Summary
Book I: Neither principles nor ideas are innate
Locke begins by defining his inquiry into the origin, certainty and extent of human knowledge. He rejects universal assent as proof of innate principles and observes that supposedly self-evident maxims are not consciously known by every person.
Speculative principles become intelligible only after their component ideas have been acquired. Practical principles show still greater diversity across societies. Natural desires and capacities do not amount to fully formed moral rules written into the mind.
Book II: Of ideas
All ideas arise from sensation or reflection. The senses provide ideas of external qualities, while reflection supplies awareness of mental activities. The mind receives simple ideas and actively combines, compares and abstracts them.
Locke distinguishes primary qualities, such as solidity and extension, from secondary qualities, such as colour and sound. Bodies possess powers to produce sensory ideas whose character depends upon the perceiver.
Perception, retention, discerning, comparison, composition and abstraction explain the development of thought. Complex ideas divide into modes, substances and relations. Space, duration, number, infinity, pleasure, pain and power are analysed through the operations that construct their ideas.
In discussing freedom, Locke separates the liberty of an agent from the freedom of the will considered as a faculty. Human action is often governed by uneasiness and desire, though reflection can suspend immediate choice.
Ideas of substances collect qualities presumed to coexist in an unknown support. Identity depends upon the kind of being considered. Personal identity consists in continuity of consciousness, which grounds the attribution of actions and responsibility.
The final chapters assess clear, distinct, real, adequate, true and false ideas and explain how habitual connections can produce misleading associations.
Book III: Of words
Words serve as signs of ideas and make communication possible. General words apply to many particulars because the mind abstracts common features and leaves individual circumstances aside.
Locke distinguishes nominal essences from real essences. A nominal essence is the abstract idea by which a kind is named; the real essence is the internal constitution producing observable properties. In natural substances, this constitution is usually unknown.
Language is imperfect because speakers attach different ideas to the same words, use vague classifications and lack access to real constitutions. Abuses arise when words are used without ideas, meanings shift unnoticed, obscurity is cultivated or verbal definitions are mistaken for knowledge of things.
Remedies include declaring meanings, using terms consistently and supporting definitions with examples. Philosophical clarity requires attention to both ideas and the signs used for them.
Book IV: Of knowledge and probability
Knowledge consists in perceiving agreement or disagreement among ideas. It concerns identity, relations, coexistence and real existence. Intuitive knowledge is immediate; demonstrative knowledge uses intermediate proofs; sensitive knowledge assures us of present external things.
Human knowledge is limited. Mathematical and moral relations may allow demonstration, while the necessary coexistence of qualities in substances usually remains beyond us because real constitutions are hidden.
Locke defends the reality of knowledge when ideas conform to their archetypes or when necessary relations among ideas are correctly perceived. General truths depend upon well-defined abstract ideas rather than words alone.
We know our own existence intuitively, God’s existence demonstratively and external objects sensitively. Knowledge can be improved by acquiring clear ideas, discovering intermediate relations and adopting sound methods rather than relying mechanically upon maxims.
Judgement supplies assent where certainty is absent. Probability depends upon conformity with experience and credible testimony, so belief should vary with the balance of evidence. Reason evaluates proofs and probabilities; faith concerns genuine revelation but cannot overturn clear knowledge or contradiction.
Enthusiasm mistakes inward confidence for divine authority. Wrong assent also arises from ignorance, prejudice, passion and failure to consider available reasons. Locke concludes by dividing inquiry into natural philosophy, practical philosophy and the doctrine of signs.