An Essay Concerning Human Understanding

by John Locke

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John Locke’s An Essay Concerning Human Understanding is the first major presentation of the empirical theory of knowledge that was to play such an important role in British philosophy. The author had studied at Oxford, and later he became a medical doctor. Although he did not practice much, he was greatly interested in the developments current in medical and physical science, and there is some evidence that he first began to formulate his theory of knowledge in terms of considerations arising from medical researches of the day. Locke was a member of the Royal Society of England, where he came into contact with many of the important experimental scientists, such as Robert Boyle and Sir Isaac Newton. A discussion with some of his friends seems to have been the immediate occasion of the writing of An Essay Concerning Human Understanding, in which Locke attempted to work out a theory of knowledge in keeping with the developing scientific findings and outlook.

The completed version of the work dates from the period when Locke, along with his patron, the Anthony Ashley Cooper, the third earl of Shaftesbury, was a political refugee in Holland. After the Glorious Revolution of 1688, Locke returned to England and was quickly recognized as the leading spokesperson for the democratic system of government that was emerging in his homeland. The essay, first published in the same year as Locke’s famous work in political philosophy, Two Treatises of Government, quickly established the author as the foremost spokesperson for the new empirical philosophical point of view that was to dominate English philosophy from then on.

Seeking the Origins of Human Knowledge

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The question to which Locke addressed himself in his essay is that of “the origin[s], certainty, and extent of human knowledge, together with the grounds and degrees of belief, opinion, and assent.” By using what he called “this historical, plain method,” Locke hoped to discover where our ideas and our knowledge come from, what we are capable of knowing about, how certain our knowledge actually is, and when we may be justified in holding opinions based on our ideas. The value of such an undertaking, Locke asserted, is that one would thus know the powers and the limits of human understanding, so that “the busy mind of man” would then restrict itself to considering only those questions with which it was actually capable of dealing and would “sit down in a quiet ignorance of those things” beyond the reach of its capacities.

Before commencing his investigations, Locke pointed out that human beings do, in fact, have adequate knowledge to enable them to function in the condition in which they find themselves. Therefore, even if the result of seeking the origin, nature, and extent of our knowledge leads us to the conclusion that we are unable to obtain complete certitude on various matters, this should not be grounds for despair, for skepticism, or for intellectual idleness. People have wasted too much time, Locke insisted, in bemoaning their intellectual situation or in disputing in areas in which satisfactory conclusions are impossible. Instead, he said, we should find out our abilities and our limitations, and then operate within them.

The Question of Innate Knowledge

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The first book of An Essay Concerning Human Understanding deals with one theory about the origin of our ideas, the thesis that our knowledge is based upon certain innate principles that are supposed to be “stamped upon the mind of man.” Locke severely criticized this theory, especially in the form in which it had been presented by thinkers such as Herbert of Cherbury. Adherents of this theory of innate ideas had maintained that the universal agreement of...

(This entire section contains 229 words.)

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humankind regarding certain principles showed that these must be innate. Locke argued in opposition that the fact of universal agreement would be insufficient evidence as to the source of the principles in question. He also argued that, in fact, there actually are no principles that are universally agreed to, since children and idiots do not seem to know or believe the principles that are usually cited as examples of innate ideas. The way in which children acquire knowledge about the principles in question, through the learning process, further indicates that they are not born with innate ideas.

After having criticized the innate idea theory, Locke turned next to the positive side of his investigation. We do have ideas (an idea being defined as whatever is the object of the understanding when a person thinks); this is beyond any possible doubt. Then, if the ideas are not innate, where do they come from?

Empirical Origins of Knowledge

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The second book of An Essay Concerning Human Understanding begins the development of a hypothesis about the origins of human knowledge, namely, the empirical theory. Let us suppose, Locke said, that the mind initially is just a blank tablet (a tabula rasa). Where, then, does it obtain its ideas? From experience, Locke proclaimed. Experience comprises two sources of ideas, sensation and reflection. We receive many, if not most, of our ideas when our sense organs are affected by external objects. We receive other ideas by reflection when we perceive the operations of our minds on the ideas that we have already received. Sensation provides us with ideas of qualities, such as the ideas of yellow or of heat. Reflection provides us with ideas such as those of thinking, willing, and doubting. These two sources, Locke insisted, give us all the ideas that we possess. If anyone has any doubts about this, let that person simply inspect his or her own ideas and see if there are any that have not come to him or her either by sensation or reflection. The development of children also provides a further confirmation of this empirical theory of the origin of human knowledge. As the child receives more ideas from sensation, and reflects on them, that child’s knowledge gradually increases.

