An Essay Concerning Human Understanding by John Locke

Context

An Essay Concerning Human Understanding was first published in December 1689 (dated 1690) and immediately established John Locke as the leading philosopher of the English-speaking world. Locke had worked on the book for nearly twenty years, and it grew out of discussions with friends about the limits of human knowledge — questions that had been given new urgency by the scientific revolution and by the rise of mechanical philosophy. The Essay is the founding document of British empiricism: its project is to determine the origin, certainty, and extent of human knowledge by tracing all knowledge to experience.

The book's central claim is that the mind is a "white paper" (tabula rasa) at birth — that there are no innate ideas, and that all the materials of thought come from experience. This was a direct attack on the rationalist tradition, which held that the mind possesses innate principles and ideas. Locke's alternative was a genetic account: all ideas arise from either sensation (the perception of external objects) or reflection (the perception of the mind's own operations). The Essay's account of the mind as experience-driven was to become the starting point for the empiricism of Berkeley and Hume, and its account of personal identity remains a classic of the philosophy of mind.

Core Arguments

The Rejection of Innate Ideas

Book I of the Essay argues that there are no innate ideas or principles. Locke attacks the rationalist claim that certain principles — "Whatever is, is" or "It is impossible for the same thing to be and not to be" — are imprinted on the mind from birth. His arguments are empirical: children and the mentally impaired do not assent to these principles, which suggests they are learned; and the principles that are universally assented to can be explained by experience and education. The rejection of innate ideas is the foundation of Locke's empiricism: if there are no innate ideas, then all ideas must come from experience.

The Theory of Ideas

Book II develops the theory of ideas. All ideas derive from sensation or reflection: sensation supplies ideas of external objects (yellow, cold, round, sweet), and reflection supplies ideas of the mind's own operations (perceiving, thinking, doubting, willing). Simple ideas are the atoms of thought, received passively from experience; complex ideas — substances, modes, relations — are constructed by the mind from simple ideas. This genetic account of the understanding was revolutionary: it made the mind's content fully explainable by experience and its combinations, without any innate furniture.

Primary and Secondary Qualities

The Essay's account of perception distinguishes primary qualities — solidity, extension, figure, motion — which exist in the objects themselves and resemble our ideas of them, from secondary qualities — color, sound, taste, smell — which are powers in objects to produce ideas in us and do not resemble our ideas. The distinction was Locke's way of accommodating the mechanical philosophy: the real properties of bodies are their primary qualities, and the sensory qualities we experience are effects of those properties on our senses. The distinction was later attacked by Berkeley, who argued that primary qualities are as mind-dependent as secondary ones — a critique that helped drive the development of idealism.

Personal Identity

The Essay's account of personal identity, in Book II Chapter 27, is one of the most influential passages in the history of philosophy. Locke defines a person as "a thinking intelligent being, that has reason and reflection, and can consider itself as itself, the same thinking thing, in different times and places." Personal identity, he argues, consists in consciousness: a person is the same person as long as their consciousness reaches back to past thoughts and actions. The self is constituted by memory and the continuity of consciousness, not by the sameness of a substance (whether material or immaterial). This "psychological continuity" account of personal identity is the ancestor of all modern theories of personal identity, and it was taken up and transformed by Hume, Derek Parfit, and contemporary philosophers.

Key Concepts

The Essay's key concepts — the tabula rasa, sensation and reflection, simple and complex ideas, primary and secondary qualities, the personal identity criterion of consciousness, and the limits of knowledge — became the vocabulary of British empiricism and shaped the philosophy of mind for three centuries. Locke's account of the self as constituted by consciousness rather than substance was the first modern statement of the psychological theory of personal identity, and his claim that knowledge is limited to what experience can supply defined the empiricist project.

Legacy & Influence

The Essay is the foundation of modern empiricism and one of the most influential books in the history of philosophy. Its account of the mind as experience-driven shaped psychology, epistemology, and the philosophy of mind, and its rejection of innate ideas set the agenda for the empiricist tradition that runs from Locke through Berkeley and Hume to the logical empiricists of the twentieth century. Its account of personal identity initiated the debate that continues today in the psychological continuity theory and the bundle theory of the self. The Essay's theory of knowledge — that knowledge is the perception of agreement or disagreement among ideas — also framed the epistemological questions that dominated early modern philosophy. Kant, who called Locke's project an "anatomy of the understanding," built his critical philosophy in response to the problems the Essay raised. The book's influence on the philosophy of mind is direct: it is the origin of the question "what is the self?" in its modern form.

Reading Guide

The Essay is long (four books) and can be read selectively. Book I (innate ideas) is the historical prelude. Book II (ideas) contains the theory of ideas and the famous Chapter 27 on personal identity. Book III (words) treats language. Book IV (knowledge) presents the theory of knowledge and its limits. For the philosophy of mind, the essential readings are Book II Chapters 1–9 (sensation and reflection, simple and complex ideas, primary and secondary qualities) and Chapter 27 (identity and diversity). The answer pages on personal identity and the psychological continuity theory provide accessible introductions to Locke's legacy.

Related Works

The Essay's empiricism was radicalized by George Berkeley in Principles of Human Knowledge (1710) and transformed by David Hume in A Treatise of Human Nature. Its account of personal identity is the ancestor of Derek Parfit's Reasons and Persons and of the modern bundle theory of the self. Its epistemology is the target of Kant's Critique of Pure Reason, and its philosophy of mind is the historical background of the modern philosophy of mind.