Kant has said that the business of philosophy is to answer three questions: What can I know? What ought I to do? and For what may I hope? But it is pretty plain that these three resolve themselves, in the long run, into the first. For rational expectation and moral action are alike based upon beliefs; and a belief is void of justification, unless its subject-matter lies within the boundaries of possible knowledge, and unless its evidence satisfies the conditions which experience imposes as the guarantee of credibility.
Fundamentally, then, philosophy is the answer to the question, What can I know? and it is by applying itself to this problem, that philosophy is properly distinguished as a special department of scientific research. What is commonly called science, whether mathematical, physical, or biological, consists of the answers which mankind have been able to give to the inquiry, What do I know? They furnish us with the results of the mental operations which constitute thinking; while philosophy, in the stricter sense of the term, inquires into the foundation of the first principles which those operations assume or imply.
But though, by reason of the special purpose of philosophy, its distinctness from other branches of scientific investigation may be properly vindicated, it is easy to see that, from the nature of its subject-matter, it is intimately and, indeed, inseparably connected with one branch of science. For it is obviously impossible to answer the question, What can we know? unless, in the first place, there is a clear understanding as to what is meant by knowledge; and, having settled this point, the next step is to inquire how we come by that which we allow to be knowledge; for, upon the reply, turns the answer to the further question, whether, from the nature of the case, there are limits to the knowable or not. While, finally, inasmuch as What can I know? not only refers to knowledge of the past or of the present, but to the confident expectation which we call knowledge of the future; it is necessary to ask, further, what justification can be alleged for trusting to the guidance of our expectations in practical conduct.
It surely needs no argumentation to show, that the first problem cannot be approached without the examination of the contents of the mind; and the determination of how much of these contents may be called knowledge. Nor can the second problem be dealt with in any other fashion; for it is only by the observation of the growth of knowledge that we can rationally hope to discover how knowledge grows. But the solution of the third problem simply involves the discussion of the data obtained by the investigation of the foregoing two.
Thus, in order to answer three out of the four subordinate questions into which What can I know? breaks up, we must have recourse to that investigation of mental phenomena, the results of which are embodied in the science of psychology.
Psychology is a part of the science of life or biology, which differs from the other branches of that science, merely in so far as it deals with the psychical, instead of the physical, phenomena of life.
As there is an anatomy of the body, so there is an anatomy of the mind; the psychologist dissects mental phenomena into elementary states of consciousness, as the anatomist resolves limbs into tissues, and tissues into cells. The one traces the development of complex organs from simple rudiments; the other follows the building up of complex conceptions out of simpler constituents of thought. As the physiologist inquires into the way in which the so-called “functions” of the body are performed, so the psychologist studies the so-called “faculties” of the mind. Even a cursory attention to the ways and works of the lower animals suggests a comparative anatomy and physiology of the mind; and the doctrine of evolution presses for application as much in the one field as in the other.
But there is more than a parallel, there is a close and intimate connexion between psychology and physiology. No one doubts that, at any rate, some mental states are dependent for their existence on the performance of the functions of particular bodily organs. There is no seeing without eyes, and no hearing without ears. If the origin of the contents of the mind is truly a philosophical problem, then the philosopher who attempts to deal with that problem, without acquainting himself with the physiology of sensation, has no more intelligent conception of his business than the physiologist, who thinks he can discuss locomotion, without an acquaintance with the principles of mechanics; or respiration, without some tincture of chemistry.
On whatever ground we term physiology, science, psychology is entitled to the same appellation; and the method of investigation which elucidates the true relations of the one set of phenomena will discover those of the other. Hence, as philosophy is, in great measure, the exponent of the logical consequences of certain data established by psychology; and as psychology itself differs from physical science only in the nature of its subject-matter, and not in its method of investigation, it would seem to be an obvious conclusion, that philosophers are likely to be successful in their inquiries, in proportion as they are familiar with the application of scientific method to less abstruse subjects; just as it seems to require no elaborate demonstration, that an astronomer, who wishes to comprehend the solar system, would do well to acquire a preliminary acquaintance with the elements of physics. And it is accordant with this presumption, that the men who have made the most important positive additions to philosophy, such as Descartes, Spinoza, and Kant, not to mention more recent examples, have been deeply imbued with the spirit of physical science; and, in some cases, such as those of Descartes and Kant, have been largely acquainted with its details. On the other hand, the founder of Positivism no less admirably illustrates the connexion of scientific incapacity with philosophical incompetence. In truth, the laboratory is the fore-court of the temple of philosophy; and whoso has not offered sacrifices and undergone purification there, has little chance of admission into the sanctuary.