Ideas: Simple and Complex

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Having thus answered the question concerning the origin of our ideas, Locke proceeded to investigate the nature of the ideas that we possess. All of our ideas are either simple or complex. A simple idea is one that is uncompounded, that contains nothing but one uniform appearance, and that cannot be distinguished into different ideas. An example of a simple idea would be the smell of a rose. A complex idea, in contrast, is one that is composed of two or more simples, such as a yellow and fragrant idea. The simples, Locke insisted, can neither be created nor be destroyed by the mind. The mind has the power to repeat, compare, and unite the simples, thereby creating new complex ideas. However, the mind cannot invent simple ideas that it has not experienced. The simples, in the Lockean theory of knowledge, are the building blocks from which all of our complex and compounded ideas can be constructed and accounted for.

Many of the simple ideas are conveyed by one sense, such as the ideas of colors, sounds, tastes, smells, and touches. One crucial case for which Locke argued is the idea of solidity, which he claimed we receive by touch. This idea is that of a basic quality of bodies. It is not the same as the space that bodies occupy, nor is it the same as the subjective experience of hardness that we receive when we feel objects. Instead, for Locke, solidity is akin to the fundamental physical notion of “mass” in Newtonian physics. It is that which makes up bodies. To those who doubted that they were actually acquainted with such an idea, Locke suggested that they place a physical object, such as a ball, between their hands and then try to join them. Such an experience, presumably, would provide a complete and adequate knowledge of solidity—or at least as complete and adequate an idea as we are capable of obtaining of any simple idea. The importance of this idea in Locke’s theory will be seen shortly with regard to his theory of primary and secondary qualities.

Some of our ideas are conveyed by two or more senses. Locke included in this group the ideas of space or extension, figure, rest, and motion, which, he said, we receive by means of both sight and touch. Other ideas come from reflection. Still others are the result of both reflection and sensation. Included in this latter group are the ideas of pleasure and pain and the idea of power (which we gain from reflecting on our experience of our own ability to move parts of ourselves at will).

Qualities of Objects

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If these are the types of ideas that we possess, classified according to their sources, can we distinguish those ideas that resemble actual features, or qualities of objects, and those that do not? The qualities of objects are divided by Locke into two categories, the primary and the secondary ones. The primary ones are those that are inseparable from bodies no matter what state the object may be in. This group includes solidity, extension, figure, mobility, and number. In contrast, the secondary qualities “are nothing in the objects themselves, but the powers to produce various sensations in us by their primary qualities,” such as the power of an object, through the motion of its solid, extended parts, to produce sounds, tastes, and odors in us when we are affected by it.

Thus, in Locke’s theory, objects possess primary qualities, the basic ingredients of Newtonian physics, and they possess secondary ones, which are actually the powers of the primary qualities to cause us to perceive features, such as colors, odors, and so on, which are not “in” the objects themselves. In terms of this distinction, we can say that our ideas of primary qualities resemble the characteristics of existing objects outside us, whereas our ideas of secondary qualities do not. The primary qualities of things are really in them, whereas the secondary qualities, as perceived sensations, are only in the observer. If there were no observers, only the primary qualities and their powers would exist. Hence, the rich, colorful, tasteful, noisy, odorous world of our experience is only the way we are affected by objects, not the way objects actually are. This distinction between our ideas of primary and secondary qualities led Locke to argue that some of our ideas give us genuine information about reality, while others do not.

