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 reminding us. As regards kinds of knowledge, he finds that “all

knowledge we are capable of” must be assertion or denial of some one of three sorts of relation among our ideas themselves, or else of relations between our ideas and reality that exists independently of us and our ideas. Accordingly, knowledge is concerned either with (a) relations of identity and difference among ideas, as when we say that “blue is not yellow”; or (b) with mathematical relations, as that “two triangles upon equal bases between two parallels must be equal”; or (c) in assertions that one quality does or does not coexist with another in the same substance, as that “iron is susceptible of magnetical impressions, or that ice is not hot”; or (d) with ontological reality, independent of our perceptions, as that “God exists” or “I exist” or “the universe exists.” The first sort is analytical; mathematical and ethical knowledge represents the second; physical science forms the third; real knowledge of self, God and the world constitutes the fourth.

Locke found important differences in the way in which knowledge of any sort is reached. In some instances the known relation is self-evident, as when we judge intuitively that a circle cannot be a triangle, or that three must be more than two. In other cases the known relation is perceived to be intellectually

necessary through the medium of premisses, as in a mathematical demonstration. All that is strictly knowledge is reached in these two ways. But there is a third sort, namely sense-perception, which hardly deserves the name. For “our perceptions of the particular existence of finite beings without us” go beyond mere probability, yet they are not purely rational. There is nothing self-contradictory in the supposition that our perceptions of things external are illusions, although we are somehow unable to doubt them. We find ourselves inevitably “conscious of a different sort of perception,” when we actually see the sun by day and when we only imagine the sun at night.

Locke next inquired to what extent knowledge—in the way either of intuitive certainty, demonstrative certainty, or sense perception—is possible, in regard to each of the four (already mentioned) sorts of knowable relation. There is only one of the four in which our knowledge is co-extensive with our ideas. It is that of “identity and diversity”: we cannot be conscious at all without distinguishing, and every affirmation necessarily implies negation. The second sort of knowable relation is sometimes intuitively and sometimes demonstrably discernible. Morality, Locke thinks, as well as mathematical quantity, is capable of being demonstrated. “Where there is no property there is no injustice,” is an example of a proposition “as certain as any demonstration in Euclid.” Only we are more apt to be biassed, and thus to leave reason in abeyance, in dealing with questions of morality than in dealing with problems in mathematics.

Turning from abstract mathematical and moral relations to concrete relations of coexistence and succession among phenomena—the third sort of knowable relation—Locke finds the light of pure reason disappear; although these relations form “the greatest and most important part of what we desire to know.” Of these, including as they do all inductive science, he reports that demonstrable knowledge “is very short, if indeed we have any at all”; and are not thrown wholly on presumptions of probability, or else left in ignorance. Man cannot attain perfect and infallible science of bodies. For natural science depends, he thinks, on knowledge of the relations between their secondary qualities on the one hand, and the mathematical qualities of their atoms on the other, or else “on something yet more remote from our comprehension.” Now, as perception of these atoms and their relations is beyond us, we must be satisfied with inductive presumptions, for which “experimental verification” affords, after all, only conclusions that wider experience may prove to be inadequate. But this moral venture Locke accepts as “sufficient for our purposes.”

Our knowledge under Locke’s fourth category of relations—real existence—includes (a) intuitive perceptions of our own existence; (b) demonstrable certainty of the existence of God; and (c) actual perception of the existence of surrounding things, as long as, but only as long as the things are present to sense. “If I doubt all other things, that very doubt makes me perceive my own existence, and will not suffer me to doubt of that” (iv. 9. 3). Faith in the existence of God is virtually with Locke an expression of faith in the principle of active causality in its ultimate universality. Each person knows that he now exists, and is convinced that he had a beginning; with not less intuitive certainty he knows that “nothing can no more produce any real being than it can be equal to two right angles.” His final conclusion is that there must be eternally “a most powerful and most knowing Being, in which, as the origin of all, must be contained all the perfections that can ever after exist,” and out of which can come only what it has already in itself; so that as the cause of my mind, it must be Mind. There is thus causal necessity for Eternal Mind, or what we call “God.” This is cautiously qualified thus in a letter to Anthony Collins, written by Locke a few months before he died: “Though I call the thinking faculty in me ‘mind,’ yet I cannot, because of that name, equal it in anything to that infinite and incomprehensible Being, which, for want of right and distinct conceptions, is called ‘Mind’ also.” But the immanence of God in the things and persons that compose the universal order, with what this implies, is a conception foreign to Locke, whose habitual conception was of an extra-mundane deity, the dominant conception in the 18th century.

