The Nexus Archive
Liberal Political Philosophy · Treatise
An Essay Concerning Human Understanding
by John Locke · The Philosopher of Consent · Enlightenment, 1632-1704
Locke's full inquiry into ideas, language, knowledge, probability, faith, reason, and the limits of human understanding.
- Brutal
- 1690
- ~20 hr 15 min read
- 45 chapter guides
- Public domain
Start reading · Read the The Epistle to the Reader — III guide free
Before the text
Editor's Prelude
The mind turns on itself
Prepared by the NEXUS Editors
Locke says this immense book began with a smaller embarrassment. Five or six friends had gathered in his room to discuss another subject, but their argument stalled. Before they could settle the question before them, he suggested, they needed to ask what the human understanding was equipped to know. That pause became a project that occupied him for years.
The Essay proceeds through numbered sections, patient distinctions and ordinary things: a piece of gold, a hand in water, a child learning a name. An idea here means whatever the mind has before it in thinking. Locke follows those materials through experience, memory and language, repeatedly testing how a confident assertion is formed.
The stakes reach into conduct and belief. People must act where certainty runs out, and they often demand certainty from others while borrowing their own opinions. Locke seeks enough understanding to guide a life, together with the humility to recognize its limits.
The opening pages
The Epistle to the Reader — I
The Epistle to the Reader
READER,
I have put into thy hands what has been the diversion of some of my idle and heavy hours. If it has the good luck to prove so of any of thine, and thou hast but half so much pleasure in reading as I had in writing it, thou wilt as little think thy money, as I do my pains, ill bestowed. Mistake not this for a commendation of my work; nor conclude, because I was pleased with the doing of it, that therefore I am fondly taken with it now it is done. He that hawks at larks and sparrows has no less sport, though a much less considerable quarry, than he that flies at nobler game: and he is little acquainted with the subject of this treatise—the UNDERSTANDING—who does not know that, as it is the most elevated faculty of the soul, so it is employed with a greater and more constant delight than any of the other. Its searches after truth are a sort of hawking and hunting, wherein the very pursuit makes a great part of the pleasure. Every step the mind takes in its progress towards Knowledge makes some discovery, which is not only new, but the best too, for the time at least.
For the understanding, like the eye, judging of objects only by its own sight, cannot but be pleased with what it discovers, having less regret for what has escaped it, because it is unknown. Thus he who has raised himself above the alms-basket, and, not content to live lazily on scraps of begged opinions, sets his own thoughts on work, to find and follow truth, will (whatever he lights on) not miss the hunter’s satisfaction; every moment of his pursuit will reward his pains with some delight; and he will have reason to think his time not ill spent, even when he cannot much boast of any great acquisition.
This, Reader, is the entertainment of those who let loose their own thoughts, and follow them in writing; which thou oughtest not to envy them, since they afford thee an opportunity of the like diversion, if thou wilt make use of thy own thoughts in reading. It is to them, if they are thy own, that I refer myself: but if they are taken upon trust from others, it is no great matter what they are; they are not following truth, but some meaner consideration; and it is not worth while to be concerned what he says or thinks, who says or thinks only as he is directed by another. If thou judgest for thyself I know thou wilt judge candidly, and then I shall not be harmed or offended, whatever be thy censure. For though it be certain that there is nothing in this Treatise of the truth whereof I am not fully persuaded, yet I consider myself as liable to mistakes as I can think thee, and know that this book must stand or fall with thee, not by any opinion I have of it, but thy own. If thou findest little in it new or instructive to thee, thou art not to blame me for it. It was not meant for those that had already mastered this subject, and made a thorough acquaintance with their own understandings; but for my own information, and the satisfaction of a few friends, who acknowledged themselves not to have sufficiently considered it.
Chapter guides · 45
The Epistle to the Reader — III
A word needs a settled meaning
Locke wants each important word to stand for a definite idea throughout an argument.
What determined means
Locke explains his preference for determined over clear and distinct. A simple idea is a particular appearance in the mind. A complex idea joins a definite set of parts. Giving either a name works only if that name keeps pointing to the same idea.
The closing edition notices
Language has fewer words than thought has possible combinations. Even so, Locke argues, steady meanings would end many disputes. The Epistle closes with notices about additions to later editions, including chapters on association and enthusiasm and changes to the discussion of power.
Book I, Chapter I
Knowing how far the mind reaches
Limited understanding can still give people enough guidance to live.
The inquiry takes shape
Locke sets aside questions about the physical substance of the mind. He asks where ideas come from, what people can know with certainty, and how they judge when proof is unavailable. His sailor needs a sounding line long enough for the waters he must navigate.
What an idea is
Human understanding need not answer every possible question to serve a purpose. Locke compares its available light to a candle that can guide ordinary work. He ends by naming his basic subject: an idea is whatever the mind has before it when it thinks. Its origin is the first question.
Book I, Chapter II
Learning a truth does not make it innate
Immediate agreement with a statement does not prove that its truth was present in the mind from birth.
Book I, Chapter III
Familiar beliefs can seem inborn
Education and social pressure can make learned principles feel too obvious to question.
Book I, Chapter IV
Understanding cannot be borrowed
Authority cannot give someone knowledge in place of their own understanding.
Book II, Chapter I
The mind receives its first materials
Experience supplies basic ideas that thought can develop but cannot invent at will.
Book II, Chapter VII
A few ingredients can make much
Pleasure, pain, power and other simple ideas give thought materials for many combinations.
Book II, Chapter VIII
One object can affect people differently
Locke separates a body’s features from its powers to produce sensations and change other bodies.
Book II, Chapter XI
Reason can follow a mistaken starting point
A connected argument can still go wrong when its first ideas or assumptions are mistaken.
