Perception is the first step toward knowledge. After perception the most indispensable faculty is retention, the prolonged consciousness of present ideas and the revival of those which have disappeared, or, as it were, have been put aside. For an idea to be "in the memory" means that the mind has the capacity to reproduce it at will, whereupon it recognizes it as previously experienced. If our ideas are not freshened up from time to time by new impressions of the same sort they gradually fade out, until finally (as the idea of color in one become blind in early life) they completely disappear. Ideas impressed upon the mind by frequent repetition are rarely entirely lost. Memory is the basis for the intellectual functions of discernment and comparison, of composition, abstraction, and naming. Since, amid the innumerable multitude of ideas, it is not possible to assign to each one a definite sign, the indispensable condition of language is found in the power of abstraction, that is, in the power of generalizing ideas, of compounding many ideas into one, and of indicating by the names of the general ideas, or of the classes and species, the particular ideas also which are contained under these. Here is the great distinction between man and the brute. The brute lacks language because he lacks (not all understanding whatever, e.g., not a capacity, though an imperfect one, of comparison and composition, but) the faculty of abstraction and of forming general ideas. The object of language is simply the quick and easy communication of our thoughts to others, not to give expression to the real essence of objects. Words are not names for particular things, but signs of general ideas; and abstracta nothing more than an artifice for facilitating intellectual intercourse. This abbreviation, which aids in the exchange of ideas, involves the danger that the creations of the mind denoted by words will be taken for images of real general essences, of which, in fact, there are none in existence, but only particular things. In order to prevent anyone to whom I am speaking from understanding my words in a different sense from the one intended, it is necessary for me to define the complex ideas by analyzing them into their elements, and, on the other hand, to give examples in experience of the simple ideas, which do not admit of definition, or to explain them by synonyms. Thus much from Locke's philosophy of language, to which he devotes the third book of the Essay.
Complex ideas, which are very numerous, may be divided into three classes:
Modes, Substances, and Relations.
Modes (states, conditions) are such combinations of simple ideas which do not "contain in them the supposition of subsisting by themselves, but are considered as dependencies on, or affections of substances." They fall into two classes according as they are composed of the same simple ideas, or simple ideas of various kinds; the former are called simple, the latter mixed, modes. Under the former class belong, for example, a dozen or a score, the idea of which is composed of simple units; under the latter, running, fighting, obstinacy, printing, theft, parricide. The formation of mixed modes is greatly influenced by national customs. Very complicated transactions (sacrilege, triumph, ostracism), if often considered and discussed, receive for the sake of brevity comprehensive names, which cannot be rendered by a single expression in the language of other nations among whom the custom in question is not found. The elements most frequently employed in the formation of mixed modes are ideas of the two fundamental activities, thinking and motion, together with power, which is their source. Locke discusses simple modes in more detail, especially those derived from the ideas of space, time, unity, and power. Modifications of space are distance, figure, place, length; since any length or measure of space can be repeated to infinity, we reach the idea of immensity. As modes of time are enumerated succession (which we perceive and measure only by the flow of our ideas), duration, and lengths or measures of duration, the endless repetition of which yields the idea of eternity. From unity are developed the modes of numbers, and from the unlimitedness of these the idea of infinity. No idea, however, is richer in modes than the idea of power. A distinction must be made between active power and passive power, or mere receptivity. While bodies are not capable of originating motion, but only of communicating motion received, we notice in ourselves, as spiritual beings, the capacity of originating actions and motions. The body possesses only the passive power of being moved, the mind the active power of producing motion. This latter is termed "will." Here Locke discusses at length the freedom of the will, but not with entire clearness and freedom from contradictions (cf. below).
Modes are conditions which do not subsist of themselves, but have need of a basis or support; they are not conceivable apart from a thing whose properties or states they are. We notice that certain qualities always appear together, and habitually refer them to a substratum as the ground of their unity; in which they subsist or from which they proceed. Substance denotes this self-existent "we know not what," which has or bears the attributes in itself, and which arouses the ideas of them in us. It is the combination of a number of simple ideas which are presumed to belong to one thing. From the ideas of sensation the understanding composes the idea of body, and from the ideas of reflection that of mind. Each of these is just as clear and just as obscure as the other; of each we know only its effects and its sensuous properties; its essence is for us entirely unknowable. Instead of the customary names, material and immaterial substances, Locke recommends cogitative and incogitative substances, since it is not inconceivable that the Creator may have endowed some material beings with the capacity of thought. God,—the idea of whom is attained by uniting the ideas of existence, power, might, knowledge, and happiness with that of infinity,—is absolutely immaterial, because not passive, while finite spirits (which are both active and passive) are perhaps only bodies which possess the power of thinking.