Obvious as these considerations may appear to be, it would be wrong to ignore the fact that their force is by no means universally admitted. On the contrary, the necessity for a proper psychological and physiological training to the student of philosophy is denied, on the one hand, by the “pure metaphysicians,” who attempt to base the theory of knowing upon supposed necessary and universal truths, and assert that scientific observation is impossible unless such truths are already known or implied: which, to those who are not “pure metaphysicians,” seems very much as if one should say that the fall of a stone cannot be observed, unless the law of gravitation is already in the mind of the observer.
On the other hand, the Positivists, so far as they accept the teachings of their master, roundly assert, at any rate in words, that observation of the mind is a thing inherently impossible in itself, and that psychology is a chimera — a phantasm generated by the fermentation of the dregs of theology. Nevertheless, if M. Comte had been asked what he meant by “physiologic cérebrale,” except that which other people call “psychology;” and how he knew anything about the functions of the brain, except by that very “observation intérieure,” which he declares to be an absurdity — it seems probable that he would have found it hard to escape the admission, that, in vilipending psychology, he had been propounding solemn nonsense.
It is assuredly one of Hume’s greatest merits that he clearly recognised the fact that philosophy is based upon psychology; and that the inquiry into the contents and the operations of the mind must be conducted upon the same principles as a physical investigation, if what he calls the “moral philosopher” would attain results of as firm and definite a character as those which reward the “natural philosopher.”14 The title of his first work, a “Treatise of Human Nature, being an Attempt to introduce the Experimental method of Reasoning into Moral Subjects,” sufficiently indicates the point of view from which Hume regarded philosophical problems; and he tells us in the preface, that his object has been to promote the construction of a “science of man.”
“’Tis evident that all the sciences have a relation, greater or less, to human nature; and that, however wide any of them may seem to run from it, they still return back by one passage or another. Even Mathematics, Natural Philosophy, and Natural Religion are in some measure dependent on the science of MAN; since they lie under the cognizance of men, and are judged of by their powers and qualities. ’Tis impossible to tell what changes and improvements we might make in these sciences were we thoroughly acquainted with the extent and force of human understanding, and could explain the nature of the ideas we employ and of the operations we perform in our reasonings. . . . To me it seems evident that the essence of mind being equally unknown to us with that of external bodies, it must be equally impossible to form any notion of its powers and qualities otherwise than from careful and exact experiments, and the observation of those particular effects which result from its different circumstances and situations. And though we must endeavour to render all our principles as universal as possible, by tracing up our experiments to the utmost, and explaining all effects from the simplest and fewest causes, ’tis still certain we cannot go beyond experience; and any hypothesis that pretends to discover the ultimate original qualities of human nature, ought at first to be rejected as presumptuous and chimerical. . . .
“But if this impossibility of explaining ultimate principles should be esteemed a defect in the science of man, I will venture to affirm, that it is a defect common to it with all the sciences, and all the arts, in which we can employ ourselves, whether they be such as are cultivated in the schools of the philosophers, or practised in the shops of the meanest artisans. None of them can go beyond experience, or establish any principles which are not founded on that authority. Moral philosophy has, indeed, this peculiar disadvantage, which is not found in natural, that in collecting its experiments, it cannot make them purposely, with premeditation, and after such a manner as to satisfy itself concerning every particular difficulty which may arise. When I am at a loss to know the effects of one body upon another in any situation, I need only put them in that situation, and observe what results from it. But should I endeavour to clear up in the same manner any15 doubt in moral philosophy, by placing myself in the same case with that which I consider, ’tis evident this reflection and premeditation would so disturb the operation of my natural principles, as must render it impossible to form any just conclusion from the phenomenon. We must, therefore, glean up our experiments in this science from a cautious observation of human life, and take them as they appear in the common course of the world, by men’s behaviour in company, in affairs, and in their pleasures. Where experiments of this kind are judiciously collected and compared, we may hope to establish on them a science which will not be inferior in certainty, and will be much superior in utility, to any other of human comprehension.”—(I. pp. 7–11.)
