An Essay Concerning Human Understanding | Study Guide

John Locke

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An Essay Concerning Human Understanding | Vol. 2, Book 4, Chapters 20–21 : Of Knowledge and Probability | Summary



Vol. 2, Book 4, Chapter 20: Of Wrong Assent, or Error

Locke now raises the question: how do people come to believe false or improbable things? One cause of such "error," he says, is a "want of proofs," meaning a lack of exposure to the tools needed for proper reasoning. Most people, he says, lack the educational opportunities to correctly assess the truth of statements in science, politics, and other learned pursuits. Others live under oppressive regimes which limit their access to information. Still others lack the skill or the will to reason about the evidence presented to them. Finally, there are some who rely on "doubtful propositions" or supposedly authoritative opinions as though they were established truths. Once such opinions are "riveted" into the mind, they are extremely difficult to remove.

Vol. 2, Book 4, Chapter 21: Of the Division of the Sciences

In this short concluding chapter, Locke proposes to divide the sciences into three major branches. ("Science" here means not just the physical sciences, but any attempt to advance human knowledge.) He gives the name physica to sciences which aim at "the knowledge of things," which includes what we today would call physics, as well as such areas as theology. The second category, practica, concerns "applying our own powers and actions, for the attainment of things good and useful." Locke uses ethics as an example of this branch. The third and final category is semiotica, the study of signs, which includes grammar and logic. Once these three categories are set forth and a few words said about their usefulness, the Essay abruptly ends.


Although it may seem cursory and even tacked-on, Locke's "Division of the Sciences" is in some ways a fitting capstone to the rest of the Essay. The exercise of grouping knowledge into different branches was nothing new in Locke's time. In the Middle Ages and well into the Renaissance, curricula were organized according to definitions which, like Locke's, might now appear arbitrary. The term liberal arts originally referred to one such arrangement in which seven distinct fields of study were identified. Grammar, logic, and rhetoric were the trivium, while arithmetic, geometry, astronomy, and music constituted the quadrivium. The system reflected the high premium placed on language and communication in the monasteries and academies of medieval Europe.

Locke's more tentative system likewise emerges from the demands of his own time. As Locke reminds his readers throughout the Essay, the late 17th century was an era of transformative change in philosophy, mathematics, and the physical sciences. Locke is happy to acknowledge recent advances in these fields; he even calls Sir Isaac Newton's Principia a book "never enough to be praised." Ultimately, however, Locke sees moral philosophy—and not "natural philosophy," as science was often called—as the field with the greatest potential to advance the human condition. Accordingly, Locke modestly proposes that we reorganize our view of knowledge to include an applied component, one concerned with how we are to conduct our lives. Entirely in keeping with this belief, Locke nominates ethics—and not, say, architecture or engineering—as the "most considerable" of the practical sciences.

Given his lifelong hatred of rhetorical "wrangling" and pedantic formal logic, it is unsurprising that Locke also creates a category to deal with "the study of signs." His purpose in doing so is at least twofold. For one thing, grouping grammar and logic under this heading makes it clear that these are tools for understanding human communication, not for discovering fundamental truths or deciding how we ought to live. Locke is also, as he showed in the final chapters of Book 3, acutely aware of how fallible and misleading language can be. In proposing a sustained study of words and their functions, he invites the reader to resume the investigation where he left off.

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