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Locke: Knowledge and its Limits


The idea of a particular substance is the complex idea of a set of coexisting qualities and powers, together with the supposition that there is some unknown substrate upon which they all depend. Locke is derisive about the confused idea of this something, "we know not what," that is supposed by scholastic philosophers. (Essay II xxiii 2) But he cannot eliminate the concept of substance altogether, since he, too, must account for the existence and coherence of just this group of features.

About species or kinds of substances, Locke offers a more sophisticated explanation. Our complex idea of a specific kind of substance—"gold" or "horse," for example—is the collection of features by reference to which we classify individual substances as belonging to that kind. (Essay II xxiii 6-8) These nominal essences, developed for our convenience in sorting things into kinds, rely heavily upon the secondary qualities and powers that are the most obvious features of such things in our experience—the color, weight, and malleability of gold, for example, and the shape, noises, and movements of horses.

As a corpuscularian, Locke supposed that individual substances must also have real essences, the primary qualities of their insensible parts, which cause all of their qualities. But since we cannot observe the "real inner constitutions" of things, we cannot use them for purposes of classification, nor can we even understand their causal influence on our perception. (Essay III vi 6) Since Locke doubted that real essences could ever be discovered, he was thrown back on the supposition of an underlying reality which we cannot know.

This account imposes a severe limitation on the possibilities of our knowledge of substances. According to Locke, the mechanical operations of nature remain hidden to us. Careful observation and experimentation may support a reliable set of generalizations about the appearances of the kinds of things we commonly encounter, but we cannot even conceive of their true natures.

Personal Identity

Among our ideas of relations, the strongest is that of identity. Locke held that the criteria for identity depend upon the kind of thing we are considering. Substantial identity requires the unique spatio-temporal history that is just the existence of each substance, but this is not the only consideration in all cases.

The identity of the tree outside my window, for example, does not depend on the substantial identity of its parts (in fact, they change from day to day and season to season); what matters in this case is the organization of those parts into a common life. A similar explanation, Locke held, accounts for the identity of animals and human beings. (Essay II xxvii 4-6) We recognize living bodies at different times by the organization of their material parts rather than by their substantial composition.

In analogous fashion, Locke explained personal identity independently of identity of substance. The idea of the person is that of a moral agent who can be held responsible for his or her actions. (Essay II xxvii 9) But Locke used a series of hypothetical examples to show that the identity of an underlying immaterial substance or soul is neither necessary nor sufficient for personal identity in this sense. Even the identity of the same human body (though we may rely upon that for third-person attributions of identity) is not truly relevant. The only thing that does matter, on Locke's view, is that the person self-consciously appropriates actions as its own.

This is, as Locke says, a "forensic" notion of personal identity; its aim is to secure the justice and effectiveness of moral sanctions. (Essay II xxvii 26) If, and only if, I now remember having committed a particular act in the past can I be justly punished for having done so. If, and only if, I project myself into the future can the prospect of punishment or reward influence my deliberations about how to act now. Locke's way of thinking about personal identity has shaped discussions of the issue ever since.


Locke devoted Book III of the Essay to a discussion of language. His basic notion is clear: words signify ideas. Thus, the meaning of a word is always the idea it signifies in the minds of those who use it. (Essay III ii 2) Of course, those ideas are presumed in turn to represent things, but the accuracy of that representation does not directly affect the meaning of the word. The names of substances, for example, signify the complex ideas Locke called their nominal essences, not the real nature of the substances themselves. Thus, common names for substances are general terms by means of which we classify things as we observe them to be; we can agree upon the meaning of such terms even though we remain ignorant of the real essences of the things themselves.

The chief point of Locke's theory of language was to eliminate the verbal disputes that arise when words are used without clear signification. It is always reasonable to ask for the meaning of a word, that is, to know what idea it signifies. If a speaker cannot supply the idea behind the word, then it has no meaning. Many of the academic squabbles that obstruct advancement in human knowledge, Locke believed, could be dissolved by careful attention to the meaning of words.

Knowledge and its Degrees

Having provided a thorough account of the origins of our ideas in experience, Locke opens Book IV of the Essay with a deceptively simple definition of knowledge. Knowledge is just perception of the agreement or disagreement of our ideas. (Essay IV i 2) We know the truth of a proposition when we become aware of the relation between the ideas it conjoins. This can occur in any of three distinct ways, each with its characteristic degree of certainty.

Intuitive knowledge involves direct and immediate recognition of the agreement or disagreement of two ideas. (Essay IV ii 1) It yields perfect certainty, but is only rarely available to us. I know intuitively that three is not the same as seven.

