An Essay Concerning Human Understanding

Summary

Locke says in the Epistle to the Reader that the Essay began when five or six friends meeting in his rooms found themselves at a standstill over questions of morality and revealed religion, and he proposed that they first examine what objects human understanding is fitted to deal with. Nearly twenty years later the result was a long, patient survey of where ideas come from, how they combine, how words signify them and how far knowledge and probable belief can reach. Book I attacks the doctrine of innate principles. If any speculative or practical truths were stamped on the mind, children, 'idiots' and whole nations would assent to them, yet none commands universal assent, and even the idea of God differs widely between peoples. Book II, the longest and most influential, supposes the mind to be white paper and derives every idea from two sources: sensation of external objects and reflection on the mind's own operations. Simple ideas are passively received; the mind actively compounds, compares and abstracts them into complex ideas of modes, substances and relations. Along the way Locke distinguishes primary qualities such as solidity, extension and figure, which resemble their ideas, from secondary qualities such as colour, taste and warmth, which are only powers in bodies to produce sensations. He reports William Molyneux's question whether a man born blind and newly sighted could tell a globe from a cube by sight alone, and agrees that he could not. He explains madness as right reasoning from wrongly joined ideas, analyses power and the will at great length, and concedes that our idea of substance is only a supposed something we know not what. The chapter on identity, added in 1694, separates the identity of a man, a living organism, from the identity of a person, which extends exactly as far as consciousness reaches back, with thought experiments about a prince's soul in a cobbler's body and a waking and sleeping Socrates. The final chapter of Book II, added in the fourth edition, names the association of ideas and treats chance connexions made by custom as a kind of madness found in nearly everyone. Book III examines language: words stand for ideas in the speaker's mind, general terms name abstract ideas, and the 'real essences' of natural kinds are unknown, so much dispute is merely verbal. Book IV defines knowledge as the perception of agreement or disagreement between ideas, ranks intuition, demonstration and sensitive knowledge, and admits that most of life runs on probability, judgment and faith regulated by reason, closing with a warning against religious enthusiasm.

Top Lessons

• Experience writes on a mind that begins blank

  Locke denies that any principles are inborn and traces every idea to sensation or reflection. The practical upshot is that education, environment and early impressions shape minds profoundly, for good or ill.

• A person reaches as far as memory and consciousness reach

  Locke separates the human organism from the person, an accountable self constituted by consciousness of past actions. This makes identity a question of psychological continuity, with direct consequences for punishment and responsibility.

• Chance associations can feel like nature

  In Book II, Chapter XXXIII Locke shows how ideas joined by accident or repetition harden into antipathies, fears and prejudices that reason cannot dissolve, and how time can loosen some of them. He urges parents and teachers to prevent such connexions early.

• Most of life runs on probability, so judge assent by grounds

  Book IV narrows certain knowledge and leaves most belief to probable judgment. Locke asks that assent be proportioned to evidence and warns that strong inner conviction, as in enthusiasm, proves nothing about truth.

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Medical Lens

Locke was a trained physician and close associate of Thomas Sydenham, and the Essay carries Sydenham's preference for observation over speculative theory. For psychiatry it offers an influential cognitive theory of madness (correct reasoning from wrongly joined ideas), which shaped eighteenth-century writing on insanity and the later concept of partial insanity or monomania. The chapter on association reads as a proto-theory of conditioned fear, phobia and context-dependent memory, and its section on grief makes a still-relevant point about the limits of rational consolation. The identity chapter's cases of memory loss, drunkenness and sleepwalking remain central to debates on dementia, dissociation and criminal responsibility. Equating Locke's examples with modern diagnoses is an interpretive move, not a historical claim about his intentions.

• Book II, Ch. IX, §8

  Molyneux's problem: whether a man born blind, newly given sight, could distinguish a globe from a cube without touching them; Locke and Molyneux answer no.

• Book II, Ch. XI, §12–13

  Difference between 'naturals' (intellectual disability) and madmen; delusions such as believing oneself a king or made of glass; partial madness in otherwise sober people.

• Book II, Ch. XXI

  The will determined by present uneasiness; the drunkard who sees his health and estate decaying yet returns to drink.

• Book II, Ch. XXIII

  Opium's colour, taste and soporific or anodyne virtues as powers grounded in hidden primary qualities.

• Book II, Ch. XXVII

  Personal identity through consciousness; cases of forgetting, drunkenness, sleepwalking and alternating consciousness, and their bearing on punishment.

• Book II, Ch. XXXIII, §7–16

  Associations producing antipathies (surfeit of honey), fear of the dark, rumination on injury, aversion to places of illness, unending grief, aversion to a surgeon who cured madness, and context-bound dancing.