But Leibniz, on the other hand, felt as if he were dealing, in philosophical matters at least, with foemen hardly worthy of his steel. Locke, he says, had subtlety and address, and a sort of superficial metaphysics; but he was ignorant of the method of mathematics,—that is to say, from the standpoint of Leibniz, of the method of all science. We have already seen that he thought the examination of a work which had been the result of the continued labor of Locke was a matter for the leisure hours of his courtly visits. Indeed, he would undoubtedly have felt about it what he actually expressed regarding his controversy with Clarke,—that he engaged in it

      “Ludus et jocus, quia in philosophia

       Omnia percepi atque animo mecum ante peregi.”

      He regarded the English as superficial and without grasp of principles, as they thought him over-deep and over-theoretical.

      From this knowledge of the external circumstances of the work of Leibniz and its relation to Locke, it is necessary that we turn to its internal content, to the thought of Leibniz as related to the ideas of Locke. The Essay on the Human Understanding is, as the name implies, an account of the nature of knowledge. Locke tells us that it originated in the fact that often, when he had been engaged in discussions with his friends, they found themselves landed in insoluble difficulties. This occurred so frequently that it seemed probable that they had been going at matters from the wrong side, and that before they attempted to come to conclusions about questions they ought to examine the capacity of intelligence, and see whether it is fitted to deal with such questions. Locke, in a word, is another evidence of that truth which lies at the basis of all forms of philosophical thought, however opposed they may be to one another,—the truth that knowledge and reality are so organic to each other that to come to any conclusion about one, we must know something about the other. Reality equals objects known or knowable, and knowledge equals reality dissolved in ideas,—reality which has become translucent through its meaning.

      Locke’s Essay is, then, an account of the origin, nature, extent, and limitations of human knowledge. Such is its subject-matter. What is its method? Locke himself tells us that he uses the “plain historical method.” We do not have to resort to the forcing of language to learn that this word “historical” contains the key to his work. Every page of the Essay is testimony to the fact that Locke always proceeds by inquiring into the way and circumstances by which knowledge of the subject under consideration came into existence and into the conditions by which it was developed. Origin means with Locke, not logical dependence, but temporal production; development means temporal succession. In the language of our day, Locke’s Essay is an attempt to settle ontological questions by a psychological method. And as we have before noticed, Leibniz meets him, not by inquiry into the pertinence of the method or into the validity of results so reached, but by the more direct way of impugning his psychology, by substituting another theory of the nature of mind and of the way in which it works.

      The questions with which the discussion begins are as to the existence of innate ideas, and as to whether the soul always thinks,—questions which upon their face will lead the experienced reader of to-day to heave a sigh in memory of hours wasted in barren dispute, and which will create a desire to turn elsewhere for matter more solid and more nutritive. But in this case, under the form which the discussion takes at the hands of Leibniz, the question which awaits answer under the meagre and worn-out formula of “innate ideas” is the function of intelligence in experience.

      Locke denies, and denies with great vigor, the existence of innate ideas. His motives in so doing are practical and theoretical. He sees almost every old idea, every hereditary prejudice, every vested interest of thought, defended on the ground that it is an innate idea. Innate ideas were sacred, and everything which could find no defence before reason was an innate idea. Under such circumstances he takes as much interest in demolishing them as Bacon took in the destruction of the “eidols.” But this is but a small portion of the object of Locke. He is a thorough-going empiricist; and the doctrine of innate ideas appears to offer the greatest obstacle to the acceptance of the truth that all the furnishing of the intellect comes from experience. Locke’s metaphors for the mind are that it is a blank tablet, an empty closet, an unwritten book. The “innate idea” is only a sentence written by experience, but which, deified by a certain school of philosophers, has come to be regarded as eternally imprinted upon the soul.

      Such, indeed, is Locke’s understanding of the nature of innate ideas. He conceives of them as “characters stamped, as it were, upon the mind of man, which the soul has received in its first being and brings into the world with it;” or they are “constant impressions which the souls of men receive in their first beings.” They are “truths imprinted upon the soul.” Having this conception of what is meant by “innate ideas,” Locke sets himself with great vigor, and, it must be confessed, with equal success, to their annihilation.

      His argument is somewhat diffuse and scattered, but in substance it is as follows: Whatever is in the mind, the mind must be conscious of. “To be in the mind and not to be perceived, is all one as to say that anything is and is not in the mind.” If there be anything in the mind which is innate, it must be present to the consciousness of all, and, it would seem, of all at all times, savages, infants, and idiots included. And as it requires little philosophical penetration to see that savages do not ponder upon the principle that whatever is, is; that infants do not dwell in their cradle upon the thought of contradiction, or idiots ruminate upon that of excluded middle,—it ought to be evident that such truths cannot be innate. Indeed, we must admit, with Locke, that probably few men ever come to the explicit consciousness of such ideas, and that these few are such as direct their minds to the matter with some pains. Locke’s argument may be summed up in his words: If these are not notions naturally imprinted, how can they be innate? And if they are notions naturally imprinted, how can they be unknown?

      But since it may be said that these truths are in the mind, but in such a way that it is only when they are proposed that men assent to them, Locke goes on to clinch his argument. If this be true, it shows that the ideas are not innate; for the same thing is true of a large number of scientific truths, those of mathematics and morals, as well as of purely sensible facts, as that red is not blue, sweet is not sour, etc.,—truths and facts which no one calls innate. Or if it be said that they are in the mind implicitly or potentially, Locke points out that this means either nothing at all, or else that the mind is capable of knowing them. If this is what is meant by innate ideas, then all ideas are innate; for certainly it cannot be denied that the mind is capable of knowing all that it ever does know, or, as Locke ingenuously remarks, “nobody ever denied that the mind was capable of knowing several truths.”

      It is evident that the force of Locke’s contention against innate ideas rests upon a certain theory regarding the nature of innate ideas and of the relations of consciousness to intelligence. Besides this, there runs through his whole polemic the assertion that, after all, innate ideas are useless, as experience, in the sense of impressions received from without, and the formal action of intelligence upon them, is adequate to doing all they are supposed to do. It is hardly too much to say that the nerve of Locke’s argument is rather in this positive assertion than in the negations which he brings against this existence. Leibniz takes issue with him on each of these three points. He has another conception of the very nature of innate ideas; he denies Locke’s opinions about consciousness; he brings forward an opposed theory upon the relation of experience to reason. This last point we shall take up in a chapter by itself, as its importance extends far beyond the mere question as to the existence of ideas which may properly be called innate. The other two questions, as to the real character of innate ideas and the relation of an idea to consciousness, afford material to occupy us for the present.

      The metaphor which Locke constantly uses is the clew to his conception of innate ideas. They are characters stamped or imprinted upon the mind, they exist in the mind. The mind would be just what it is, even if they had no existence. It would not have quite so much “in” it, but its own nature would not be changed. Innate ideas he conceives as bearing a purely external relation to mind. They are not organic to it, nor necessary instruments through which it expresses itself; they are mechanically impressed upon it. But what the “intellectual” school had meant by innate ideas was precisely that the relation of ideas to intelligence is not