The Idea of Substance

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In the remainder of the second book of An Essay Concerning Human Understanding, Locke surveyed the various other kinds of ideas that we possess, those gained by reflection, those that are complexes, and so on. The most important, in terms of his theory and in terms of later philosophy, is the complex idea of substance. The idea of substance originates from the fact that in our experience a great many simple ideas constantly occur together. We then presume them to belong to one thing because we cannot imagine how these simple ideas can subsist by themselves. Therefore, we accustom ourselves to suppose that there must be some substratum in which the ideas subsist, and we call this substratum a substance. When we ask ourselves what idea we actually have of a substance, we find that our idea is only that of a something to which the constantly conjoined ideas belong. When we try to find out what this something is, we discover that we do not know, except that we suppose it must be a something that can support or contain the qualities that could produce the collection of simple ideas in us. If we attempt to find out something more definite about the nature of substance, we discover that we cannot. What do color and weight belong to? If we answer, to the extended solid parts, then to what do these belong? It is, Locke suggested, similar to the case of the Indian philosopher who said that the world is supported by a great elephant. When asked what supported the elephant, he replied that it rested on a great tortoise. When asked what the tortoise rested on, he conceded and said, “I know not what.” This, Locke asserted, is all that can finally be said of the nature of substance. It is something—we know not what—which we suppose is the support of the qualities that we perceive or that affect us.

Each constantly conjoined group of qualities we assume belongs to some particular substance, which we name “horse,” “gold,” “man,” and so on. We possess no clear idea of substance, either in the case of physical things or in the case of spiritual things. However, we find that we cannot believe that either the physical qualities or the mental ones that we always experience together can exist without belonging to something. Therefore, although we have no definite ideas, we assume that there must be both bodies and spirits underlying and supporting the qualities that give rise to our ideas. Our inability to obtain clear ideas of substances, however, forever prevents us from gaining genuine knowledge about the real nature of things.

Ideas and Reality

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At the end of the second book of An Essay Concerning Human Understanding, Locke evaluated what he had discovered about the nature of our ideas. This evaluation commences the examination of the problem of the extent and certitude of our knowledge, which is developed at length in the fourth book. Our ideas are real, Locke contended, when they have a foundation in nature and when they conform with the real character of things. In this sense, all simple ideas are real because they must be the result of genuine events and things (since the mind cannot create them but receives them from experience). However, not all real ideas are necessarily adequate representations of what does in fact exist. Ideas of primary qualities are both real and adequate. Ideas of secondary qualities are real but only partially represent what is outside us. They represent powers that exist but not features corresponding to the ones that we perceive. The ideas of substances that we have are very inadequate, because we are never sure that we are aware of all the qualities that are joined together in one substance, nor are we sure of why they are so joined. Hence, some of our ideas tell us what is really outside us, whereas other ideas, caused by what is outside us or by our reflection on our ideas, do not adequately represent “real” objects.

Later philosophers, such as George Berkeley and David Hume, were to argue that once Locke had admitted that some of our ideas were neither representative of reality nor adequate to portray reality, he could not then be certain that any of our ideas actually correspond to real features of the world. Hence, they contended that Locke, in trying to build from an empirical theory of knowledge to genuine knowledge of reality, had actually laid the groundwork for a skeptical denial of the contention that people can know anything beyond the ideas in their own minds. Locke’s theory rested on maintaining that our ideas of primary qualities resemble genuine characteristics of reality. However, the opponents argued, primary qualities are really no different from secondary qualities, as we know them, and hence we have no assurance from the ideas themselves that some are real and adequate and others are not.

Words and Language

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The third book of An Essay Concerning Human Understanding appears to deal with some unrelated topics, those concerning the nature of words and language. This book, which evoked the interest of those concerned with linguistic philosophy, covers problems normally dealt with in anthropology, psychology, linguistics, and philosophy.

Two points that are raised are of central importance to Locke’s main theme of the nature and extent of our knowledge and played a role in the later history of empirical philosophy. One of these is Locke’s theory concerning the meaning and referrent of general terms, such as “man” and “triangle.” All things that exist, Locke asserted, are particular, but by abstracting from our ideas of things, by separating from them particular details or features, we finally form a general idea. In this way, we arrive at the general abstract ideas about which we reason. Berkeley and Hume both challenged Locke on this point and insisted that we do not, in fact, possess any abstract general ideas. Hence, they insisted that an empirical account of our ideas of so-called general terms must be developed from the particular ideas that we have.

