No one has yet succeeded in inventing a philosophy at once credible and self-consistent. Locke aimed at credibility, and achieved it at the expense of consistency. Most of the great philosophers have done the opposite. A philosophy which is not self-consistent cannot be wholly true, but a philosophy which is self-consistent can very well be wholly false. The most fruitful philosophies have contained glaring inconsistencies, but for that very reason have been partially true. There is no reason to suppose that a self-consistent system contains more truth than one which, like Locke's, is obviously more or less wrong.
Not only Locke's valid opinions, but even his errors, were useful in practice. Take, for example, his doctrine as to primary and secondary qualities. The primary qualities are defined as those that are inseparable from body, and are enumerated as solidity, extension, figure, motion or rest, and number. The secondary qualities are all the rest: colour, sounds, smells, etc. The primary qualities, he maintains, are actually in bodies; the secondary qualities, on the contrary, are only in the percipient. Without the eye, there would be no colours; without the ear, no sounds, and so on. For Locke's view as to secondary qualities there are good grounds--jaundice, blue spectacles, etc. But Berkeley pointed out that the same arguments apply to primary qualities. Ever since Berkeley, Locke's dualism on this point has been philosophically out of date. Nevertheless, it dominated practical physics until the rise of quantum theory in our own day. Not only was it assumed, explicitly or tacitly, by physicists, but it proved fruitful as a source of many very important discoveries. The theory that the physical world consists only of matter in motion was the basis of the accepted theories of sound, heat, light, and electricity. Pragmatically, the theory was useful, however mistaken it may have been theoretically. This is typical of Locke's doctrines.
Locke's philosophy, as it appears in the Essay, has throughout certain merits and certain demerits. Both alike were useful: the demerits are such only from a theoretical standpoint. He is always sensible, and always willing to sacrifice logic rather than become paradoxical. He enunciates general principles which, as the reader can hardly fail to perceive, are capable of leading to strange consequences; but whenever the strange consequences seem about to appear, Locke blandly refrains from drawing them. To a logician this is irritating; to a practical man, it is a proof of sound judgement. Since the world is what it is, it is clear that valid reasoning from sound principles cannot lead to error; but a principle may be so nearly true as to deserve theoretical respect, and yet may lead to practical consequences which we feel to be absurd. There is therefore a justification for common sense in philosophy, but only as showing that our theoretical principles cannot be quite correct so long as their consequences are condemned by an appeal to common sense which we feel to be irresistible. The theorist may retort that common sense is no more infallible than logic. But this retort, though made by Berkeley and Hume, would have been wholly foreign to Locke's intellectual temper.
A characteristic of Locke, which descended from him to the whole Liberal movement, is lack of dogmatism. Some few certainties he takes over from his predecessors: our own existence, the existence of God, and the truth of mathematics. But wherever his doctrines differ from those of his forerunners, they are to the effect that truth is hard to ascertain, and that a rational man will hold his opinions with some measure of doubt. This temper of mind is obviously connected with religious toleration, with the success of parliamentary democracy, with laissez-faire, and with the whole system of liberal maxims. Although he is a deeply religious man, a devout believer in Christianity who accepts revelation as a source of knowledge, he nevertheless hedges round professed revelations with rational safeguards. On one occasion he says: "The bare testimony of revelation is the highest certainty," but on another he says: "Revelation must be judged by reason." Thus in the end reason remains supreme.
His chapter "Of Enthusiasm" is instructive in this connection. "Enthusiasm" had not then the same meaning as it has now; it meant the belief in a personal revelation to a religious leader or to his followers. It was a characteristic of the sects that had been defeated at the Restoration. When there is a multiplicity of such personal revelations, all inconsistent with each other, truth, or what passes as such, becomes purely personal, and loses its social character. Love of truth, which Locke considers essential, is a very different thing from love of some particular doctrine which is proclaimed as the truth. One unerring mark of love of truth, he says, is "not entertaining any proposition with greater assurance than the proofs it is built upon will warrant." Forwardness to dictate, he says, shows failure of love of truth. "Enthusiasm, laying by reason, would set up revelation without it; whereby in effect it takes away both reason and revelation, and substitutes in the room of it the ungrounded fancies of a man's own brain." Men who suffer from melancholy or conceit are likely to have "persuasions of immediate intercourse with the Deity." Hence odd actions and opinions acquire Divine sanction, which flatters "men's laziness, ignorance, and vanity." He concludes the chapter with the maxim already quoted, that "revelation must be judged of by reason."
What Locke means by "reason" is to be gathered from his whole book. There is, it is true, a chapter called "Of Reason," but this is mainly concerned to prove that reason does not consist of syllogistic reasoning, and is summed up in the sentence: "God has not been so sparing to men to make them barely two-legged creatures, and left it to Aristotle to make them rational." Reason, as Locke uses the term, consists of two parts: first, an inquiry as to what things we know with certainty; second, an investigation of propositions which it is wise to accept in practice, although they have only probability and not certainty in their favour. "The grounds of probability," he says, "are two: conformity with our own experience, or the testimony of other's experience." The King of Siam, he remarks, ceased to believe what Europeans told him when they mentioned ice.
