Credited as the founder of immaterialism, Berkeley still stands as one of the most influential empiricists

George Berkeley (1685–1753) was an Anglo-Irish philosopher and Church of Ireland bishop (Bishop of Cloyne). He is best known for his theory of immaterialism — also called ‘subjective idealism’ or ‘empirical idealism’ — the claim that so-called “material objects” are nothing more than ideas perceived by minds. Since human perception is intermittent, Berkeley argued that the world endures because God perceives it continuously. He is typically grouped with the other two major British empiricists, John Locke and David Hume.

To understand Berkeley’s text, it is essential to situate him within the history of early modern philosophy. He rejects rationalism — the view that reason alone is the supreme source of knowledge — and instead belongs to the empiricist tradition, which holds that knowledge is derived from sensory experience. Yet Berkeley is not simply following Locke; he is writing in response to him, with much of his project being devoted to criticizing and Locke’s account of empiricism.

Berkeley’s foundational text is A Treatise Concerning the Principles of Human Nature (1710). His text has 156 numbered sections. However, this blog post will focus on sections 1–33, where the case for immaterialism is concisely laid out. (Much of what follows these sections elaborates and answers objections.)

At any rate, Berkeley numbered each section to structure his argument but provided no titles. For clarity, I have added my own headings that capture the main idea of each passage.

For this blog, I’m using Jonathan Bennett’s Early Modern Texts edition of The Principles of Human Knowledge. While the language is modernized for accessibility, Bennett’s version remains faithful to Berkeley’s original arguments, and the editorial notes and translation choices are consistently clear and well-justified.

Here is the question that Berkeley is attempting to answer: “If a tree falls in the forest and no one is around to hear it, does it make a sound?”

Section-by-Section

I. Ideas as the Basis of Knowledge

Anyone who surveys the objects of human knowledge will easily see that they are all ideas … imprinted on the senses … perceived by attending to one’s own emotions … or formed out of ideas of the first two types.

Berkeley begins by stating that all human knowledge consists solely of ideas. These arise from three sources: (1) the senses, (2) reflection on one’s own emotions, and (3) imagination, which combines the first two sources. What we call an “object” is simply a cluster of these ideas that appear together. For example, to encounter an apple is to encounter a combination of color, shape, smell, taste, and texture — all of which are ideas in the mind. The name “apple” refers to this cluster of ideas.

II. Mind as Perceiver

This perceiving, active entity is what I call ‘mind’, ‘spirit’, ‘soul’, or ‘myself’… The existence of an idea consists in its being perceived.

Alongside the variety of ideas we encounter, there must also be something that perceives and acts upon them. Berkeley names this perceiving subject the “mind, spirit, soul, or myself.” Unlike ideas, which are passive and dependent, the mind is active — it wills, imagines, and remembers. The mind is not itself an idea but the entity in which ideas exist and by which they are perceived.

III. To Exist Is to Be Perceived

For unthinking things, to exist is to be perceived; so they couldn’t possibly exist out of the minds or thinking things that perceive them.

This section is the crux of Berkeley’s immaterialism. Just as everyone accepts that thoughts and emotions exist only in the mind, Berkeley insists that sensory ideas — colors, sounds, shapes, and the like — also exist only insofar as they are perceived. To say that a table exists is simply to say it is being perceived with certain qualities (color, size, texture). Because the table is not itself a thinking thing, its existence cannot be independent; it depends entirely on a mind to perceive it.

IV. Contradiction of Unperceived Objects

For what are houses, mountains, rivers etc. but things we perceive by sense? … Isn’t it plainly contradictory that these, either singly or in combination, should exist unperceived?

At this point, Berkeley acknowledges the common belief that material objects exist independently of perception. Houses, mountains, and rivers are thought to stand on their own, whether or not anyone observes them. But he argues that this is a contradiction: such objects are nothing more than collections of sensory ideas, and ideas by their very nature cannot exist unperceived. A house and mountain can only be conceptualized in terms of ideas. To suppose otherwise is to imagine an idea existing without a mind — an impossibility.

