An Essay Concerning Human Understanding By John Locke Summary and Analysis Book II: Of Ideas, Chapters 12-33


In the first eleven chapters of Book II, Locke has presented an account of simple ideas. In the remaining chapters of this book, he makes an analysis of complex ideas. One of the main differences between simple ideas and complex ideas is the fact that in the former the mind is relatively passive, whereas in the latter it is active. While it is true that in the simple ideas which are derived from reflection the mind is active, in one respect it is an activity which has to do with the materials that have entered the mind involuntarily. In other words, what has been received is quite independent of the will of the conscious subject. This is not what happens in the case of complex ideas, for here the mind exerts its power over simple ideas and produces whatever content it takes in order to complete all that is contained in one's store of knowledge.

This activity of the mind takes place in three different ways. The first of these consists in bringing together a number of simple ideas so as to form a single compound one. This type of activity is illustrated in such ideas as beauty, gratitude, mankind, army, or the universe. The second way in which the mind produces complex ideas is that of comparing simple ideas with one another. Here the ideas remain separate and distinct rather than being merged together to form a single one. It is from this activity that we derive the ideas of relationship, such as greater than, less than, to the right of, more costly than. The third way consists in separating ideas from all the others with which they have been associated in real experience. This is the opposite of the method of compounding. It is a process of abstracting from a number of particulars the elements which they have in common.

These processes or activities of the mind are sufficient to produce what may be designated as an infinite number of combinations, although they are all derived from the materials that were received into the mind either by sensation or by reflection. This wide variety of ideas can be classified under three heads which are known, respectively, as modes, substances, and relations.

By modes, the author means those complex ideas which refer to objects that do not exist by themselves but are always dependent on, or are affections of, some substance. This includes such ideas as triangle, gratitude, murder, and so on. Modes may be classified still further as simple and mixed, depending on whether they are combinations of the same kind of simple ideas, as in the case of such terms as a score or a dozen, or compounded of different kinds of simple ideas, such as we have in the case of beauty or theft.

By substances is meant that type of combination of simple ideas which are usually interpreted to mean particular things subsisting by themselves. This is illustrated in the ideas such as wood, lead, man, sheep, and the like.

By relations are meant the complex ideas that are obtained when simple ideas are compared with one another. These are the elements out of which all human knowledge is composed.

The four kinds of simple ideas and the three classes of complex ideas may be combined in an almost limitless number of different ways. It is comparable to what may be done with the twenty-six letters of the English alphabet when they are arranged in all of the different combinations that make up the words and sentences found in all of the books produced in that language.

To illustrate the way in which these kinds of complex ideas are formed in the human mind, Locke explains what happens in the case of such ideas as space, immensity, number, infinity, power, substance, cause and effect, personal identity, moral evaluations, and the sense in which ideas may be said to be true or false. A few of his examples will be sufficient to make clear the essential elements that are involved in his theory of knowledge.

The idea of space is derived from both the sense of sight and the sense of touch. When it is considered with reference to the length that exists between any two objects, it is called distance, and when considered with reference to length, breadth, and thickness, it is called extension. Each different distance is a modification of space, or in other words, a simple mode of this idea. It is in this manner that we speak of an inch, foot, yard, mile, or any number of such units combined into a single idea. The power of repeating or doubling any of these ideas beyond any defined limit is what is meant by immensity.

Another kind of distance has to do with the succession of ideas as they occur and disappear in our minds. This is what is called duration and forms the basis for our ideas of time and of eternity. It is by reflecting on the appearing of various ideas one after another that we get the idea of succession. Motion would not be perceived at all without a train of successive ideas. When the motion is too slow or too swift to be perceived by the senses, it produces the idea of a stationary object.

Any portions of duration which are not distinguished and which therefore cannot be measured do not belong to the idea of time. It is in this sense that we use the phrase "before all time" and "when time shall be no more." The succession of ideas that occur with the revolutions of the heavenly bodies constitutes the most proper unit for the measurement of time. Infinity of duration, or that which goes beyond any definite limits, is what is meant by eternity.

With reference to the idea of substance, Locke tells us that the occurrence in our minds of a great number of simple ideas which are always found together, and which therefore give rise to the impression that they belong together, are combined by the mind into a single complex idea. This is due to the fact that we are incapable of imagining how these ideas can subsist by themselves, and we therefore accustom ourselves to suppose that there is some substratum wherein they do subsist, which is what we call substance. If, however, we stop to examine the idea of substance, we will find that it contains nothing except that of an unknown something in which it is supposed that those qualities which have been sensed really exist. This is the general or obscure idea of substance.

