An Essay Concerning Human Understanding | Study Guide

John Locke

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An Essay Concerning Human Understanding | Summary



An Essay on Human Understanding opens with two letters, the first of which is to the book's dedicatee, Thomas Herbert, Earl of Pembroke, a leading English politician of his day. The second addresses the reader directly, and offers a defense of the book and its arguments from various perspectives. The rest of the work is divided into four books, as follows.

Book 1: Of Innate Notions

Locke begins his exploration of human knowledge by refuting a widely held belief of his time. He argues that there are no innate ideas in the human mind—none that people bring into the world at birth. Instead, he says, the mind is initially a blank slate, and ideas are imprinted on it only through experience. This goes even for the most fundamental principles of reasoning, including speculative principles (i.e., statements about what's true) and practical principles (i.e., assertions about how to behave).

Book 2: Of Ideas

Locke now presents his own theories regarding ideas: what they are, how they are acquired, and how they relate to reality. Ideas, Locke says, are the "objects of thinking," and they arise in the mind through just two means: sensation and reflection. Sensation is the sensory experience of the external world, which gives rise to ideas like "hot," "cold," "bitter," and "sweet." Reflection is the mind's awareness of its own operation and gives rise to more abstract ideas. Locke further classes ideas into simple ideas, which are directly and immediately perceived, and complex ideas, which are built up by the mind from simple components.

To illustrate his arguments Locke explores basic philosophical ideas relating to time, space, and number. Even these complex concepts, he says, ultimately come from a combination of sensation and reflection and are built up from specific simple ideas. Abstract relationships, like cause and effect or sameness and difference, likewise come from the mind's reflection on its simple ideas. This process of assembly, Locke argues, explains why people sometimes yoke together seemingly unrelated ideas. For example, someone who has eaten too much honey as a child might be nauseated, even in adulthood, by the smell or taste of honey. This happens not because of any essential relationship between honey and queasiness, but because the mind puts ideas together to reflect its own experiences.

Book 3: Of Words

Locke next discusses the relationship between language and experience. Although words are not necessary for having ideas, he says, they are important because they allow us to communicate ideas. Carefulness with language is essential, Locke maintains, because an idea can be inaccurately communicated to others even if it is clear to the individual. The opposite sometimes happens as well, especially in religious and philosophical debates: people use a word without having a clear and distinct idea of what it means. Locke cautions that both types of mistakes are a serious hindrance to the advancement of knowledge. Loosely defined words may, he admits, be adequate for the business of everyday life, but precision is essential in philosophical discussions.

Locke is unconvinced by the idea that names reflect the true essence of things. Instead, he asserts, words reflect people's attempts to group things together based on their similar traits. He uses the word "gold" as an example to discuss its essence as opposed to its qualities and the way it may be used by a child, an adult layperson, and a chemist. According to Locke, his example shows that names denote bundles of ideas that can easily vary from person. The variation is evident with a concrete word like "gold," but it's even greater when abstract concepts like "justice" or "frugality" are discussed. By keeping this in mind, he contends, we can avoid many of the difficulties that arise in communication.

Book 4: Of Knowledge and Probability

Locke concludes the Essay with an investigation of the nature of knowledge, which he defines as an awareness of the relationships among ideas. He maintains that human knowledge is extremely limited, and that many things called "knowledge" are really belief or conjecture. This, as Locke is well aware, flies in the face of the grand claims that are often made about the potential of the human mind.

Locke's seeming pessimism about human knowledge comes in part from the strictness with which he uses the word "knowledge." For Locke, knowledge implies both total certainty and the ability to establish something beyond a doubt. The term "probability," in contrast, refers to things established as merely likely to be true, even if the likelihood is very high. For example, Locke regards it as probable, but not totally certain, that Julius Caesar won the Roman Civil War. It's probable because reputable historians say so and nobody has an obvious reason to lie about such a thing. It can't be "certain knowledge," however, because nobody alive in Locke's time can remember the events of that long-ago era.

Finally Locke turns his attention to the related concept of reason, which he defines as the ability to discover truths and establish connections among them. As with knowledge itself, Locke argues that people are generally too optimistic about what human reason can achieve. Nonetheless, he insists that reason—not tradition, popular opinion, or wishful thinking—must be the basic standard for those who seek truth.

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