In 1688 the Irish scientist and philosopher William Molyneux wrote to John Locke with a deceptively simple puzzle. Imagine a man born blind who has learned to tell a cube from a sphere of the same metal and size by touch alone. If he were suddenly given sight, could he say which is which without touching them? Locke included the question in his Essay, and George Berkeley took it up in his theory of vision. It condenses the central dispute of empiricism: what exactly does experience give us, and what do we merely assume it gives us? This article follows the answers of Locke and Berkeley, two of the three names that textbooks group under the label “British empiricism” (the third, David Hume, has an article of his own).

A word of method first. The Locke–Berkeley–Hume triad is a later historiographical construction, consecrated by textbooks; the authors themselves did not see themselves as members of a school, and Berkeley in particular uses empiricist principles to reach a metaphysical conclusion, the non-existence of matter, that Locke would never have accepted. The unity of the tradition lies less in a shared set of theses than in a shared problem: how to explain human knowledge from the “ideas” that experience supplies to the mind.

1. The Context: What “Empiricism” Means Here

Modern empiricism does not come from nowhere. Francis Bacon (1561–1626) had already proposed, in the Novum Organum (1620), a science founded on disciplined observation and gradual induction (see the article on Francis Bacon and the empirical method). Thomas Hobbes, for his part, held that all knowledge derives from sensation. But it is with Locke that the epistemological question receives systematic form: before asking what we can know, we must examine the capacity of the understanding itself.

The background is twofold. On one side stands continental rationalism, the Cartesian doctrine that the intellect possesses clear and distinct ideas that do not depend on the senses (see Cogito, ergo sum and Leibniz’s Monadology). On the other stands the new corpuscular science of Robert Boyle and Isaac Newton, which described the physical world in terms of particles in motion, an image Locke absorbs and whose philosophical consequences Berkeley will judge dangerous.

2. Locke: Life and Work

John Locke was born in Wrington, Somerset, in 1632 and died at Oates, Essex, in 1704. Educated at Oxford, he studied medicine, moved in Boyle’s circle, and served as secretary and adviser to Anthony Ashley Cooper, later Earl of Shaftesbury, which drew him into English politics and then into exile in the Netherlands (1683–1689). Back in England after the Glorious Revolution, he published within a few months the works that made his name: the Letter Concerning Toleration (1689), the Two Treatises of Government (1689, dated 1690 on the title page), and the Essay Concerning Human Understanding, issued in December 1689 but dated 1690. His political theory is treated in the article on the social contract in Hobbes, Locke, and Rousseau, and his account of personal identity in Personal Identity: Locke, Hume, and Parfit. Our concern here is the Essay, in four books: innate ideas (I), ideas (II), words (III), and knowledge (IV). More on the author can be found in the profile of John Locke.

Locke states the purpose of the work: to inquire into the origin, certainty, and extent of human knowledge, together with the grounds and degrees of belief, opinion, and assent. The method he announces is descriptive, a “plain historical method” of examining how the mind comes by its ideas, and the attitude is one of modesty: recognizing the limits of the understanding would prevent fruitless disputes.

3. The Critique of Innate Ideas (Book I)

Book I of the Essay attacks the thesis that the mind comes into the world with theoretical or practical principles “stamped” upon it. Three arguments stand out.

The first is the argument from universal consent. Defenders of innatism held that certain principles, such as “whatsoever is, is” or “it is impossible for the same thing to be and not to be,” are accepted by all humankind and are therefore innate. Locke replies that no such assent exists, since children and people with intellectual disabilities do not know these principles, and that even if it did, it could be explained in other ways than innateness.

The second is the argument from moral diversity: practical principles vary across peoples and ages, so there is no universally accepted moral rule. For this Locke draws on the reports of travelers and early ethnographers of his day.

The third is a dilemma about consciousness: if an idea is in the mind, the mind must perceive it, and to say that an idea is in the mind though the mind has never perceived it seems contradictory.

The exact targets of Book I are debated by scholars, since Locke rarely names his adversaries. What matters is the positive conclusion: if there are no innate ideas, the entire content of the mind must come from experience.

Tabula rasa: A Note on Precision

This position is often summed up by the expression tabula rasa (“blank slate”). Yet the expression does not appear literally in the Essay. Locke asks us to suppose the mind to be, “as we say, white paper, void of all characters,” without any ideas (Essay II.1.2). The image of an empty writing surface goes back to Aristotle (De Anima III.4), who compares the intellect to a writing tablet on which nothing is actually written, and the Latin phrase circulated in the scholastic tradition. Attributing “the tabula rasa” to Locke is correct as to doctrine but inexact as to text.