A more definite and specific idea of substance is derived when the mind brings together the combinations of simple ideas that have been associated in particular experiences. It is in this way that we come to think of such substances as gold, water, man, horse, and so forth. If we should be asked concerning the nature of these substances, we could only reply in terms of the simple ideas that have been associated with them.

The idea of substance is associated with our mental experiences just as much as it is with those experiences that have to do with the external, or what we usually call the outside, world. It is just as impossible to imagine thinking, reasoning, comparing, or abstracting as processes that subsist by themselves as it is to imagine weight, size, or motion subsisting without some substance in which the activity takes place. Thus we come to think of spiritual substances in the same manner that we think of material substances.

The idea of power is one of the elements associated with the idea of substance. The mind is informed through the senses of the alteration of simple ideas that are observed in connection with external objects. Reflecting on these changes that take place and that which makes the changes possible, it comes by the idea of power. One thinks that fire has the power to burn things, the sun has the power to melt wax, gold has the power to be melted, and so on. Powers are of two kinds depending on whether they are able to make or to receive changes. The former are known as active powers and the latter as passive powers.

Power may be included as one of the simple ideas that belong to the class of relations. It is complex only insofar as it is combined with the idea of substance. Our clearest idea of active power is derived from spirit, or we may say from the activity that takes place in our own minds. Whenever a change is observed, the mind must possess the power to make that change. The power that is thus involved can be experienced directly, but we cannot in the same way observe the power that may be present in external objects. We can form some notion of their powers only through analogy with that which takes place in our own minds.

The idea of causality, or that of a casual relationship existing between things, is closely allied with this idea of power. In observing the changes or alterations that take place as one sensation follows another, we think not only of a substance in which the qualities we have sensed exist, but that this substance is also the cause of that which we have experienced.

We do not, however, derive the idea of a cause and effect relation from what we have experienced alone. It is impossible to see or hear a causal relation or to experience one through any sensation. Where then do we get the idea of a cause? Locke tells us that we get it from reflection on the processes which take place in our own minds. He says, "The idea of the beginning of motion we have only from reflection on what passes in ourselves; where we find by experience that, barely by willing it, barely by a thought of the mind, we can move the parts of our bodies, which before were at rest."

Since the idea of a causal relationship means that the same sequence of events will occur in the future that have been observed in the past, we can only say that the mind interprets the external objects to have the power to bring about this orderly sequence of events. There is no sensory confirmation that the future will be like the past, and for this reason we do not have certainty in our knowledge concerning it. All that we do have is a high degree of probability which is based entirely on what has taken place in the past.

Of all the problems which arise in connection with complex ideas, there is no one that is more puzzling than that of personal identity. The problem is a crucial one, for unless we can establish the fact that it is the same person who experiences a series of events, all attempts to derive a satisfactory theory of knowledge will be in vain. How can an individual whose body, mind, and actions are never exactly the same during any two successive periods of time be said to be the same person? This problem was not discussed in any thoroughgoing fashion in the first edition of Locke's Essay, but in response to a suggestion from one of his critics, a chapter was added in the second edition for the purpose of dealing with it in a more detailed manner.

To say that a person, or for that matter any particular object, can change and still remain the same as it was before appears to be a direct violation of the law of non-contradiction. Those who believe in an immortal soul that remains always the same while inhabiting changing bodies would seem to have a solution for this problem, but Locke sees plenty of difficulties involved in this conception. Then, too, he has gone to great lengths to disprove the existence of innate ideas, which may be regarded as a corollary of the belief in immortal souls. He therefore attempts to find a solution on the basis of his empirical theory of knowledge.

He begins by making a clear distinction between what is meant by identity and what is meant by diversity. Most of the confusion, he tells us, has been due to the fact that people have not been clear in their own minds about what it is that remains identical with itself and what it is that changes from time to time. Obviously, the identity is not to be found in the physical elements of which one's material body is composed, and the same is true of the specific contents included in one's mind. What does persist throughout the changing states of one's physical and mental existence is the type of organization that binds all of these states into a single unit, which we designate as the person.