Book II, Chapter XIII
Space and body are different ideas
Distinguishing two ideas does not by itself prove what exists outside the mind.
Book II, Chapter XIV
Time can extend beyond its measures
The mind can apply a measure of duration to events that occurred before that measure existed.
Book II, Chapter XVII
An endless addition is not a finished picture
Locke distinguishes the ability to keep adding from a complete positive idea of infinity.
Book II, Chapter XX
Passions turn on pleasure and pain
Love, fear, anger and other passions differ in how the mind considers possible goods and harms.
Book II, Chapter XXI: Power — Sections 35–43
Knowing the better good may not move us
A felt need can govern the next action even when someone recognizes a greater good.
Book II, Chapter XXI: Power — Sections 72–75
A revised account of choice
Locke ends the discussion of power by reviewing his argument and acknowledging a change in his own thinking.
Book II, Chapter XXII
An action’s name may leave its cause unknown
Naming an action or its result does not necessarily explain how it happens.
Book II, Chapter XXIII
Knowing qualities leaves their support obscure
Our ideas of bodies and spirits can collect known features without revealing the substance beneath them.
Book II, Chapter XXVII
Person is a term of responsibility
Locke connects personal identity with the ownership of actions and the justice of reward and punishment.
Book II, Chapter XXIX
One clear part can hide a confused whole
Knowing one feature of an idea exactly does not mean the entire idea is distinct.
Book II, Chapter XXXII
Error enters through a claim of agreement
An idea becomes mistaken when the mind treats it as matching a standard that it does not match.
Book II, Chapter XXXIII
Habit can make error feel reasonable
People can sincerely defend a mistaken view when custom has joined ideas that do not belong together.
Book III, Chapter I
Speech needs more than sounds
Language uses sounds as signs of thought and general names for many particular things.
Book III, Chapter III
Nature supplies resemblances; minds make classes
A name groups things under selected features rather than revealing their hidden constitution.
Book III, Chapter IV
A definition cannot supply every experience
Words can clarify a known sensation’s name without giving someone a wholly new simple idea.
Book III, Chapter V
A fluent word can hide a thin idea
Knowing how to use a word in conversation may leave its full meaning unsettled.
Book III, Chapter VI
A definition differs from a discovery
A statement about gold may repeat a chosen meaning or claim something further about the material itself.
Book III, Chapter VIII
A thing and an abstract quality differ
Saying that something has a quality does not make two distinct abstract ideas identical.
Book III, Chapter IX
Uncertain language calls for modest interpretation
A text’s authority does not give its reader an infallible understanding of its words.
Book III, Chapter X
Familiar sounds do not guarantee shared ideas
A speaker can assume that a common word has conveyed much more than the hearer actually understands.
Book III, Chapter XI
Consistent words still need explanation
A writer should keep a word’s meaning steady and make necessary changes of sense clear.
Book IV, Chapter I
Knowledge is a perceived agreement
Locke calls something knowledge when the mind sees how its ideas agree or differ.
Book IV, Chapter II
Present sensation has its own evidence
Locke treats awareness of external things as knowledge, though its certainty differs from intuition and demonstration.
Book IV, Chapter III
A regular effect can leave its connection unknown
Experience can establish what happens without revealing why it must happen.
Book IV, Chapter IV
Names cannot settle what things are
Ideas of substances need observed patterns, rather than combinations that merely seem possible.
Book IV, Chapter VI
An object’s qualities depend on its surroundings
Locke argues that understanding a body requires more than looking within its surface.
Book IV, Chapter VII
True maxims cannot rescue shifting definitions
An unquestionable rule can still be used in a misleading argument when its terms change meaning.
Book IV, Chapter VIII
A tidy argument may add no discovery
Coherent relations between names do not by themselves increase knowledge of things.
Book IV, Chapter XI
Past observation differs from present existence
Memory can preserve knowledge of what existed without proving that it still exists now.
Book IV, Chapter XII
Experiments improve life without settling everything
Locke values tested inquiry while warning against treating a useful hypothesis as certain proof.
Book IV, Chapter XIV
Judgment makes action possible under uncertainty
A person often needs to decide without possessing a demonstration.
Book IV, Chapter XVI
Repeated testimony is not new testimony
A claim does not gain independent support simply because many writers repeat the same original report.
Book IV, Chapter XVII
Exposing an error does not prove a truth
Reason needs evidence, rather than submission won through shame, ignorance or an opponent’s mistake.
Book IV, Chapter XVIII
Faith still requires a judgment of evidence
Locke gives revelation authority over uncertain opinions while preserving the force of clear knowledge.
Book IV, Chapter XX
Conviction can protect itself from inquiry
A belief can survive contrary evidence when habit, interest or allegiance prevents a fair examination.
Book IV, Chapter XXI
Things, actions and signs
Locke ends by dividing the objects of understanding according to what thought studies and what it seeks.
Questions readers ask
- How long does it take to read An Essay Concerning Human Understanding?
- About 20 hr 15 min in total.
- How difficult is An Essay Concerning Human Understanding?
- Brutal. A very long treatise; about 20 hours of reading.
- Can I read An Essay Concerning Human Understanding for free?
- An Essay Concerning Human Understanding is part of Nexus Premium, so reading it needs an account and a Premium subscription. The opening above and two chapter guides on this page are free to read.
- How many chapter guides are there?
- 45 chapter guides, written by the Nexus editors. Two open in full here; the rest show their opening line.
About John Locke
John Locke was an English philosopher whose writings on knowledge, government, toleration, and rights helped shape liberal political thought and the American founding.
“What makes political power legitimate?”
Locke is one of the central sources for modern arguments about rights, liberty, property, and constitutional government.
Source text: public domain. Chapter guides: original Nexus writing. An Essay Concerning Human Understanding is part of The Nexus Archive, guided Quests through the great works.