4. The Doctrine of Ideas (Book II)

Locke calls “idea” whatever is the immediate object of thought, “whatsoever is the object of the understanding when a man thinks.” Ideas come from two sources: sensation, which gives us ideas of the qualities of bodies (yellow, cold, hard), and reflection, the perception of the operations of our own mind (perceiving, doubting, believing, willing). There is no third source.

4.1 Simple and Complex Ideas

Simple ideas are received passively: the mind can neither create nor destroy them. They are like the atoms of thought, each containing a single uniform appearance. Complex ideas are built by the mind out of simple ones through three operations: combination (joining several simple ideas into one, as in the idea of an “apple”), comparison (setting ideas side by side to obtain ideas of relation), and abstraction (separating an idea from the circumstances that accompany it, which yields general ideas). Complex ideas divide into modes, substances, and relations. Modes (such as number, duration, beauty) depend on other things; ideas of substance represent things that subsist by themselves; relations result from comparison.

4.2 Primary, Secondary, and Tertiary Qualities

In chapter 8 of Book II, Locke distinguishes the idea (what is in the mind) from the quality (the power in the body that produces the idea). Among qualities he separates:

  • Primary qualities: inseparable from bodies whatever changes they undergo: solidity, extension, figure, motion or rest, and number. Our ideas of them, Locke says, resemble the qualities in bodies.
  • Secondary qualities: powers of bodies to produce sensations in us, such as colors, sounds, tastes, and smells, by virtue of the primary qualities of their minute, insensible parts. Our ideas of them do not resemble anything in bodies.
  • Tertiary qualities (later terminology): powers to produce changes in other bodies, such as the power of fire to melt wax.

The distinction rests on corpuscular physics: if the world is made of particles with shape, size, and motion, then color and taste are the way these arrangements affect us. Here lie the roots of a debate still alive in the philosophy of perception and of mind (see the mind-body problem).

4.3 Substance: “I Know Not What”

The idea of substance is, for Locke, among the most problematic. We observe that certain simple ideas (color, shape, weight, solubility) always appear together, and we suppose there is something that supports them. Of this “substance in general,” however, we have no clear idea: in the Essay Locke says that by the word “substance” we signify only “an uncertain supposition of we know not what” (I.4.18), and in chapter 23 of Book II he compares the substratum to the elephant that, in the Indian fable, holds up the world and itself rests on a tortoise. This does not lead Locke to deny substance; it leads him to say that we know only its qualities. Whether this “something I know not what” is a poorly digested scholastic residue, and what role it plays in Locke’s theory, is among the most disputed topics among commentators. For the concept in historical perspective, see Substance: Aristotle, Descartes, and Spinoza.

Locke further distinguishes the real essence of a thing (the internal constitution on which its properties depend, generally unknown) from its nominal essence (the abstract complex idea to which we give a name and by which we classify). Our species are, to a large extent, the work of our understanding and of language.

5. Language and Knowledge (Books III and IV)

Book III is a landmark in modern philosophy of language. For Locke, words are sensible signs of ideas: they signify, in the first instance, the ideas in the speaker’s mind, not things directly. The thesis has consequences. Communication is fallible, because each person attaches somewhat different ideas to the same word, and many philosophical disputes arise from the misuse of words. The chapter on the “abuse of words” is a critique of the empty terminology of scholasticism. General terms do not correspond to existing universal essences but to abstract ideas that the mind forms.

In Book IV knowledge is defined as the perception of the connection and agreement, or disagreement and repugnancy, of any of our ideas (IV.1.2, paraphrased). There are three degrees: the intuitive (immediate perception of the agreement of two ideas), the demonstrative (agreement perceived through a chain of intuitions), and the sensitive (the existence of external objects, of lower certainty than the others). What these means cannot reach falls into the domain of probability, where reason weighs evidence, and of faith, whose authority, for Locke, cannot contradict reason. The thesis has political weight: it supports religious toleration and the critique of “enthusiasm.”

6. Berkeley: From the Critique of Locke to Immaterialism

George Berkeley was born in 1685 in Kilkenny, Ireland, and died in Oxford in 1753. An Anglican clergyman, he became Bishop of Cloyne in 1734. He wrote his main works while still young: An Essay towards a New Theory of Vision (1709), A Treatise Concerning the Principles of Human Knowledge (1710), and the Three Dialogues between Hylas and Philonous (1713). Later he published De Motu (1721), Alciphron (1732), a critique of the foundations of Newton’s infinitesimal calculus in The Analyst (1734), and Siris (1744). For biographical detail see the profile of George Berkeley.