V. The Futility of Abstraction

Light and colours, heat and cold, extension and shapes … what are they but so many sensations, notions, ideas, or sense impressions? And can any of these be separated, even in thought, from perception?

Berkeley highlights that perceptual qualities — like color, heat, or shape — cannot exist apart from their being perceived. The color red, for instance, has no independent existence as an abstract idea outside of perception. While Berkeley concedes that the mind can abstract objects that are capable of existing separately (such as imagining a torso without limbs), perceptual qualities themselves cannot be detached from the act of perceiving. In short, to conceive of a quality like red without reference to perception is impossible.

VI. Self-Evident

… For them to exist is for them to be perceived or known; consequently so long as they aren’t actually perceived … they must either have no existence at all or else exist in the mind of some eternal spirit.

Berkeley emphasizes that his claim is self-evident: to genuinely reflect on it is to see its truth. Everything in heaven and earth depends on perception for its existence. Bodies cannot exist on their own; their being requires a mind to perceive them. If not perceived by finite spirits, these objects must either cease to exist altogether or continue in existence through the perception of an eternal spirit that observes them at all times.

VII. Spirits as Substances

Now it is plainly self-contradictory to suppose that an idea might exist in an unperceiving thing, for to have an idea is just the same as to perceive…

For Berkeley, the logical conclusion is that the only substances that exist must be spirits — minds that perceive. All perceptual qualities — from colors and shapes to smells and motion — are only ideas in a given sense impression. A chair only exists insofar as a perceiver perceives its qualities: the shape, size, color, etc. If something does not perceive, then that thing is not capable of having ideas, because having an idea is the same as perceiving it. Thus, a thing that does not perceive cannot be a substance with its own qualities, because to suppose that would require saying that the qualities exist unperceived — which is contradictory.

VIII. Copies of the Material World

… The only thing an idea can resemble is another idea; a colour or shape can’t be like anything but another colour or shape.

At this point, some may object that although ideas exist only in the mind, there might still be independent, material objects outside the mind that our ideas copy or resemble. For instance, the idea of an apple could be seen as just a mental image of a “real apple” that exists beyond perception. This allows, at least in theory, for a world of ideas that are mere representations of an underlying material world.

Berkeley rejects this objection. He argues that an idea can only resemble another idea: colors can resemble other colors, shapes other shapes, and so on. But it is incoherent to say that a visible color could resemble something invisible, or that tangible hardness could resemble something intangible.

From here, Berkeley show that the objections must fall into one of two categories:

1. Things outside the mind are perceivable. If so, then they are themselves ideas — and Berkeley’s point is confirmed that nothing exists beyond ideas.
2. Things outside the mind are not perceivable. If so, then it is meaningless to claim that our ideas resemble them, since resemblance is only conceivable between perceivable things.

In either case, there can be no notion of an idea resembling a material object, because there are no material objects — only ideas.

IX. Berkeley’s Rejection of Locke’s Primary/Secondary Qualities

… Extension, shape, and motion are quite clearly nothing but ideas existing in the mind, and … consequently neither they nor things from which they are copied can exist in an unperceiving substance.

In this section, Berkeley addresses Locke’s distinction between primary and secondary qualities. Locke holds that primary qualities (extension, solidity, motion, rest, number) exist in matter outside the mind, while secondary qualities (color, taste, sound, etc.) do not exist in matter but are produced in us through the operation of primary qualities. Berkeley rejects this framework entirely. He argues that primary qualities are no less ideas than secondary ones, since they too exist only in perception. And because ideas can only resemble other ideas, it makes no sense to suppose that primary qualities could inhere in an unthinking substance. Thus, the very notion of “matter” collapses.