In his chapter "Of Degrees of Assent" he says that the degree of assent we give to any proposition should depend upon the grounds of probability in its favour. After pointing out that we must often act upon probabilities that fall short of certainty, he says that the right use of this consideration "is mutual charity and forbearance. Since therefore it is unavoidable to the greatest part of men, if not all, to have several opinions, without certain and indubitable proofs of their truth; and it carries too great an imputation of ignorance, lightness, or folly, for men to quit and renounce their former tenets presently upon the offer of an argument which they cannot immediately answer and show the insufficiency of; it would, methinks, become all men to maintain peace and the common offices of humanity and friendship in the diversity of opinions, since we cannot reasonably expect that any one should readily and obsequiously quit his own opinion, and embrace ours with a blind resignation to an authority which the understanding of man acknowledges not. For, however it may often mistake, it can own no other guide but reason, nor blindly submit to the will and dictates of another. If he you would bring over to your sentiments be one that examines before he assents, you must give him leave at his leisure to go over the account again, and, recalling what is out of his mind, examine the particulars, to see on which side the advantage lies; and if he will not think over arguments of weight enough to engage him anew in so much pains, it is but what we do often ourselves in the like case; and we should take it amiss if others should prescribe to us what points we should study: and if he be one who wishes to take his opinions upon trust, how can we imagine that he should renounce those tenets which time and custom have so settled in his mind that he thinks them self-evident, and of an unquestionable certainty; or which he takes to be impressions he has received from God himself, or from men sent by him? How can we expect, I say, that opinions thus settled should be given up to the arguments or authority of a stranger or adversary? especially if there be any suspicion of interest or design, as there never fails to be where men find themselves ill-treated. We should do well to commiserate our mutual ignorance, and endeavour to remove it in all the gentle and fair ways of information, and not instantly treat others ill as obstinate and perverse because they will not renounce their own and receive our opinions, or at least those we would force upon them, when it is more than probable that we are no less obstinate in not embracing some of theirs. For where is the man that has uncontestable evidence of the truth of all that he holds, or of the falsehood of all he condemns; or can say, that he has examined to the bottom all his own or other men's opinions? The necessity of believing without knowledge, nay, often upon very slight grounds, in this fleeting state of action and blindness we are in, should make us more busy and careful to inform ourselves than to restrain others. . . . There is reason to think, that if men were better instructed themselves, they would be less imposing on others."
I have dealt hitherto only with the latest chapters of the Essay, where Locke is drawing the moral from his earlier theoretical investigation of the nature and limitations of human knowledge. It is time now to examine what he has to say on this more purely philosophical subject.
Locke is, as a rule, contemptuous of metaphysics. A propos of some speculation of Leibniz's, he writes to a friend: "You and I have had enough of this kind of fiddling." The conception of substance, which was dominant in the metaphysics of his time, he considers vague and not useful, but he does not venture to reject it wholly. He allows the validity of metaphysical arguments for the existence of God, but he does not dwell on them, and seems somewhat uncomfortable about them. Whenever he is expressing new ideas, and not merely repeating what is traditional, he thinks in terms of concrete detail rather than of large abstractions. His philosophy is piecemeal, like scientific work, not statuesque and all of a piece, like the great Continental systems of the seventeenth century.
Locke may be regarded as the founder of empiricism, which is the doctrine that all our knowledge (with the possible exception of logic and mathematics) is derived from experience. Accordingly the first book of the Essay is concerned in arguing, as against Plato, Descartes, and the scholastics, that there are no innate ideas or principles. In the second book he sets to work to show, in detail, how experience gives rise to various kinds of ideas. Having rejected innate ideas, he says:
"Let us then suppose the mind to be, as we say, white paper, void of all characters, without any ideas; how comes it to be furnished? Whence comes it by that vast store, which the busy and boundless fancy of man has painted on it with an almost endless variety? Whence has it all the materials of reason and knowledge? To this I answer in one word, from experience: in that all our knowledge is founded, and from that it ultimately derives itself" (Book II, Ch. I, Sec. 2).
Our ideas are derived from two sources, (a) sensation, and (b) perception of the operation of our own mind, which may be called "internal sense." Since we can only think by means of ideas, and since all ideas come from experience, it is evident that none of our knowledge can antedate experience.
Perception, he says, is "the first step and degree towards knowledge, and the inlet of all the materials of it." This may seem, to a modern, almost a truism, since it has become part of educated common sense, at least in English-speaking countries. But in his day the mind was supposed to know all sorts of things a priori, and the complete dependence of knowledge upon perception, which he proclaimed, was a new and revolutionary doctrine. Plato, in the Theaetetus, had set to work to refute the identification of knowledge with perception, and from his time onwards almost all philosophers, down to and including Descartes and Leibniz, had taught that much of our most valuable knowledge is not derived from experience. Locke's thorough-going empiricism was therefore a bold innovation.
