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An Essay Concerning Human Understanding is one of John Locke's two most famous works, the other being his Second Treatise on Civil Government. First appearing in 1689, the essay concerns the foundation of human knowledge and understanding. He describes the mind at birth as a blank slate (tabula rasa), filled later through experience. The essay was one of the principal sources of empiricism in modern philosophy, and influenced many enlightenment philosophers, such as David Hume and Bishop Berkeley.

Locke drafted the Essay over a period of about 18 years. In the "Epistle to the Reader," Locke writes that the germ of the essay sprang from a conversation with friends. At a point where this discourse seemed stuck, Locke remarked that it could not proceed without a close examination of "our own abilities and...what objects our understandings were, or were not, fitted to deal with." This conversation occurred around 1671, and in that year Locke formulated two drafts of the Essay. He would continue to work on it for nearly two decades, clarifying and expanding his basic position. Though dated 1693, the book actually first appeared in 1689.

Book II of the Essay sets out Locke's theory of ideas, including his distinction between passively acquired simple ideas, such as "red," "sweet," "round," etc., and actively built complex ideas, such as numbers, causes and effects, abstract ideas, ideas of substances, identity, and diversity. Locke also distinguishes between the truly existing primary qualities of bodies, like shape, motion and the arrangement of minute particles, and the secondary qualities that are "powers to produce various sensations in us"

such as "red" and "sweet." These secondary qualities, Locke claims, are dependent on the primary qualities. He also offers a theory of personal identity, offering a largely psychological criterion. Book III is concerned with language, and Book IV with knowledge, including intuition, mathematics, moral philosophy, natural philosophy ("science"), faith, and opinion.


Locke's main thesis is that the mind of a newborn is a blank slate and that all ideas are developed from experience. Book I of the Essay is devoted to an attack on the doctrine of innate ideas. Locke allowed that some ideas are in the mind from an early age, but argued that such ideas are furnished by the senses starting at birth: for instance, differences between colors or tastes. If we have a universal understanding of a concept like sweetness, it is not because this is an innate idea, but because we are all exposed to sweet tastes at an early age.

Along these lines, Locke also argued that people have no innate principles. Locke contended that innate principles would rely upon innate ideas, which do not exist. For instance, we cannot have an innate sense that God should be worshipped, when we cannot even agree on a conception of God or whether God exists at all.

One of Locke's fundamental arguments against innate ideas is the very fact that there is no truth to which all people attest. He took the time to argue against a number of propositions that rationalists offer as universally accepted truth, for instance the principle of identity, pointing out that at the very least children and idiots are often unaware of these propositions.

Whereas Book I is intended to reject the doctrine of innate ideas proposed by Descartes and the rationalists, Book II explains that every idea is derived from experience either by sensation – direct sensory information – or reflection – mental construction.


The close of Book II suggests that Locke discovered a close relationship between words and ideas that prompted him to include a book on language before moving on to discuss knowledge. Book III addresses definitions, names, and the imperfections and abuses of verbal communication. According to Locke's theory of language, the human mind does not have the capacity to grant a name to every single thing it sees, such as individual crows or grains of sand. Man therefore groups ideas into 'general terms', based on the distinguishing features of each individual thing. All cows, for example, have the properties of 'eating grass', 'being milked' etc, and are therefore processed as one large group. When an object or idea has a personal connection to an individual, only then does the need arise for particular names. Hence - to put it crudely - when a specific cow is important to a person, it is brought out of the realm of general ideas, and given a proper name, such as 'Daisy'.

When looking at language, Locke posed that there were two primary uses for language: to identify and store our thoughts, and to communicate those thoughts verbally. Looking further at communication, Locke distiguished between two possible uses of verbal communication. Civil communication related to everyday necessary communication to conduct daily affairs; philosophical communication, he said "may serve to convey the precise notions of things, and to express in general propositions certain and undoubted truths, which the mind may rest upon and be satisfied with in its search after true knowledge."

Book III also included Locke's thoughts on some of the major problems in defining terms. Some them include when "the ideas they stand for are very complex, and made up of a great number of ideas put together; where the ideas they stand for have no certain connection to nature; when the signification of the word is referred to a standard, which standard is not easy to be known; where the signification of the word and the real essence of the thing are not exactly the same."

Locke also posited some situations in which language is abused. After stating that the three ends of language were "to convey our ideas, to do it with quickness, and to convey the knowledge of things," Locke noted that words fail when "[they] are used without any ideas, when complex ideas are without names annexed to them, when the same sign is not put for the same idea, when words are diverted from their common use, and when they are names of fantastical imaginations." In connecting the ends and the abuses, the implication seems to be that failure to heed the ends of communication can lead to the abuses mentioned, among others. He sums up the section with this passage: "He that hath names without ideas, wants meaning in his words, and speaks only empty sounds. He that hath complex ideas without names for them, wants liberty and dispatch in his expressions, and is necessitated to use periphrases. He that uses his words loosely and unsteadily will either be not minded or not understood. He that applies his names to ideas different from their common use, wants propriety in his language, and speaks gibberish. And he that hath the ideas of substances disagreeing with the real existence of things, so far wants the materials of true knolwedge in his understanding, and hath instead thereof chimeras."