One of the general terms that Locke claimed gained some meaning from the abstracting process is “substance.” However, when he analyzed what we might mean by the term, Locke distinguished between what he called “the nominal essence” and “the real essence” of a substance. The nominal essence is that abstract general idea of a substance formed by abstracting the basic group of features that constantly occur together. The real essence, in contrast, is the nature of the object that accounts for its having the properties that it does. The nominal essence describes what properties a substance has, whereas the real essence explains why it has these properties. Unfortunately, Locke pointed out, we can never know the real essence of anything, since our information, which we abstract from, deals only with the qualities that we experience and never with the ultimate causes that account for the occurrence of these properties. Thus, our knowledge of things is sure to be sharply curtailed because of the fact that we will never discover the reasons why things have the characteristics that they have.

Knowledge as Comparison of Ideas

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The fourth and last book of An Essay Concerning Human Understanding deals with knowledge in general, with the scope of knowledge, and with the question of how certain we can be of such knowledge. Our knowledge deals only with ideas, since these are the only items with which the mind is directly acquainted. What constitutes knowledge, according to Locke, is the perception of the agreement or disagreement of two ideas. Ideas may agree or disagree in four ways. They may possibly be identical or diverse. They may be related in some respect. They may agree in coexisting in the same subject or substance. They may agree or disagree in having a real existence outside the mind. All of our knowledge, Locke insisted, falls under these headings. We know either that some ideas are the same or different, or that they are related, or that they always coexist, or that they really exist independently of our minds.

If these are the kinds of items that we can know about, how can we gain such knowledge? One source of our knowledge is intuition, the direct and immediate perception of the agreement or disagreement of any two ideas. The mind “sees” that black is not white and that a circle is not a triangle. Also, “this kind of knowledge is the clearest and most certain that human frailty is capable of.” Anyone who demands more certainty than that gained by intuition “demands he knows not what, and shows only that he has a mind to be a sceptic, without being able to be so.” All certain knowledge depends upon intuition as its source and guarantee.

The Problem of Certitude

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We acquire knowledge not only by directly inspecting ideas but also through demonstrations. According to Locke, when we know by demonstration, we do not see immediately that two ideas agree or disagree, but we see immediately by means of connecting two ideas with others until we are able to connect them with each other. This process is actually a series of intuitions, and each step in a demonstration is therefore certain. However, because the steps occur successively in the mind, error is possible if we forget the previous steps or if we assume that one has occurred if it actually has not. Intuition and demonstration are the only two sources of certain knowledge.

However, there is another source of knowledge that has a degree of certitude assuring us of truths about particular experiences. This kind of knowledge goes beyond bare probability but does not reach genuine certainty. It is called “sensitive knowledge,” which is the assurance that we have on the occurrence of specific experiences, that certain external objects actually exist that cause or produce these experiences. We cannot reasonably believe, Locke insisted, that all of our experiences are imaginary or are just part of a dream. Hence, we have sensitive knowledge, a degree of assurance that something real is going on outside us.

The Extent of Human Knowledge

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In terms of these kinds of knowledge, types of sources, and degrees of certainty, it is now possible to outline the extent of human knowledge and to evaluate what we can actually know about the real world. We can gain knowledge only to the extent that we can discover agreements or disagreements among our ideas. Because we can neither intuit nor demonstrate all the relations that ideas can have with one another, our knowledge is not even as extensive as our ideas. In almost all cases, we can determine with certainty whether our ideas are identical or different from one another. We can tell if our ideas are related to others only when we can discover sufficient intermediary ideas. In fields such as mathematics, we keep expanding our knowledge as more connections between ideas are intuited or demonstrated. The areas in which we seem to be most limited in gaining knowledge are those dealing with the coexistence and real existence of ideas. Because we can never know the real essence of any substance, we can never know why any two ideas must necessarily coexist. We never discover why particular secondary qualities occur when a specific arrangement of primary qualities exists. We are aware of the fact that certain ideas occur over and over again in combination, but we do not know why they do this. With regard to real existence, we are, Locke maintained, intuitively certain of our own existence and demonstratively certain that God exists. We are only sensitively certain that anything else exists, which means that we have serious assurance that objects other than ourselves and God exist only when we have experiences that we feel must be caused by something outside us. Our assurance in these cases is limited to the actual moment when we are having these experiences. Once an experience is over, we have no certitude at all that the object that caused the experience still continues to exist. All that we can know about an object when we know that it exists is that at such times it actually possesses the primary qualities that we perceive, together with the power to produce the other effects that we experience.