Turning from our knowledge of Spirit to our knowledge of Matter, nearly all that one can affirm or deny about “things external is,” according to Locke, not knowledge but venture or presumptive trust. We have, strictly speaking, no “knowledge” of real beings beyond our own self-conscious existence,

the existence of God, and the existence of objects of sense as long as they are actually present to sense. “When I see an external object at a distance, a man for instance, I cannot but be satisfied of his existence while I am looking at him. (Locke might have added that when one only ‘sees a man’ it is merely his visible qualities that are perceived; his other qualities are as little ‘actual present sensations’ as if he were out of the range of sense.) But when the man leaves me alone, I cannot be certain that he still exists.” “There is no necessary connexion between his existence a minute since (when he was present to any sense of sight) and his existence now (when he is absent from all my senses); by a thousand ways he may have ceased to be. I have not that certainty of his continued existence which we call knowledge; though the great likelihood of it puts it past doubt. But this is but probability and not knowledge” (chap. 11, § 9). Accordingly, purely rational science of external Nature is, according to Locke, impossible. All our “interpretations of nature” are inadequate; only reasonable probabilities, not final rational certainties. This boundless region affords at the best probabilities, ultimately grounded on moral faith, all beyond lies within the veil. Such is Locke’s “plain, matter-of-fact” account of the knowledge of the Real that is open to man.

We learn little from Locke as to the rationale of the probabilities on which man thus depends when he deals with the past, the distant or the future. The concluding chapters of the fourth book contain wise advice to those whose lives are passed in an ever-changing environment, for avoiding the frequent risk of error in their conclusions, with or without the help of syllogism, the office of which, as a means of discovery, is here critically considered.

Investigation of the foundation of inductive inference was resumed by Hume where Locke left it. With a still humbler view of human reason than Locke’s, Hume proposed as “a subject worthy of curiosity,” to inquire into “the nature of that evidence which assures us of any real existence and matter

of fact, beyond the present testimony of our senses and the records of our memory; a part of philosophy that has been little cultivated either by the ancients or the moderns.” Hume argues that custom is a sufficient practical explanation of this gradual enlargement of our objective experience, and that no deeper explanation is open to man. All beyond each present transitory “impression” and the stores of memory is therefore reached blindly, through custom or habitual association. Associative tendency, individual or inherited, has since been the favourite constructive factor of human experience in Empirical Philosophy. This factor is not prominent in Locke’s Essay. A short chapter on “association of ideas” was added to the second book in the fourth edition. And the tendency to associate is there presented, not as the fundamental factor of human knowledge, but as a chief cause of human error.

Kant’s critical analysis of pure reason is more foreign to Locke than the attempts of 18th- and 19th-century associationists and evolutionists to explain experience and science. Kant’s aim was to show the necessary rational constitution of experience. Locke’s design was less profound. It was his distinction to present to the modern world, in his own “historical plain method,” perhaps the largest assortment ever made by any individual of facts characteristic of human understanding. Criticism of the presuppositions implied in those facts—by Kant and his successors, and in Britain more unpretentiously by Reid, all under the stimulus of Hume’s sceptical criticism—has employed philosophers since the author of the Essay on Human Understanding collected materials that raised deeper philosophical problems than he tried to solve. Locke’s mission was to initiate modern criticism of the foundation and limits of our knowledge. Hume negatively, and the German and Scottish schools constructively, continued what it was Locke’s glory to have begun.

—The Essay concerning Human Understanding has passed through more editions than any classic in modern philosophical literature. Before the middle of the 18th century it had reached thirteen, and it has now passed through some forty editions, besides being translated into Latin, French, Dutch, German and modern Greek. There are also several abridgments. In addition to those criticisms which appeared when Locke was alive, among the most important are Leibnitz’s Nouveaux Essais sur l’entendement humain—written about 1700 and published in 1765, in which each chapter of the Essay of Locke is examined in a corresponding chapter by Leibnitz; Cousin’s “École sensualiste: système de Locke,” in his Histoire de la philosophie au XVIII&#8202;e siècle (1829); and the criticisms in T. H. Green’s Introduction to the Philosophical Works of Hume (1874). The Essay, with Prolegomena, biographical, critical and historical, edited by Professor Campbell Fraser and published by the Oxford Clarendon Press in 1894, is the only annotated edition, unless the Nouveaux Essais of Leibnitz may be reduced to this category.

The Letters on Toleration, Thoughts on Education and The