Locke's chief work, An Essay concerning Human Understanding, which had been planned as early as 1670, was published in 1689-90, a short abstract of it having previously appeared in French in Le Clerc's Bibliothèque Universelle, 1688. His theoretical works include, further, the two posthumous treatises, On the Conduct of the Understanding (originally intended for incorporation in the fourth edition of the Essay, which, however, appeared in 1700 without this chapter, which probably had proved too extended) and the Elements of Natural Philosophy. To political and politico-economic questions Locke contributed the two Treatises on Government, 1690, and three essays on money and the coinage. In the year 1689 appeared the first of three Letters on Tolerance, followed, in 1693, by Some Thoughts on Education, and, in 1695, by The Reasonableness of Christianity as delivered in the Scriptures. The collected works appeared for the first time in 1714, and in nine volumes in 1853; the philosophical works (edited by St. John) are given in Bonn's Standard Library (1867-68).[1]
[Footnote 1: Lord King and Fox Bourne have written on Locke's life, 1829 and 1876. A comparison of Locke's theory of knowledge with Leibnitz's critique was published by Hartenstein in 1865, and one by Von Benoit (prize dissertation) in 1869, and an exposition of his theory of substance by De Fries in 1879. Victor Cousin's Philosophie de Locke has passed through six editions. [Among more recent English discussions reference may be made to Green's Introduction to Hume's Treatise on Human Nature, 1874 (new ed. 1890), which is a valuable critique of the line of development, Locke, Berkeley, Hume; Fowler's Locke, in the English Men of Letters, 1880; and Fraser's Locke, in Blackwood's Philosophical Classics, 1890.—TR.]
%(a) Theory of Knowledge.%—Locke's theory of knowledge is controlled by two tendencies, one native, furnished by the Baconian empiricism, and the other Continental, supplied by the Cartesian question concerning the origin of ideas. Bacon had demanded the closest connection with experience as the condition of fruitful inquiry. Locke supports this commendation of experience by a detailed description of the services which it renders to cognition, namely, by showing that, in simple ideas, perception supplies the material for complex ideas, and for all the cognitive work of the understanding. Descartes had divided ideas, according to their origin, into three classes: those which are self-formed, those which come from without, and those which are innate (p. 79), and had called this third class the most valuable. Locke disputes the existence of ideas in the understanding from birth, and makes it receive the elements of knowledge from the senses, that is, from without. He is a representative of sensationalism,—not in the stricter sense, first put into the term by those who subsequently continued his endeavors, that thought arises from perception, that it is transformed sensation—but in the wider sense, that thought is (free) operation with ideas, which are neither created by it nor present in it from the first, but given to it by perception, that, consequently, the cognitive process begins with sensation and so its first attitude is a passive one. From the standpoint of the Cartesian problem, which he solves in a sense opposite to Descartes, Locke supplements the empiricism of Bacon by basing it on a psychologically developed theory of knowledge. That in the course of the inquiry he introduces a new principle, which causes him to diverge from the true empirical path, will appear in the sequel.
The question "How our ideas come into the mind" receives a negative answer (in the first book of the Essay): "There are no innate principles in the mind"[1] The doctrine of the innate character of certain principles is based on their universal acceptance. The asserted agreement of mankind in regard to the laws of thought, the principles of morality, the existence of God, etc., is neither cogent as an argument nor correct in fact. In the first place, even if there were any principles which everyone assented to, this would not prove that they had been created in the soul; the fact of general consent would admit of a different explanation. Granted that no atheists existed, yet it would not necessarily follow that the universal conviction of the existence of God is innate, for it might have been gradually reached in each case through the use of the reason—might have been inferred, for instance, from the perception of the purposive character of the world. Second, the fact to which this theory of innate ideas appeals is not true. No moral rule can be cited which is respected by all nations. The idea of identity is entirely unknown to idiots and to children. If the laws of identity and contradiction were innate they must appear in consciousness prior to all other truths; but long before a child is conscious of the proposition "It is impossible for the same thing to be and not to be," it knows that sweet is not bitter, and that black is not white. The ideas first known are not general axioms and abstract concepts, but particular impressions of the senses. Would nature write so illegible a hand that the mind must wait a long time before becoming able to read what had been inscribed upon it? It is often said, however, that innate ideas and principles may be obscured and, finally, completely extinguished by habit, education, and other extrinsic circumstances. Then, if they gradually become corrupted and disappear, they must at least be discoverable in full purity where these disturbing influences have not yet acted; but it is especially vain to look for them in children and the ignorant. Perhaps, however, these possess such principles unconsciously; perhaps they are imprinted on the understanding, without being attended to? This would be a contradiction in terms. To be in the mind or the understanding simply means "to be understood" or to be known; no one can have an idea without being conscious of it. Finally, if the attempt be made to explain "originally in the mind" in so wide a sense that it would include all truths which man can ever attain or is capable of discovering by the right use of reason, this would make not only all mathematical principles, but all knowledge in general, all sciences, and all arts innate; there would be no ground even for the exclusion of wisdom and virtue. Therefore, either all ideas are innate or none are. This is an important alternative. While Locke decides for the second half of the proposition, Leibnitz defends the first by a delicate application of the concept of unconscious representation and of implicit knowledge, which his predecessor rejects out of hand.
[Footnote 1: According to Fox Bourne this first book was written after the others. Geil (Ueber die Abhängigkeit Lockes von Descartes, Strassburg, 1887, chap, iii.) has endeavored to prove that, since the arguments controverted are wanting in Descartes, the attack was not aimed at Descartes and his school, but at native defenders of innate ideas, as Lord Herbert of Cherbury and the English Platonists (Cudworth, More, Parker, Gale). That along with these the Cartesian doctrine was a second and chief object of attack is shown by Benno Erdmann in his discussion of the treatises by G. Geil and R. Sommer (Lockes Verhältnis zu Descartes, Berlin, 1887) in the Archiv für Geschichte der Philosophie, ii, pp. 99-121.]