The crucial factor that determines the identity of a person existing at one moment with the person who exists at another moment is the phenomenon of memory. Consciousness at one moment of what has happened over a long series of experiences constitutes not only a unity of these experiences but an awareness of the continuing process that makes this unity possible. The identity is not in the process alone nor in the particular states of body and mind taken by themselves. Rather, it is the combination of these factors viewed as a single unifying process in which the differences are relatively unimportant, and for the purpose of establishing an identity they can be ignored. It is in this sense that we may speak of the justice that is involved when punishments or rewards are administered to an individual for deeds which were performed at some time in the past.

Ideas of relations are of several different kinds. Some of them are known as proportional. Others are called natural. One of the most important of the various classes of relations is the one that is generally designated as moral. Ideas of good and bad are, in Locke's judgment, derived solely from pleasure and pain. He says, "moral good and evil is the conformity or disagreement of our voluntary actions to some law where by good or evil is drawn on us, from the will and power of the lawmaker." It is the power of the lawgiver to administer either rewards or punishments, and this is what makes it a matter of pleasure and pain.

In his discussion of the truth and falsity of ideas, Locke calls attention to the fact that in the strict sense of these words, ideas are neither true nor false. In this respect, they are like the names that we assign to given objects. They are an effective means of communication, but we cannot say that the name is necessarily like any of the qualities found in the object. The same is true of our ideas.

Nevertheless, it is customary to speak of one's ideas as being true or false, and there is a sense in which it is legitimate to do so. It is important, however, to indicate the sense in which ideas may be true and the sense in which it is not true. Ideas may be true in the sense that they refer to real objects in the external world. Locke calls these objects archetypes.

We cannot say that the sensations in our minds are like the qualities in the objects in any respect except that of having the power to cause these sensations. Simple ideas are the ones that are most likely to be true in this respect. Complex ideas which are formed by the processes of combining, comparing, and abstracting may be said to be true in the sense that they are adequate for communicating to another person's mind ideas which are like the ones in our own mind. They cannot be said to be true in the sense that they are like some object that is external to the mind. It is on this point that Locke differs from the rationalists, who have always insisted that universals refer to realities which exist independent of our human minds. For Locke, they are merely creations of the mind which serve a useful purpose in enabling human beings to communicate with one another.


Locke's account of complex ideas is an attempt to explain the processes by which the mind arrives at all of its various conceptions concerning both itself and the world to which it belongs. It was a tremendous task, the pursuit of which involved not only an enormous amount of detailed analyses but also uncovered a multitude of problems that were more difficult than he had imagined when the work was started. Having proved, at least to his own satisfaction, that innate ideas do not exist, he found it necessary to account for such ideas as selfhood, cause and effect relationships, personal identity, class names, abstract principles, and all those objects that are designated by class names or universals on the basis of sense perceptions and the reflections of the mind on these sensations.

The basic conviction on which he constructed his entire theory of knowledge was that all of the kinds of complex ideas are derived from simple ones which precede in point of time the combinations, comparisons, and abstractions that are formed. This type of analysis necessarily had implications for the field of psychology, and it can be said that the psychological approach to philosophical problems which became dominant during the two centuries that followed Locke was due in no small measure to his influence.

The apparent success of Locke's work during the period which immediately followed its publication was due in part to the fact that he was able to use the new empirical method and still retain his belief in the validity of many of those ideas which his rationalistic predecessors had supposed it impossible to defend on any other than non-empirical grounds. The fact that he was not able to do this with complete consistency was not discovered all at once. It required the work of several of his more critical successors to bring these inconsistencies to light. However, in fairness to Locke, it should be remembered that he was a pioneer in this field, and the sciences of his day had not at that time achieved the progress that they acquired in later years.

Throughout the entire Essay, it is evident that Locke never questioned the existence of an external world which is independent of the mind that perceives it. His problem was that of determining the extent to which one's ideas about that world could be regarded as true. In the case of simple ideas, he believed it was possible to maintain a real correspondence between the sensations that occur in one's mind and the qualities that exist in the outside world.

This is what may roughly be regarded as an example of the correspondence theory of truth, and it is in this respect that he may be considered as one of the forerunners of what is now known as critical realism. One of the major difficulties involved in this conception arises from the fact that not all of the qualities present in sensation can be said to exist in the external object. The primary qualities, such as size, weight, and motion, may be regarded as present in the object, but the secondary qualities of color, sound, taste, and touch are only in the mind of the receiving subject.