All science starts with hypotheses — in other words, with assumptions that are unproved, while they may be, and often are, erroneous; but which are better than nothing to the seeker after order in the maze of phenomena. And the historical progress of every science depends on the criticism of hypotheses — on the gradual stripping off, that is, of their untrue or superfluous parts — until there remains only that exact verbal expression of as much as we know of the fact, and no more, which constitutes a perfect scientific theory.
Philosophy has followed the same course as other branches of scientific investigation. The memorable service rendered to the cause of sound thinking by Descartes consisted in this: that he laid the foundation of modern philosophical criticism by his inquiry into the nature of certainty. It is a clear result of the investigation started by Descartes, that there is one thing of which no doubt can be entertained, for he who should pretend to doubt it would thereby prove its existence; and that is the momentary consciousness we call a present thought or feeling; that is safe, even if all other kinds of certainty are merely more or less probable inferences. Berkeley and Locke, each in his way, applied philosophical criticism in other directions; but they always, at any rate professedly, followed the Cartesian maxim of admitting no propositions to be true but such as are clear, distinct, and evident, even while their arguments stripped off many a layer of hypothetical assumption which their great predecessor had left untouched. No one has more clearly stated the aims of the critical philosopher than Locke, in a passage of the famous Essay concerning Human Understanding, which, perhaps, I ought to assume to be well known to all English readers, but which so probably is unknown to this full-crammed and much examined generation that I venture to cite it:
“If by this inquiry into the nature of the understanding I can discover the powers thereof, how far they reach, to what things they are in any degree proportionate, and where they fail us, I suppose it may be of use to prevail with the busy mind of man to be more cautious in meddling with things exceeding its comprehension: to stop when it is at the utmost extent of its tether; and to sit down in quiet ignorance of those things which, upon examination, are proved to be beyond the reach of our capacities. We should not then, perhaps, be so forward, out of an affectation of universal knowledge, to raise questions and perplex ourselves and others with disputes about things to which our understandings are not suited, and of which we cannot frame in our minds any clear and distinct perception, or whereof (as it has, perhaps, too often happened) we have not any notion at all. . . . Men may find matter sufficient to busy their heads and employ their hands with variety, delight, and satisfaction, if they will not boldly quarrel with their own constitution and throw away the blessings their hands are filled with because they are not big enough to grasp everything. We shall not have much reason to complain of the narrowness of our minds, if we will but employ them about what may be of use to us: for of that they are very capable: and it will be an unpardonable, as well as a childish peevishness, if we under-value the advantages of our knowledge, and neglect to improve it to the ends for which it was given us, because there are some things that are set out of the reach of it. It will be no excuse to an idle and untoward servant who would not attend to his business by candlelight, to plead that he had not broad sunshine. The candle that is set up in us shines bright enough for all our purposes. . . . Our business here is not to know all things, but those which concern our conduct.”16
Hume develops the same fundamental conception in a somewhat different way, and with a more definite indication of the practical benefits which may be expected from a critical philosophy. The first and second parts of the twelfth section of the Inquiry are devoted to a condemnation of excessive scepticism, or Pyrrhonism, with which Hume couples a caricature of the Cartesian doubt; but, in the third part, a certain “mitigated scepticism” is recommended and adopted, under the title of “academical philosophy.” After pointing out that a knowledge of the infirmities of the human understanding, even in its most perfect state, and when most accurate and cautious in its determinations, is the best check upon the tendency to dogmatism, Hume continues:—
“Another species of mitigated scepticism, which may be of advantage to mankind, and which maybe the natural result of the PYRRHONIAN doubts and scruples, is the limitation of our inquiries to such subjects as are best adapted to the narrow capacity of human understanding. The imagination of man is naturally sublime, delighted with whatever is remote and extraordinary, and running, without control, into the most distant parts of space and time in order to avoid the objects which custom has rendered too familiar to it. A correct judgment observes a contrary method, and, avoiding all distant and high inquiries, confines itself to common life, and to such subjects as fall under daily practice and experience; leaving the more sublime topics to the embellishment of poets and orators, or to the arts of priests and politicians. To bring us to so salutary a determination, nothing can be more serviceable than to be once thoroughly convinced of the force of the PYRRHONIAN doubt, and of the impossibility that anything but the strong power of natural instinct could free us from it. Those who have a propensity to philosophy will still continue their researches; because they reflect, that, besides the immediate pleasure attending such an occupation, philosophical decisions are nothing but the reflections of common life, methodised and corrected. But they will never be tempted to go beyond common life, so long as they consider the imperfection of those faculties which they employ, their narrow reach, and their inaccurate operations. While we cannot give a satisfactory reason why we believe, after a thousand experiments, that a stone will fall or fire burn; can we ever satisfy ourselves concerning any determination which we may form with regard to the origin of worlds and the situation of nature from and to eternity?”—(IV. pp. 189 — 90.)