This assessment of the extent of our knowledge indicates, according to Locke, that we can never know enough to develop a genuine, certain science of bodies or of spirits, because our information about their existence and their natures is so extremely limited. However, we can obtain sufficient knowledge and probable information to satisfy our needs in this world, so we should not despair or become skeptical just because investigation has revealed how limited our knowledge actually is and how uncertain it is in many areas.

Foundations of Empiricism

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Locke’s An Essay Concerning Human Understanding represents the first major modern presentation of the empirical theory of knowledge. In developing an account of human knowledge in terms of how it is derived from experience, what its nature is, and how limited it is, Locke provided the basic pattern of future empirical philosophy. In attempting to justify some basis for maintaining that we can have some knowledge of some aspects of reality, Locke raised many of the problems that have remained current in philosophical discussions up to the present time. Empiricists after Locke, such as Berkeley and Hume, showed that if one consistently followed out the thesis that all of our knowledge comes from experience, one could not be certain that substances exist or that anything exists beyond the ideas directly perceived. Locke’s Essay is the source of many of the methods, ideas, and problems that have prevailed in philosophy, especially in British and American epistemology, ever since its first publication.


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Additional Reading

Ayers, Michael. Locke. New York: Routledge, 1999. An excellent biographical introduction to the thoughts of the philosopher, clearly presented and requiring no special background. Bibliography.

Brantley, Richard E. Locke, Wesley, and the Method of English Romanticism. Gainesville: University of Florida Press, 1984. Brantley alleges that John Locke influenced John Wesley, the founder of the Methodist Church, and that Wesley’s work influenced the eighteenth century Romantic poets.

Chappell, Vere, ed. John Locke: Theory of Knowledge. New York: Garland, 1992. Thirty critical essays about the philosopher and his ideas concerning knowledge, reprinted from their original locations. This book is volume 8 in a series; Locke’s philosophy of politics is contained in volume 9.

Kramer, Matthew H. John Locke and the Origins of Private Property. Cambridge, England: Cambridge University Press, 1997. A detailed analysis of Locke’s theories about the rewards of labor and the relationships between labor and ownership.

Lennon, Thomas M. The Battle of the Gods and Giants. Princeton, N.J.: Princeton University Press, 1993. This book is about René Descartes and Pierre Gassendi, but a forty-page chapter and a few pages at the end of the book are devoted to Locke’s inheritance from both philosophers.

Schouls, Peter A. Reasoned Freedom: John Locke and the Enlightenment. Ithaca, N.Y.: Cornell University Press, 1992. Compares Locke to Descartes, then invites comparison to other philosophers, such as Michael Oakeshott, by presenting Locke’s defense of reason and his explications of freedom, self-determination, and education.

Seliger, M. The Liberal Politics of John Locke. London: George Allen and Unwin, 1968. Seliger attempts to explain the ambiguities and contradictions of Locke and thereby explain his system. He has more argument with opinions about Locke than he has with Locke.

Tully, James. A Discourse on Property: John Locke and His Adversaries. Cambridge, England.: Cambridge University Press, 1980. Locke’s theories of property have long caused argument among political thinkers. This book is Tully’s attempt to reconsider what Locke said.

Zuckert, Michael P. Natural Rights and the New Republicanism. Princeton, N.J.: Princeton University Press, 1994. Locke (and many others) wrote about “natural rights.” This book about the thought of a number of philosophers features text about Locke and his influence on the Americans, on questions about natural law, on government, and on property.


Critical Essays