Locke apparently recognized this difficulty, for in some parts of his discussion he insists that we can know nothing about the independent character of that which is external to the mind. In other parts of his discussion, he departs from this position and says that we can know something about it. We not only know that external objects exist but that they have the power to cause the sensations which occur in the mind. In the case of the primary qualities, what exists in the mind is said to be like that which exists in the objects, but with the secondary qualities this is not true. All that can be said with reference to them is that the objects possess whatever power it takes to produce the sensations. Even this much is not warranted on the basis of Locke's method, for as we have indicated before, the logical consequence of his method is complete skepticism about what is external to the mind even though he does not pursue the method to this extent.

The significance of Locke's method is even more apparent in the light of his treatment concerning complex ideas. It is in this area that he abandons any attempt to show a correspondence between ideas and the objects for which they stand. In fact, so far as the most of his examples are concerned, he denies that there are any concrete objects to which they refer. They are only creations of the mind, which are useful for the purposes of communication, but they have no existence that is separate from, or independent of, the mind which conceives them.

This is a point of view that had important consequences for the future development of epistemological theories. It meant a change of direction in the course of inquiry. Instead of trying to discover the nature of the object to which the ideas might refer, the purpose of the investigation is to understood to be that of discovering the way in which these ideas are formed in the human mind. This can be seen in the analysis that Locke makes of the ideas of space and time.

According to the Newtonian conception, which was generally accepted in Locke's day, both space and time have an existence in the outside, or external, world. In contrast with this view, Locke shows how they are derived from the mind's reflection on the particular sensations that have occurred and the order and manner of their appearance and disappearance. In this way he anticipated the subjective theories of space and time which were developed at a later time by Immanuel Kant.

Locke's account of class names, or what had long been known as universals, brings to light one of the most important implications of his theory. Rationalistic thinkers had always insisted that because ideas of this kind are eternal and unchanging, they are the only ones that can rightfully be called real. In contrast with them, it was believed that the ideas derived from sense perceptions can be called real only insofar as the universal ideas are present in them. In other words, that which changes was regarded as unreal, and only the permanent and unchanging was real. Locke's theory reverses this conception entirely. For him, only that which is revealed through the senses is real. Since only particular objects are revealed in this way, it follows that universals must be regarded as mere abstractions which have no independent existence.

This was a revival of the medieval doctrine known as nominalism, according to which universals are merely names that stand for nothing other than the particular ideas that have been present in the mind. Had Locke remained true to this position throughout his entire discussion, he could not have maintained his belief in substances, either material or spiritual. Some recognition of this fact seems to be implied in the ambiguity that is involved in his conception of substance. Actually he uses this term with three different meanings. Sometimes he speaks of substances, both spiritual and material, as though they existed independent of any mind. At other times, he writes as though substances were nothing more than the ideas that have been created by human minds. Finally, he speaks of substance as an unknown substratum, "a something we know not what."

The problem concerning personal identity is another one in which the implications of Locke's method lead to a conclusion that he does not accept. Apparently he wants to retain the belief that it is the same person who passes through the successive stages of infancy, childhood, adolescence, and adulthood. Certainly there is no sense impression that indicates an object of this kind. To be sure, Locke has a kind of explanation to account for continuity of experiences that are included in one's life as a whole. Each moment of one's life, he tells us, acts in a causal way to determine what the next moment will be like. Hence, there is a sense in which the person at one moment in life may be said to be responsible for deeds that were done earlier.

But the question still remains concerning what it is that constitutes the unity that binds together the successive moments of existence. On the basis of Locke's empirical method, we can only say that the person is a complex idea made up by the mind out of a series of simple ideas. It does not refer to a reality that is other than the particular sensations of which it is composed. This is scarcely sufficient to account for a personality who is morally responsible for the deeds he performs, and yet this is the sense in which the term person is generally used and understood. Once more it is the question of whether universals can be regarded as real. Since universals and particulars are correlatives in the sense that neither one is meaningful apart from the other, there seems to be no good reason why one of them should be considered as real in preference to the other.

The same thing may be said with reference to the idea of causality. On this point Locke seems to be reluctant to admit the logical consequence of his empirical method. Since the notion of necessity that is involved in the idea of a causal relation is something that cannot be experience by the senses or discovered by the reflection of the mind on sensations, there is no basis for claiming that it has any real existence. The scientists of Locke's day had assumed without question that causation in the sense of a necessary connection between events was a characteristic of the external world. Locke accepts their position and continues to hold it even though no support can be found for it in the actual experiences of either sensation or reflection.

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