But further, it is the business of criticism not only to keep watch over the vagaries of philosophy, but to do the duty of police in the whole world of thought. Wherever it espies sophistry or superstition they are to be bidden to stand; nay, they are to be followed to their very dens and there apprehended and exterminated, as Othello smothered Desdemona, “else she’ll betray more men.”
Hume warms into eloquence as he sets forth the labours meet for the strength and the courage of the Hercules of “mitigated scepticism.”
“Here, indeed, lies the justest and most plausible objection against a considerable part of metaphysics, that they are not properly a science, but arise either from the fruitless efforts of human vanity, which would penetrate into subjects utterly inaccessible to the understanding, or from the craft of popular superstitions, which, being unable to defend themselves on fair ground, raise these entangling brambles to cover and protect their weakness. Chased from the open country, these robbers fly into the forest, and lie in wait to break in upon every unguarded avenue of the mind and overwhelm it with religious fears and prejudices. The stoutest antagonist, if he remits his watch a moment, is oppressed; and many, through cowardice and folly, open the gates to the enemies, and willingly receive them with reverence and submission as their legal sovereigns.
“But is this a sufficient reason why philosophers should desist from such researches and leave superstition still in possession of her retreat? Is it not proper to draw an opposite conclusion, and perceive the necessity of carrying the war into the most secret recesses of the enemy? . . . The only method of freeing learning at once from these abstruse questions, is to inquire seriously into the nature of human understanding, and show, from an exact analysis of its powers and capacity, that it is by no means fitted for such remote and abstruse subjects. We must submit to this fatigue, in order to live at ease ever after; and must cultivate true metaphysics with some care, in order to destroy the false and adulterated.”—(IV. pp. 10, 11.)
Near a century and a half has elapsed since these brave words were shaped by David Hume’s pen; and the business of carrying the war into the enemy’s camp has gone on but slowly. Like other campaigns, it long languished for want of a good base of operations. But since physical science, in the course of the last fifty years, has brought to the front an inexhaustible supply of heavy artillery of a new pattern, warranted to drive solid bolts of fact through the thickest skulls, things are looking better; though hardly more than the first faint flutterings of the dawn of the happy day, when superstition and false metaphysics shall be no more and reasonable folks may “live at ease,” are as yet discernible by the enfants perdus of the outposts.
If, in thus conceiving the object and the limitations of philosophy, Hume shows himself the spiritual child and continuator of the work of Locke, he appears no less plainly as the parent of Kant and as the protagonist of that more modern way of thinking, which has been called “agnosticism,” from its profession of an incapacity to discover the indispensable conditions of either positive or negative knowledge, in many propositions, respecting which, not only the vulgar, but philosophers of the more sanguine sort, revel in the luxury of unqualified assurance.
The aim of the Kritik der reinen Vernunft is essentially the same as that of the Treatise of Human Nature, by which indeed Kant was led to develop that “critical philosophy” with which his name and fame are indissolubly bound up: and, if the details of Kant’s criticism differ from those of Hume, they coincide with them in their main result, which is the limitation of all knowledge of reality to the world of phenomena revealed to us by experience.
The philosopher of Königsberg epitomises the philosopher of Ninewells when he thus sums up the uses of philosophy:—
“The greatest and perhaps the sole use of all philosophy of pure reason is, after all, merely negative, since it serves, not as an organon for the enlargement [of knowledge], but as a discipline for its delimitation; and instead of discovering truth, has only the modest merit of preventing error.”17
14 In a letter to Hutcheson (September 17th, 1739) Hume remarks:—“There are different ways of examining the mind as well as the body. One may consider it either as an anatomist or as a painter: either to discover its most secret springs and principles, or to describe the grace and beauty of its actions;” and he proceeds to justify his own mode of looking at the moral sentiments from the anatomist’s point of view.
15 The manner in which Hume constantly refers to the results of the observation of the contents and the processes of his own mind clearly shows that he has here inadvertently overstated the case.
16 Locke, An Essay concerning Human Understanding, Book I, chap. i, §§ 4, 5, 6.
17 Kritik der reinen Vernunft. Ed. Hartenstein, p. 256.
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