Locke's positive answer to the question concerning the origin of ideas is given in his second book. Ideas are not present in the understanding from the beginning, nor are they originated by the understanding, but received through sensation. The understanding is like a piece of white paper on which perception inscribes its characters. All knowledge arises in experience. This is of two kinds, derived either from the external senses or the internal sense. The perception of external objects is termed Sensation, that of internal phenomena (of the states of the mind itself) Reflection. External and internal perception are the only windows through which the light of ideas penetrates into the dark chamber of the understanding. The two are not opened simultaneously, however, but one after the other; since the perceptions of the sensible qualities of bodies, unlike that of the operations of the mind itself, do not require an effort of attention, they are the earlier. The child receives ideas of sensation before those of reflection; internal perception presupposes external perception.
In this distinction between sensation and reflection, we may recognize an after-effect of the Cartesian dualism between matter and spirit. The antithesis of substances has become a duality in the faculties of perception. But while Descartes had so far forth ascribed precedence to the mind in that he held the self-certitude of the ego to be the highest and clearest of all truths and the soul to be better known than the body, in Locke the relation of the two was reversed, since he made the perception of self dependent on the precedent perception of external objects. This antithesis was made still sharper in later thinking, when Condillac made full use of the priority of sensation, which in Locke had remained without much effect; while Berkeley, on the other hand, reduced external perception to internal perception.
All original ideas are representations either of the external senses or of the internal sense, or of both. And since, in the case of ideas of sensation, there is a distinction between those which are perceived by a single one of the external senses and those which come from more than one, four classes of simple ideas result: (1) Those which come from one external sense, as colors, sounds, tastes, odors, heat, solidity, and the like. (2) Those which come from more than one external sense (sight and touch), as extension, figure, and motion. (3) Reflection on the operations of our minds yields ideas of perception or thinking (with its various modes, remembrance, judging, knowledge, faith, etc.), and of volition or willing. (4) From both external and internal perception there come into the mind the ideas of pleasure and pain, existence, power, unity, and succession. These are approximately our original ideas, which are related to knowledge as the letters to written discourse; as all Homer is composed out of only twenty-four letters, so these few simple ideas constitute all the material of knowledge. The mind can neither have more nor other simple ideas than those which are furnished to it by these two sources of experience.
Locke differs from Descartes again in regard to extension and thought. Extension does not constitute the essence of matter, nor thought the essence of mind. Extension and body are not the same; the former is presupposed by the latter as its necessary condition, but it is the former alone which yields mathematical matter. The essence of physical matter consists rather in solidity: where impenetrability is found there is body, and the converse; the two are absolutely inseparable. With space the case is different. I cannot conceive unextended matter, indeed, but I can easily conceive immaterial extension, an unfilled space Further, if the essence of the soul consisted in thought, it must be always thinking. As the Cartesians maintained, it must have ideas as soon as it begins to be, which is manifestly contrary to experience. Thinking is merely an activity of the mind, as motion is an activity of the body, and not its essential characteristic. The mind does not receive ideas until external objects occasion perception in it through impressions, which it is not able to avert. The understanding may be compared to a mirror, which, without independent activity and without being consulted, takes up the images of things. Some of the simple ideas which have been mentioned above represent the properties of things as they really are, others not. The former class includes all ideas of reflection (for we are ourselves the immediate object of the inner sense); but among the ideas of sensation those only which come from different senses, hence extension, motion and rest, number, figure, and, further, solidity, are to be accounted primary qualities, i. e., such as are actual copies of the properties of bodies. All other ideas, on the contrary, have no resemblance to properties of bodies; they represent merely the ways in which things act, and are not copies of things. The ideas of secondary or derivative qualities (hard and soft, warm and cold, colors and sounds, tastes and odors) are in the last analysis caused—as are the primary—by motion, but not perceived as such. Yellow and warm are merely sensations in us, which we erroneously ascribe to objects; with equal right we might ascribe to fire, as qualities inherent in it, the changes in form and color which it produces in wax and the pain which it causes in the finger brought into proximity with it. The warmth and the brightness of the blaze, the redness, the pleasant taste, and the aromatic odor of the strawberry, exist in these bodies merely as the power to produce such sensations in us by stimulation of the skin, the eye, the palate, and the nose. If we remove the perceptions of them, they disappear as such, and their causes alone remain—the bulk, figure, number, texture, and motion of the insensible particles. The ground of the illusion lies in the fact that such qualities as color, etc., bear no resemblance to their causes, in no wise point to these, and in themselves contain naught of bulk, density, figure, and motion, and that our senses are too weak to discover the material particles and their primary qualities.—The distinction between qualities of the first and second order—first advanced by the ancient atomists, revived by Galileo and Descartes on the threshold of the modern period, retained by Locke, and still customary in the natural science of the day—forms an important link in the transition from the popular view of all sense-qualities as properties of things in themselves to Kant's position, that spatial and temporal qualities also belong to phenomena alone, and are based merely on man's subjective mode of apprehension, while the real properties of things in themselves are unknowable.
Thus far the procedure of the understanding has been purely passive. But besides the capacity for passively receiving simple ideas, it possesses the further power of variously combining and extending these original ideas which have come into it from without, of working over the material given in sensation by the combination, relation, and separation of its various elements. In this it is active, but not creative. It is not able to form new simple ideas (and just as little to destroy such as already exist), but only freely to combine the elements furnished without its assistance by perception (or, following the figure mentioned above, to combine into syllables and words the separate letters of sensation). Complex ideas arise from simple ideas through voluntary combination of the latter.