All of the previous work that Locke has done in the Essay has been setting up the framework for the investigation of knowledge. In Book IV, Locke finally turns toward knowledge itself, asking what it is and in what areas we can hope to attain it. Locke defines knowledge as "the perception by reason of the connection and agreement or repulsion and disagreement between any two or more of our ideas" (IV.i.2). Because it has only to do with internal relations that hold between ideas knowledge, is not actually of the world itself. Locke identifies four different sorts of agreement and disagreement that reason can perceive in order to produce knowledge: identity and diversity (e.g. A=A); relation (e.g. a diamond is a square laid on its side); coexistence (e.g. that the area of a triangle always equals one half the base time the height); realizing that existence belongs to the very ideas themselves (e.g. the idea of God and of the self). To count as knowledge, the connection between ideas must be very strong. In the case of disagreement, the connection must be one of logical inconsistency, and in the case of agreement, it needs to be a necessary connection. For example, in order to know that A caused B you need to know that given A, B could not have failed to happen. In other words, to know that A caused B, you need to be able to deduce B given only the information that A, or derive B from A. In chapter ii Locke distinguishes between three grades or degrees of knowledge. The highest grade of knowledge is intuition. In intuition, we immediately perceive an agreement or disagreement the moment the ideas are understood. Example of intuitive knowledge are the knowledge that A=A and that all bachelors are unmarried. Understanding what it means to be a bachelor requires feeling the truth of this claim. One grade below intuition is demonstration. In demonstrative knowledge, one must go through some sort of proof to see the connection between ideas. Each step in the proof, however, must be a matter of intuition. An example of demonstrative knowledge would be any proof of geometry. Intuition and demonstration are the only truly legitimate forms of knowledge, so, ultimately all knowledge depends on intuition. There is also, however, a final grade of pseudo-knowledge. This is sensitive knowledge, which is treated at length in Chapter xi. Locke's definition of knowledge was common among 17th century thinkers. Both Rene Descartes and David Hume defined knowledge in much the same way. However, it is tempting to think that this definition is too strong. Consider the following example: I notice that every time my cat makes a sound, it comes out as "meow." In addition, I notice that this same fact holds true of all the cats I have ever come across, and from the testimony of others I gather that the same is true of all cats that anyone has ever observed. While I am tempted to say that I know that all cats say "meow," I have no knowledge of any necessary connection between the cat and the sound "meow." I do not know anything about cats that would show me why cats must say only "meow," nor do I know anything to tell me why they must say "meow" at all. According to Locke, I do not know that all cats say "meow." I may believe this strongly, but I do not know it. Whether or not Locke's definition of knowledge is too strict (and it is not clear that it is; perhaps I really do not know that all cats say "meow), he had good reasons for holding to it. To return to the example above, imagine now that I happen across a cat that makes a sound more like "greck." It turns out I did not know that cats say "meow" after all, since this cat does not. The claim that all cats say "meow" is simply not true, and it is impossible to know something that is not true. I might have thought that I knew that all cats say "meow," but I was mistaken. Is it possible to imagine my coming across such a creature? It is, so long as I do not know of any necessary connection between cats and meows. If, on the other hand, I do know of any such connection, then I know that I will never come across such a cat. To grasp a necessary connection is to know that you will never come across a disconfirming instance. And until you know that you will never come across a disconfirming instance of a rule, can you really know the rule is true? In the absence of this guarantee, there is always the chance that you will happen across something that violates the rule, proving that the rule is wrong and that you could not have known it after all. In all likelihood, this is the reasoning behind Locke's strict definition of knowledge.

Reaction, response, and influenceEdit

Locke's empiricist viewpoint was sharply criticized by rationalists. In 1704 Gottfried Leibniz wrote a rationalist response to Locke's work in the form of a chapter-by-chapter rebuttal, the Nouveaux essais sur l'entendement humain ("New Essays on Human Understanding"). At the same time, Locke's work provided crucial groundwork for the work of future empiricists such as David Hume.


  • Bizzell, Patricia, and Bruce Herzberg, eds. The Rhetorical Tradition. 2nd ed. Boston: Bedford/St. Martin's, 2001.
  • Encyclopedia of Philosophy. New York: Macmillan, 1967. s.v. "Locke, John".

See alsoEdit

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