The third book of the Essay deals with words, and is concerned, in the main, to show that what metaphysicians present as knowledge about the world is purely verbal. Chapter III, "Of General Terms," takes up an extreme nominalist position on the subject of universals. All things that exist are particulars, but we can frame general ideas, such as "man," that are applicable to many particulars, and to these general ideas we can give names. Their generality consists solely in the fact that they are, or may be, applicable to a variety of particular things; in their own being, as ideas in our minds, they are just as particular as everything else that exists.
Chapter VI of Book III, "Of the Names of Substances," is concerned to refute the scholastic doctrine of essence. Things may have a real essence, which will consist of their physical constitution, but this is in the main unknown to us, and is not the "essence" of which scholastics speak. Essence, as we can know it, is purely verbal; it consists merely in the definition of a general term. To argue, for instance, as to whether the essence of body is only extension, or is extension plus solidity, is to argue about words: we may define the word "body" either way, and no harm can result so long as we adhere to our definition. Distinct species are not a fact of nature, but of language; they are "distinct complex ideas with distinct names annexed to them." There are, it is true, differing things in nature, but the differences proceed by continuous gradations: "the boundaries of the species, whereby men sort them, are made by men." He proceeds to give instances of monstrosities, concerning which it was doubtful whether they were men or not. This point of view was not generally accepted until Darwin persuaded men to adopt the theory of evolution by gradual changes. Only those who have allowed themselves to be afflicted by the scholastics will realize how much metaphysical lumber it sweeps away.
Empiricism and idealism alike are faced with a problem to which, so far, philosophy has found no satisfactory solution. This is the problem of showing how we have knowledge of other things than ourself and the operations of our own mind. Locke considers this problem, but what he says is very obviously unsatisfactory. In one place * we are told: "Since the mind, in all its thoughts and reasonings, hath no other immediate object but its own ideas, which it alone does or can contemplate, it is evident that our knowledge is only conversant about them." And again: "Knowledge is the perception of the agreement or disagreement of two ideas." From this it would seem to follow immediately that we cannot know of the existence of other people, or of the physical world, for these, if they exist, are not merely ideas in any mind. Each one of us, accordingly, must, so far as knowledge is concerned, be shut up in himself, and cut off from all contact with the outer world.
This, however, is a paradox, and Locke will have nothing to do with paradoxes. Accordingly, in another chapter, he sets forth a different theory, quite inconsistent with the earlier one. We have, he tells us, three kinds of knowledge of real existence. Our knowledge of our own existence is intuitive, our knowledge of God's existence is demonstrative, and our knowledge of things present to sense is sensitive (Book IV, Ch. III).
In the next chapter, he becomes more or less aware of the inconsistency. He suggests that some one might say: "If knowledge consists in agreement of ideas, the enthusiast and the sober man are on a level." He replies: "Not so where ideas agree with things." He proceeds to argue that all simple ideas must agree with things, since "the mind, as has been showed, can by no means make to itself" any simple ideas, these being all "the product of things operating on the mind in a natural way." And as regards complex ideas of substances, "all our complex ideas of them must be such, and such only, as are made up of such simple ones as have been discovered to coexist in nature." Again, we can have no knowledge except (1) by intuition, (2) by reason, examining the agreement or disagreement of two ideas, (3) "by sensation, perceiving the existence of particular things" (Book IV, Ch. III, Sec. 2).
In all this, Locke assumes it known that certain mental occurrences, which he calls sensations, have causes outside themselves, and that these causes, at least to some extent and in certain respects, resemble the sensations which are their effects. But how, consistently with the principles of empiricism, is this to be known? We experience the sensations, but not their causes; our experience will be exactly the same if our sensations arise spontaneously. The belief that sensations have causes, and still more the belief that they resemble their causes, is one which, if maintained, must be maintained on grounds wholly independent of experience. The view that "knowledge is the perception of the agreement or disagreement of two ideas" is the one that Locke is entitled to, and his escape from the paradoxes that it entails is effected by means of an inconsistency so gross that only his resolute adherence to common sense could have made him blind to it.
This difficulty has troubled empiricism down to the present day. Hume got rid of it by dropping the assumption that sensations have external causes, but even he retained this assumption whenever he forgot his own principles, which was very often. His fundamental maxim, "no idea without an antecedent impression," which he takes over from Locke, is only plausible so long as we think of impressions as having outside causes, which the very word "impression" irresistibly suggests. And at the moments when Hume achieves some degree of consistency he is wildly paradoxical.
No one has yet succeeded in inventing a philosophy at once credible and self-consistent. Locke aimed at credibility, and achieved it at the expense of consistency. Most of the great philosophers have done the opposite. A philosophy which is not self-consistent cannot be wholly true, but a philosophy which is self-consistent can very well be wholly false. The most fruitful philosophies have contained glaring inconsistencies, but for that very reason have been partially true. There is no reason to suppose that a self-consistent system contains more truth than one which, like Locke's, is obviously more or less wrong.