While the ideas of substances are referred to a reality without the mind as their archetype, to which they are to conform and which they should image and represent, Relations (e.g., husband, greater) are free and immanent products of the understanding. They are not copies of real things, but represent themselves alone, are their own archetypes. We do not ask whether they agree with things, but, conversely, whether things agree with them (Book iv. 4.5). The mind reaches an idea of relation by placing two things side by side and comparing them. If it perceives that a thing, or a quality, or an idea begins to exist through the operation of some other thing, it derives from this the idea of the causal relation, which is the most comprehensive of all relations, since all that is actual or possible can be brought under it. Cause is that which makes another thing to begin to be; effect, that which had its beginning from some other thing. The production of a new quality is termed alteration; of artificial things, making; of a living being, generation; of a new particle of matter, creation. Next in importance is the relation of identity and diversity. Since it is impossible for a thing to be in two different places at the same time and for two things to be at the same time in the same place, everything that at a given instant is in a given place is identical with itself, and, on the other hand, distinct from everything else (no matter how great the resemblance between them) that at the same moment exists in another place. Space and time therefore form the principium individuationis. By what marks, however, may we recognize the identity of an individual at different times and in different places? The identity of inorganic matter depends on the continuity of the mass of atoms which compose it; that of living beings upon the permanent organization of their parts (different bodies are united into one animal by a common life); personal identity consists in the unity of self-consciousness, not in the continuity of bodily existence (which is at once excluded by the change of matter). The identity of the person or the ego must be carefully distinguished from that of substance and of man. It would not be impossible for the person to remain the same in a change of substances, in so far as the different beings (for instance, the souls of Epicurus and Gassendi) participated in the same self-consciousness; and, conversely, for a spirit to appear in two persons by losing the consciousness of its previous existence. Consciousness is the sole condition of the self, or personal identity.—The determinations of space and time are for the most part relations. Our answers to the questions "When?" "How long?" "How large?" denote the distance of one point of time from another (e.g., the birth of Christ), the relation of one duration to another (of a revolution of the sun), the relation of one extension to another well-known one taken as a standard. Many apparently positive ideas and words, as young and old, large and small, weak and strong, are in fact relative. They imply merely the relation of a given duration of life, of a given size and strength, to that which has been adopted as a standard for the class of things in question. A man of twenty is called young, but a horse of like age, old; and neither of these measures of time applies to stars or diamonds. Moral relations, which are based on a comparison of man's voluntary actions with one of the three moral laws, will be discussed below.
The inquiry now turns from the origin of ideas to their cognitive value or their validity, beginning (in the concluding chapters of the second book) with the accuracy of single ideas, and advancing (in Book iv., which is the most important in the whole work) to the truth of judgments. An idea is real when it conforms to its archetype, whether this is a thing, real or possible, or an idea of some other thing; it is adequate when the conformity is complete. The idea of a four-sided triangle or of brave cowardice is unreal or fantastical, since it is composed of incompatible elements, and the idea of a centaur, since it unites simple ideas in a way in which they do not occur in nature. The layman's ideas of law or of chemical substances are real, but inadequate, since they have a general resemblance to those of experts, and a basis in reality, but yet only imperfectly represent their archetypes. Nay, further, our ideas of substances are all inadequate, not only when they are taken for representations of the inner essences of things (since we do not know these essences), but also when they are considered merely as collections of qualities. The copy never includes all the qualities of the thing, the less so since the majority of these are powers, i.e., consist in relations to other objects, and since it is impossible, even in the case of a single body, to discover all the changes which it is fitted to impart to, or to receive from, other substances. Ideas of modes and relations are all adequate, for they are their own archetypes, are not intended to represent anything other than themselves, are images without originals. An idea of this kind, however, though perfect when originally formed, may become imperfect through the use of language, when it is unsuccessfully intended to agree with the idea of some other person and denominated by a current term. In the case of mixed modes and their names, therefore, the compatibility of their elements and the possible existence of their objects are not enough to secure their reality and their complete adequacy; in order to be adequate they must, further, exactly conform to the meaning connected with their names by their author, or in common use. Simple ideas are best off, according to Locke, in regard both to reality and to adequacy. For the most part, it is true, they are not accurate copies of the real qualities, of things, but only the regular effects of the powers of things. But although real qualities are thus only the causes and not the patterns of sensations, still simple ideas, by their constant correspondence with real qualities, sufficiently fulfill their divinely ordained end, to serve us as instruments of knowledge, i.e., in the discrimination of things.—An unreal and inadequate idea becomes false only when it is referred to an object, whether this be the existence of a thing, or its true essence, or an idea of other things. Truth and error belong always to affirmations or negations, that is, to (it may be, tacit) propositions. Ideas uncombined, unrelated, apart from judgments, ideas, that is, as mere phenomena in the mind, are neither true nor false.
