An Essay Concerning Human Understanding | Study Guide

John Locke

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An Essay Concerning Human Understanding | Vol. 2, Book 4, Chapters 5–6 : Of Knowledge and Probability | Summary



Vol. 2, Book 4, Chapter 5: Of Truth in General

Having just defined knowledge, Locke now defines truth and attempts "to observe how the mind distinguishes it from falsehood." Truth, he says, is "a right joining or separating of ... ideas or words." A mental proposition is true when it correctly connects or separates two ideas; a verbal proposition is true when it correctly connects or separates the words standing for those ideas. Falsehood is the opposite—joining together ideas or words which do not agree. Locke observes that many statements which might be called "true" do not have any referent in the real world (e.g., "All centaurs are animals.") He therefore proposes the idea of "real truth" to describe those statements in which "ideas agree to things" (i.e., to outward reality). It is true in a trivial sense that all centaurs are animals, but it is really true that all human beings are animals.

Vol. 2, Book 4, Chapter 6: Of Universal Propositions, Their Truth and Certainty

This chapter considers a special class of propositions which attempt to capture general truths. In some simple cases, Locke says, we can be confident of the truth of these "universal" propositions. For example, we can be sure that what is cold is not hot, or that what is white is not black. But when it comes to substances Locke is much more skeptical. For him, the real essence of a substance is impossible to know. Consequently, we cannot make universal statements about substances, because we don't truly know what we're talking about when we say "gold" or "snow." The statement "all gold is malleable" makes sense as part of the definition of the word "gold." If a piece of material isn't malleable, then, by this definition, it doesn't belong to the category "gold." But if we take such a statement to mean "whatever has the real essence of gold is malleable," its truth value cannot be determined. This, for Locke, places a severe limit on the usefulness of universal propositions.


Locke was hardly the first to propose a distinction between what is trivially true and what is really true. The classical Greek philosopher Aristotle, in his treatises on logic, required propositions to have existential import. That is, if an argument has "all centaurs are animals" as a premise, the assumption—under Aristotle's rules—is that there are at least some centaurs. The concept of an empty set is not invoked in Aristotle's discussion of syllogisms. Locke, however, allows for such empty sets. When he says "all centaurs are animals," he does not mean—as Aristotle might—that there exist centaurs, and all of them are animals. Rather, he means something akin to "For every centaur which exists (and there might not be any!), it is true that that centaur is an animal."

Subsequent writers have often criticized Locke's attempt to locate truth-value in the "agreement" between ideas and "things"—that is, the relationship between ideas and outward reality. At first glance, Locke's proposal seems sensible: if a proposition is true, it should conform to "the way things are." Yet as Nigel Warburton points out in Philosophy: The Classics (2006), Locke also maintains that we cannot directly know reality, only ideas. Thus in Locke's system, we have a test for the truth-value of propositions, but no way to apply it. This, as Warburton observes, was one of Bishop Berkeley's main objections to Locke's theory of knowledge. Berkeley's disagreements with Locke are discussed further in the Context section. For a general account of Berkeley's own views on knowledge and reality, Lisa Downing's article in the Stanford Encyclopedia of Philosophy (2011) is an excellent starting point.

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