Berkeley has an explicit apologetic aim: to combat the skepticism, atheism, and materialism that, in his judgment, arose from supposing a matter existing outside the mind. His paradox is that, in order to defend common sense, he must deny what many took to be common sense’s own belief: the existence of independent material substance.

6.1 The Critique of Abstract Ideas

The Introduction to the Principles (1710) attacks Locke’s doctrine of abstract general ideas. Locke held that we form the general idea of “triangle” by abstracting from particular triangles what they have in common. Berkeley objects that he cannot form an idea of a triangle that is neither right-angled, nor equilateral, nor scalene. For him a particular idea becomes general by serving as a sign for all other particular ideas of the same sort; there is no abstract mental content. This anti-abstractionist thesis is the touchstone of the rest of the work, and Hume will regard it (in the Treatise, I.1.7) as one of the great discoveries in the republic of letters.

6.2 Esse est percipi

The central argument of the Principles (§3) holds that the existence of sensible things consists in their being perceived: “their esse is percipi.” The Latin formula is usually condensed into esse est percipi (“to be is to be perceived”). The reasoning runs as follows. The immediate objects of perception are ideas. An idea can exist only in a mind that perceives it. Therefore, to suppose that sensible objects exist independently of any mind is to suppose that there are ideas that no one perceives, which is contradictory. For spirits the formula is supplemented: the being of a spirit consists in perceiving (percipere) and willing. The result is immaterialism: only spirits and ideas exist.

6.3 Against Primary Qualities

Berkeley aims the argument directly at Locke (Principles §§9–15; First Dialogue). If secondary qualities depend on the mind, so do the primary ones: one cannot conceive extension or figure without some color or tangibility, and the perception of size, shape, and motion varies with the observer just as that of heat or cold does. Another key premise is that “an idea can be like nothing but an idea” (§8): how, then, could an idea of extension resemble an unperceived extension? What remains of Locke’s matter, the unknown material substratum, becomes superfluous and, for Berkeley, unintelligible. Some scholars nevertheless ask how much Berkeley really owes to Locke’s “something I know not what”; the story that he simply eliminated Locke’s residue is disputed.

6.4 God and the Order of Nature

If all that exists are ideas and spirits, what explains the order, regularity, and independence of our sensations from our will? Berkeley answers that the ideas of sense, which do not depend on our will, must be caused by another spirit: God (Principles §§29–33). The “laws of nature” are the regularities according to which God supplies us with these ideas; “real things” are the ideas of sense, more vivid and coherent than those of imagination. God also serves as guarantor of the continuity of objects: when no finite spirit perceives a tree, it exists in the divine perception (a theme developed in the Three Dialogues). As for our knowledge of spirits, Berkeley says we have no idea of them but a notion, derived from the awareness of our own activity.

Berkeley rejects the charge of skepticism and of denying reality: in the First Dialogue Philonous insists that the tree and the table go on existing in the ordinary sense; what he denies is only the philosophical hypothesis of matter. The charge of absurdity nonetheless persisted. The anecdote of Samuel Johnson, who according to Boswell “refuted” Berkeley by kicking a stone, illustrates the misunderstanding: the immaterialist does not deny the stone’s resistance, only its interpretation as the property of an unperceived substance.

6.5 The Theory of Vision

In the Essay towards a New Theory of Vision (1709), Berkeley argues that distance, size, and situation are not seen immediately but inferred by association with tactile sensations and other signs; sight and touch have distinct objects, and the connection between them is learned, not natural. At the end he suggests that visual language is a form of discourse through which God speaks to us (§147).

7. Molyneux’s Problem

Molyneux’s puzzle links the two authors. In a letter of 1688 Molyneux put to Locke the question of the blind man who regains sight; it appeared in the second edition of the Essay (1694, II.9.8), where Locke agrees with the negative answer: the man would not be able to tell cube from sphere at once, because he has not yet learned to associate the visual appearance with tactile experience. Berkeley, in the New Theory of Vision, also answers no, and the answer follows from his thesis that the ideas of sight and touch are heterogeneous. Leibniz, in the New Essays, gives a different answer, claiming that the man could tell them apart through intelligible geometrical properties.

The question is both empirical and philosophical. Operated cases of congenital cataract, such as that of William Cheselden (1728), have been invoked in the debate, though their interpretation is controversial. A study by Richard Held and colleagues (2011) of people in India who gained sight after surgery concluded that, immediately after recovery, they could not match seen objects to touched ones, but learned to do so quickly. This result is generally presented as favoring the negative answer, though some question the philosophical conclusions drawn from it.