Knowledge is defined as the "perception of the connexion and agreement, or disagreement and repugnancy" of two ideas; truth, as "the right joining or separating of signs, i.e., ideas or words." The object of knowledge is neither single ideas nor the relations of ideas to things, but the relations of ideas among themselves. This view was at once paradoxical and pregnant. If all cognition, as Locke suggests in objection to his own theory, consists in perceiving the agreement or disagreement of our ideas, are not the visions of the enthusiast and the reasonings of sober thinkers alike certain? are not the propositions, A fairy is not a centaur, and a centaur is a living being, just as true as that a circle is not a triangle, and that the sum of the angles of a triangle is equal to two right angles? The mind directly perceives nothing but its own ideas, but it seeks a knowledge of things! If this is possible it can only be indirect knowledge—the mind knows things through its ideas, and possesses criteria which show that its ideas agree with things.
Two cases must be clearly distinguished, for a considerable number of our ideas, viz., all complex ideas except those of substances, make no claim to represent things, and consequently cannot represent them falsely. For mathematical and moral ideas and principles, and the truth thereof, it is entirely immaterial whether things and conditions correspondent to them exist in nature or not. They are valid, even if nowhere actualized; they are "eternal truths," not in the sense that they are known from childhood, but in the sense that, as soon as known, they are immediately assented to.[1] The case is different, however, with simple ideas and the ideas of substances, which have their originals without the mind and which are to correspond with these. In regard to the former we may always be certain that they agree with real things, for since the mind can neither voluntarily originate them (e.g., cannot produce sensations of color in the dark) nor avoid having them at will, but only receive them from without, they are not creatures of the fancy, but the natural and regular productions of external things affecting us. In regard to the latter, the ideas of substances, we may be certain at least when the simple ideas which compose them have been found so connected in experience. Perception has an external cause, whose influence the mind is not able to withstand. The mutual corroboration furnished by the reports of the different senses, the painfulness of certain sensations, the clear distinction between ideas from actual perception and those from memory, the possibility of producing and predicting new sensations of an entirely definite nature in ourselves and in others, by means of changes which we effect in the external world (e.g. by writing down a word)—these give further justification for the trust which we put in the senses. No one will be so skeptical as to doubt in earnest the existence of the things which he sees and touches, and to declare his whole life to be a deceptive dream. The certitude which perception affords concerning the existence of external objects is indeed not an absolute one, but it is sufficient for the needs of life and the government of our actions; it is "as certain as our happiness or misery, beyond which we have no concernment, either of knowing or being." In regard to the past the testimony of the senses is supplemented by memory, in which certainty [in regard to the continued existence of things previously perceived] is transformed into high probability; while in regard to the existence of other finite spirits, numberless kinds of which may be conjectured to exist, though their existence is quite beyond our powers of perception, certitude sinks into mere (though well-grounded) faith.
[Footnote 1: Thus it results that knowledge, although dependent on experience for all its materials, extends beyond experience. The understanding is completely bound in the reception of simple ideas; less so in the combination of these into complex ideas; absolutely free in the act of comparison, which it can omit at will; finally, again, completely bound in its recognition of the relation in which the ideas it has chosen to compare stand to one another. There is room for choice only in the intermediate stage of the cognitive process; at the beginning (in the reception of the simple ideas of perception, a, b, c, d), and at the end (in judging how the concepts a b c and a b d stand related to each other), the understanding is completely determined.]
More certain than our sensitive knowledge of the existence of external objects, are our immediate or intuitive knowledge of our own existence and our mediate or demonstrative knowledge of the existence of God. Every idea that we have, every pain, every thought assures us of our own existence. The existence of God, however, as the infinite cause of all reality, endowed with intelligence, will, and supreme power, is inferred from the existence and constitution of the world and of ourselves. Reality exists; the real world is composed of matter in motion and thinking beings, and is harmoniously ordered. Since it is impossible for any real being to be produced by nothing, and since we obtain no satisfactory answer to the question of origin until we rise to something existent from all eternity, we must assume as the cause of that which exists an Eternal Being, which possesses in a higher degree all the perfections which it has bestowed upon the creatures. As the cause of matter and motion, and as the source of all power, this Being must be omnipotent; as the cause of beauty and order in the world, and, above all, as the creator of thinking beings, it must be omniscient. But these perfections are those which we combine in the idea of God.