Treatise Concerning Principles of Human Knowledge

Quick Facts

• Full title: A Treatise Concerning the Principles of Human Knowledge
• Author: George Berkeley
• Published: 1710, in Ireland
• Tradition: Empiricism
• Main doctrine: immaterialism, the view that there is no mind-independent material substance
• Famous slogan: esse est percipi, "to be is to be perceived"
• Main target: the idea that ordinary objects are material things existing outside every mind

The Problem

Berkeley thinks early modern philosophy has made ordinary life harder to understand than it needs to be. Philosophers say we directly know only our ideas, then they add that those ideas are caused by hidden material objects outside the mind. Berkeley asks: if we never perceive that hidden matter, why believe in it?

The problem is not whether tables, trees, colors, sounds, and pains are real. Berkeley thinks they are real. The problem is what their reality consists in. Is a table a material substance that exists behind all perception, or is it the stable group of sights, touches, and possible experiences that a mind can have?

He also thinks the theory of abstract ideas makes this confusion worse. An abstract idea is supposed to be a general idea stripped of all particular features, such as a triangle that is neither equilateral nor scalene, neither large nor small, neither black nor white. Berkeley says we never actually find such things in the mind. We have particular images and use words generally.

In One Minute

Berkeley's Treatise argues that sensible objects are not material things hidden behind experience. They are ideas perceived by minds. An "idea" here means anything immediately perceived: a color, sound, taste, shape, pain, image, or memory.

His slogan is esse est percipi: for sensible things, to exist is to be perceived. A cherry is not a bare material core plus mental decorations. It is the red color, round shape, sweet taste, smooth feel, and other perceivable qualities held together in regular experience.

This does not mean only Berkeley's own mind exists. Berkeley also believes in other finite minds and in God. God matters because our sensory experiences are not chosen by us. I can imagine a fire, but I cannot decide that a real fire will stop burning me. Berkeley explains the order and force of sense experience by saying it is given by an infinite mind.

The Main Argument

The argument starts from an empiricist premise Berkeley shares with John Locke: what we immediately perceive are ideas. When you look at an apple, you immediately have color, shape, size, smell, taste, and touch. You do not also perceive a bare material support hiding underneath those qualities.

Berkeley then asks what an ordinary object is, if not the things we perceive. A house is seen, touched, entered, measured, and remembered. A mountain is a stable range of visual and tactile possibilities. If all the directly perceived features are ideas, then the object we know is a collection of ideas.

The next step is his attack on matter. "Matter" means a mind-independent material substance: something that exists outside all perception and supports qualities such as shape, motion, and solidity. Berkeley says this notion does no real work. We never perceive it. We cannot form a clear idea of it. And if it is unlike our ideas, it cannot be what our ideas resemble.

This is why he rejects Locke's split between primary and secondary qualities. Locke treats color, taste, and sound as mind-dependent, but shape, size, motion, and number as qualities in bodies themselves. Berkeley replies that shape and motion are also perceived through sense and vary with viewpoint. A coin looks round from above and oval from an angle. A tower looks small from far away and large nearby. If color depends on perception, Berkeley thinks extension and motion do too.

Berkeley also gives a challenge: try to imagine a sensible object existing wholly unperceived. Suppose you imagine a tree in an empty field with no one looking at it. Berkeley says you are still imagining the tree. The imagined "unperceived" tree is present to your mind as an idea. This does not prove by itself that no object could exist unperceived, but it shows what Berkeley thinks is wrong with the materialist picture: it asks us to think a sensible thing apart from all possible sensing.

The final step is God. Ideas are passive; they do not decide what happens next. My idea of heat does not choose to be followed by pain when I touch flame. My private imagination is weaker and less orderly than sense experience. So Berkeley says the vivid, regular order of nature must come from an active spirit. For Berkeley, that active spirit is God. Natural laws are the regular patterns by which God orders our perceptions.

Key Ideas With Examples

Idea means anything immediately perceived or present to the mind. Redness, sweetness, pain, a remembered tune, and an imagined triangle are all ideas. Berkeley's point is not that they are unreal. It is that their being consists in being perceived.