In demonstrative knowledge we perceive the agreement or disagreement only indirectly, by means of a series of intermediate ideas. (Essay IV ii 2) Since demonstration is a chain of reasoning, its certainty is no greater than its weakest link; only if each step is itself intuitively known will the demonstration as a whole be certain. If I know that A is greater than B and that B is greater than C, then I know demonstratively that A is greater than C.

Although intuition and demonstration alone satisfy the definition of knowledge, Locke held that the belief that our sensory ideas are caused by existing things deserves the name of sensitive knowledge. (Essay IV ii 14) In the presence of a powerful, present idea of sensation, we cannot doubt that it has some real cause outside us, even though we do not know what that cause may be or how it produces the idea in us. I have only sensitive knowledge that there is something producing the odor I now smell.

Types of Knowledge

Locke distinguished four sorts of agreement or disagreement between ideas, perception of which gives us four distinct types of knowledge: (Essay IV i 3-7)

Since knowledge of identity and diversity requires only a direct comparison of the ideas involved, it is intuitive whenever the ideas being compared are clear.

Knowledge of coexistence would provide detailed information about features of the natural world that occur together in our experience, but this scientific knowledge is restricted by our ignorance of the real essences of substances; the best we can do is to rely upon careful observations of the coincidental appearance of their secondary qualities and powers.

Mathematics and morality rest upon knowledge of relation, which Locke held to be demonstrative whenever we form clear ideas and discover the links between them.

The degree of certainty in our knowledge of real existence depends wholly upon the content of our ideas in each case. Locke agreed with Descartes that we have intuitive knowledge of our own existence, and he supposed it possible to achieve demonstrative knowledge of god as the thinking creator of everything. But we have only sensitive knowledge of the existence of other things presently before our senses.

The Extent of Knowledge

The result of all of this is that our knowledge is severely limited in its extent. On Locke's definition, we can achieve genuine knowledge only when we have clear ideas and can trace the connection between them enough to perceive their agreement or disagreement. (Essay IV iii 1-6) That doesn't happen very often, especially where substances are at issue. The truths of mathematics are demonstrable precisely because they are abstract: since my ideas of lines, angles, and triangles are formed without any necessary reference to existing things, I can prove that the interior angles of any triangle add up to a straight line.

But any effort to achieve genuine knowledge of the natural world must founder on our ignorance of substances. We have "sensitive knowledge" of the existence of something that causes our present sensory ideas. But we do not have adequate ideas of the real essence of any substance, and even if we did, we would be unable to discover any demonstrative link between that real essence and the ideas it produces in us. The most careful observation can establish at best only the secondary qualities and powers that appear to coexist in our experience often enough to warrant our use of them as the nominal essence of a kind of substance. (Essay IV xi 1-7)

Locke's efforts have therefore led to a sobering conclusion. Certainty is rarely within our reach; we must often be content with probable knowledge or mere opinion. Locke ultimately recommends that we adopt significantly reduced epistemological expectations.

The Great Concernments

Despite all of these limitations, Locke believed that human knowledge is well-suited for the conduct of human life. We have all the knowledge we need to secure our "great concernments:" convenience in this life and the means for attaining a better life hereafter. (Essay IV xii 11)

Survival and comfort in daily life are attainable in spite of our ignorance of the hidden operations of nature. We don't need to know the real essences of substances in order to make use of them productively. (Indeed, Locke suggests, additional information might actually make daily life more difficult.) Surely demonstrative knowledge of the true nature of fire or food is unnecessary for my survival; my natural aversion to the pain of being burned and desire for the pleasures of eating provide ample practical guidance.

Doing the right thing is also possible, since our action is properly guided by a demonstrable morality. The truths of morality are demonstrable for the same reason that the truths of mathematics are: the mixed modes that describe possible human actions, of the moral rules that govern them, and even of the possible agents that might perform them, are all complex ideas manufactured by the mind without reference to the real existence of substantial beings, so I can prove that murder is wrong.

Finally, we have all the knowledge we need to enter into a proper relation to our creator. God's existence is demonstrable on rational grounds, and the scriptures provide us with detailed information about the divine will for our lives. (The precise boundary between reason and revelation, Locke held, is itself known only as a matter of probable knowledge or opinion.)

In the end, then, Locke believed that we have no reason to complain. Although restricted in extent, our knowledge is sufficient for our needs. (Essay IV xiv 1-2) Respecting its limits will prevent us from wasting effort on pointless wrangling. Since our experience is itself limited, an empiricist epistemology can only advise caution and modesty in our claims to know.

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