observer. There are slight indicia, which guide the judgment of the man of experience, and which no merely theoretical teaching can enable a person to detect. The occupation of the hunter or the fisher, again, requires an actual observation of the habits of the animals to be caught, in order to guide the judgment. In this minor species of warfare, a practical knowledge of the probable movements of the enemy, and of the best means of diverting or lulling his attention, of falling on him unawares, and of overwhelming him with a superior force, is as necessary, for success, as in naval or military tactics. A similar practical knowledge is a necessary qualification for those who are entrusted with the care and training of animals, as shepherds, horsebreakers, &c. For purposes of scientific observation, a training of the senses is likewise necessary. A scientific observer must be not only familiar with the terminology of his science, and be able to apply its technical terms readily to the proper objects, but he ought likewise to have acquired that delicacy, rapidity, and correctness of discernment which the habit of observation, combined with knowledge, can alone confer.1 § 8. II. In order that a person should originate sound opinions on a subject of speculation and science, he ought not merely to have devoted a long time to the assiduous study and contemplation of the subject, but his mental powers ought to be superior to the average. His mind ought to be more wide-ranging and far See this subject well explained by Dr. Whewell, Philos. of Ind. Sciences, B. XII. c. ii. §§ 26, 27. seeing. He ought to be able to take a comprehensive or synoptical view of an extensive subject, and also to trace the remote consequences of an insulated principle. That high degree of intellectual power, which we call genius, and which the ancients attributed to the inspiration of the gods, is in itself inexplicable, and can only be judged by its effects. But some ray of that light is requisite in order to enable a person to be classed among the original teachers and guides of mankind. Moreover, a man ought, in order to acquire authority as an inventor in matters of speculation, not merely to be superior in point of intellect to the average of men, but he ought also to possess the peculiar mental qualities which the subject demands. For example, in order to be pre-eminent in mathematical science, it is not sufficient for him to possess the faculties which fit him for the observation of outward nature. Nor will he be enabled to excel in speculation by powers which fit him exclusively for the solution of practical questions and the actual business of life. And, indeed, a person may be qualified to shine in one department or branch of a science, who has no aptitude for other portions of the same subject. As Pope expresses it in his Essay on Criticism: One science only will one genius fit, Such eminence of intellectual power as enables a person to discover and establish important principles in the physical and moral world is of rare occurrence, and it is not necessary to qualify one who has studied a subject, to comprehend the views and reasonings of others, and to form a competent judgment upon their soundness. Having fulfilled the previous condition of study and reflection, a person of fair capacity may accredit speculative opinions, and serve as a safe guide to others in his capacity of a discriminating judge, although he may not be able to lead them on as an inventor. Such a person may be able to separate truth from error among existing opinions, though he might not be able to discover the truth, if unknown, and proclaim it to the world. Or he may be able to arrange and systematise existing knowledge; to expound it with perspicuity; to improve subordinate parts of a science, and to correct errors in accessories, without attempting to grapple with fundamental questions. For practice again, eminent intellectual superiority is not essential; and experience alone, combined with study, if there be a fair amount of ability, will suffice. A large number of the persons practising the learned professions are justly considered as safe and prudent guides in the subjects with which they are conversant, although their general mental powers may not be greatly superior to those of the majority of the educated classes in their own country. The same may be said of sailors, architects, engineers, and other classes of persons having a special training and aptitude for judgment with respect to their own subject. In each profession, however, eminence can only be attained by remarkable ability, fitting its possessor for the peculiar department of practice which he has chosen. As opposed to unprofessional men generally, the judgment of any professional man, whatever may be his natural talents, is of weight; but among men of the same profession, the opinion of those who are peculiarly fitted by nature, in addition to the advantages of experience, is that which is most highly valued. It is not sufficient for success in practice that a person should have mastered a science, and should comprehend its theory; or even that he should attempt to apply its rules to actual cases. A certain dexterity and aptitude for the application of theory to practice is requisite. Not every man who has studied a system of rhetoric, and made some speeches, can become a successful orator; not every man who has studied law and medicine, and attended to the application of his knowledge, can be a good practising lawyer or physician. But, on the other hand, mere practical aptitude can never enable a man to practise a profession with success, unless he is well grounded in its principles. By dexterity, and a sort of legerdemain, he may conceal his defects, and divert attention from his weak points; but he never can be permanently successful. The great practitioners, whether in war, politics, law, medicine, the fine arts, or any other department of applied knowledge, have all combined a careful study of principles, and patient observation, with practical genius. Practicians, who are ignorant of the principles of their profession or art, instead of acting according to general maxims, of, which they understand the grounds, steer their course (and sometimes merely grope their way) by means of analogies; that is to say, they argue from one case or instance to another, without being able to bring both of them under one general rule or premise. The practical man, who |