Mind or spirit means an active perceiver. A mind sees, wills, imagines, remembers, and compares. Berkeley does not think a mind is another idea, because you do not perceive your mind the way you perceive a color. You know yourself as the one doing the perceiving.

Immaterialism means the denial of material substance. It does not mean denying ordinary objects. Berkeley thinks apples, books, and houses exist. He denies that they are made of an unperceived material stuff behind their perceivable qualities.

Esse est percipi means "to be is to be perceived." Berkeley applies this to sensible things, not to minds. A pain exists when it is felt. A color exists when it is seen. A table exists as a stable object of actual and possible perception within an ordered world.

Abstract ideas are supposedly featureless general ideas. Berkeley rejects them. You can use the word "triangle" generally, and you can prove something about all triangles by drawing one particular triangle. But the triangle you imagine is still particular: it has some size, some shape, and some color or line thickness.

Matter is Berkeley's name for mind-independent material substance. He thinks it is a philosopher's invention. If matter is supposed to be colorless, soundless, tasteless, untouched, and outside every mind, Berkeley asks what content is left in the idea.

Primary and secondary qualities are Locke's distinction between qualities said to be in objects themselves and qualities said to depend on perceivers. Berkeley collapses the distinction. Shape, size, and motion are no less perceived than color and taste.

Why It Matters

The Treatise is one of the sharpest tests of empiricism. If all knowledge begins with experience, Berkeley asks why philosophers believe in a kind of matter that experience never gives them. He pushes empiricism toward idealism: reality, as known by us, is made of minds and their ideas.

The book also changes the debate about perception. Berkeley is not saying, "nothing is real." He is saying ordinary experience is more secure without hidden matter. On his view, the real apple is the one you see, touch, and taste, not an unknown object standing behind those experiences.

It also made God central to Berkeley's metaphysics. God is not added as a decorative religious ending. God explains why the world is public, stable, and lawful rather than a private dream. Different people encounter the same ordered world because their perceptions are coordinated by the same divine mind.

Even critics who reject Berkeley still have to answer his questions. What do we directly perceive? Are objects known through ideas, or directly? What does "matter" add to the explanation? Can we make sense of mind-independent objects without using concepts drawn from experience?

Proponents, Critics, and Opponents

George Berkeley is the proponent. The Treatise gives his compressed argument; Three Dialogues between Hylas and Philonous later presents the same view in dialogue form.

John Locke is the main background opponent. Berkeley accepts Locke's emphasis on ideas and experience, but rejects Locke's material substance and his account of abstraction. Berkeley thinks Locke keeps too much hidden metaphysics inside an empiricist theory.

David Hume inherits the pressure Berkeley puts on substance. Hume does not simply become a Berkeleyan, but Berkeley's attack on material substance helps prepare the way for Hume's broader skepticism about substance, causation, and the self.

Materialists and realists are Berkeley's broad opponents. A materialist, in the sense Berkeley attacks, says ordinary objects exist as mind-independent material things. A realist may say the world exists independently of our minds and is not made only of ideas. Berkeley's challenge is to explain how such a world is known and what "matter" clearly means.

Many later philosophers think Berkeley's arguments are powerful but incomplete. One common criticism is that imagining an unperceived tree does not show that unperceived trees are impossible; it only shows that imagining involves a mind. Another is that appealing to God may explain order inside Berkeley's system, but will not persuade readers who do not already accept his theology.

Related Pages

• George Berkeley: author of the Treatise and main defender of immaterialism.
• Empiricism: the tradition Berkeley radicalizes by asking what experience actually gives us.
• John Locke: Berkeley's main early modern target on substance, qualities, and abstraction.
• Essay Concerning Human Understanding: Locke's major work on ideas and knowledge.
• Three Dialogues between Hylas and Philonous: Berkeley's later dialogue defense of the same basic view.
• David Hume: later empiricist shaped by the problems Berkeley pressed.