8. From Berkeley to Hume

The passage to Hume is, in a sense, the radicalization of the same program. Hume starts from the principle that all ideas derive from impressions (a more precise version of Locke’s “all ideas come from experience”) and inherits from Berkeley the rejection of abstract ideas. But he draws conclusions that neither Locke nor Berkeley would accept: the idea of substance, material or spiritual, has no corresponding impression; the “self” is not a simple substance but a bundle of perceptions; and the idea of necessary connection between cause and effect results from habit. Applying the empiricist principle to all of metaphysics thus produces a skepticism that Berkeley had tried to avoid. In a note to the Enquiry Concerning Human Understanding (section XII), Hume treats Berkeley’s arguments as essentially skeptical, since they produce no conviction. The continuation of this path is in the article on Hume, in the one on personal identity, and in the one on skepticism from Pyrrho to Hume. The profile of David Hume completes the picture.

9. Criticism and Reception

9.1 Leibniz

In the New Essays on Human Understanding (written around 1704 but published only in 1765, after the deaths of both Locke and Leibniz), Leibniz answers the Essay point by point. To the scholastic maxim “nothing is in the intellect that was not first in the senses,” he adds: “except the intellect itself.” For him, necessary truths, such as those of mathematics, cannot derive from experience alone, which gives only particular cases; they are virtually innate, like veins in marble.

9.2 Thomas Reid and Common-Sense Philosophy

Thomas Reid (1710–1796), in An Inquiry into the Human Mind on the Principles of Common Sense (1764) and the Essays on the Intellectual Powers of Man (1785), blames Hume’s skepticism on the “theory of ideas” or “ideal system,” which Descartes, Locke, and Berkeley are said to have shared: the thesis that we perceive only ideas, not objects themselves. Reid proposes a direct realism in which perception puts us in immediate contact with things, and appeals to principles of common sense that need no proof.

9.3 Kant

For Kant, empiricism errs in deriving from experience what makes experience possible. In the Critique of Pure Reason (1781; 2nd ed. 1787), the distinction between a priori and a posteriori knowledge (see A Priori and A Posteriori) answers it: experience supplies the matter, but the subject supplies the forms. Kant also discusses Berkeley, calling his idealism “dogmatic” insofar as it would declare the existence of things outside us impossible; in the “Refutation of Idealism” (2nd ed.) he argues that inner experience itself, the consciousness of my existence in time, presupposes something permanent in space. In the Prolegomena (1783), Kant protests against the label “Berkeleian idealist” attached to him by a review published in Göttingen, and insists that his transcendental idealism differs from Berkeley’s. For later developments see Philosophical Idealism: Plato, Kant, and Hegel.

9.4 Internal and Contemporary Criticisms

Against Locke, scholars question the coherence of the primary/secondary distinction and the consistency of representationalism (if we perceive only ideas, how do we know they represent anything external?). Against Berkeley, it is argued that the principle that “an idea can be like nothing but an idea” begs the question, and that the “master argument” (§23), by which we cannot conceive a thing without conceiving it, confuses the act of conceiving with the object conceived. In contemporary philosophy, phenomenalism (the thesis that material objects are logical constructions from sense data) is often presented as the heir of Berkeleian immaterialism without God, though the comparison is a matter of discussion. In the philosophy of religion, the idea of a God who sustains the order of the world by the mere act of perceiving resonates in later debates (see Philosophy of Religion).

10. Synthesis

Read together, the two authors show how a single starting point, that the content of the mind comes from experience, can be developed in opposite directions. Locke builds a moderate epistemology that admits a material reality knowable only in part, and whose language of ideas aims to be descriptive rather than speculative. Berkeley inherits that language and turns it against the hypothesis of matter in order to put God back at the center of the explanation of experience. What unites them is the centrality of ideas; what separates them is the status of the world that ideas supposedly represent. Hume’s answer, skepticism, and those of Reid and Kant, who tried to rebuild the foundations of knowledge, continue this debate.

  • An Essay Concerning Human Understanding (1689/1690) — John Locke
  • Two Treatises of Government (1689/1690) — John Locke
  • An Essay towards a New Theory of Vision (1709) — George Berkeley
  • A Treatise Concerning the Principles of Human Knowledge (1710) — George Berkeley
  • Three Dialogues between Hylas and Philonous (1713) — George Berkeley
  • New Essays on Human Understanding (1765, posthumous) — G. W. Leibniz
  • An Inquiry into the Human Mind on the Principles of Common Sense (1764) — Thomas Reid
  • Critique of Pure Reason (1781; 1787) — Immanuel Kant
  • Locke, Berkeley, Hume: Central Themes (1971) — Jonathan Bennett
  • Stanford Encyclopedia of Philosophy: entries “John Locke,” “George Berkeley,” and “Molyneux’s Problem”

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