X. Berkeley’s Continued Criticism

In short, extension, shape and motion, abstracted from all other qualities, are inconceivable. It follows that these primary qualities must be where the secondary ones are — namely in the mind and nowhere else.

Berkeley continues his critique of the primary/secondary quality distinction. He argues that primary qualities such as shape, extension, and motion cannot be conceived without also attributing secondary qualities like color. Since philosophers already concede that secondary qualities exist only in the mind, and these cannot be separated from primary qualities even in thought, it follows that primary qualities too exist only in the mind. Thus, the supposed division between the two collapses entirely.

XI. Extension and Motion are Relative

So if there is extension outside the mind, it must be neither large nor small, and extra-mental motion must be neither fast nor slow. I conclude that there is no such extension or motion.

Berkeley argues that judgments of size (large/small) and speed (fast/slow) are relative and depend entirely on the standpoint of the observer. Since these terms are mind-dependent, extension and motion outside the mind would have to exist without being large or small, fast or slow. This is an incoherent notion. From this, Berkeley concludes that extension and motion cannot exist outside perception. And because solidity requires extension, material objects cannot be solid either (meaning they cannot exist).

XII. Numbers Only Exist in the Mind

Thus, the same distance is one or three or thirty-six, depending on whether the mind considers it in terms of yards, feet or inches.

Berkeley extends his critique to number, showing that it exists only in the mind. Just as with extension and motion, number is not absolute but depends on how the mind applies a unit. The same object can be counted differently depending on perspective — for example, as one book, one chapter, or one page. This reveals that number is entirely a product of human understanding, grounded in arbitrary mental combinations rather than in any independent reality.

XIII. Unity as an Abstract Idea

I don’t find that I have any such idea corresponding to the word ‘unity’ … In short, it is an abstract idea!

Some philosophers argue that the concept of “unity” is a simple idea that always accompanies perception. For instance, when we perceive an apple, we don’t just perceive its color, taste, and shape — we also supposedly perceive it as one apple, a unity. Berkeley rejects this, insisting that he cannot find any such distinct idea of unity in his mind. If it were really a simple idea accompanying all others, it would be the most obvious of all. Instead, he concludes that “unity” is nothing more than an abstract idea.

XIV. All Qualities Are Relative

If that argument is good, then why can’t we re-apply it to prove that shape and extension don’t resemble any fixed and determinate qualities existing in matter, because they appear differently to the same eye in different positions…?

Berkeley observes that many philosophers already concede secondary qualities — like heat, cold, sweetness, or bitterness — exist only in the mind because their appearance changes with the condition of the perceiver. For example, the same food may taste sweet to a healthy palate but bitter to someone with a fever. This relativity shows such qualities cannot exist in matter itself. Berkeley argues that the same reasoning applies to primary qualities: judgments of extension as long or short, or motion as fast or slow, are equally relative. Therefore, primary qualities too must exist only in the mind.

XV. Quick Summary

… It is impossible that any colour or extension or other perceptible quality should exist in an unthinking thing outside the mind, or indeed that there should be any such thing as an object outside the mind.

At this point, Berkeley summarizes his argument: perceptible qualities such as color, taste, and sound exist only in the mind, and the same holds for extension and motion. While some philosophers conclude from this that our senses simply fail to grasp the true qualities of objects, Berkeley rejects that position. For him, there are no hidden “real” qualities in matter beyond perception; rather, matter itself is nothing more than an idea in the mind.

XVI. Incoherence of Matter

… Please explain to me what is meant by matter’s ‘supporting’ extension .… Explain ‘support’, then! Obviously it cannot be meant here in its usual or literal sense, as when we say that pillars support a building…

Berkeley critiques the notion that matter is the substratum that “supports” qualities such as extension. The argument assumes that for something to be extended, there must be a foundational substance — matter — that underlies it. Berkeley challenges this by asking: what is matter, and what does it mean to say it supports extension? If philosophers admit they have no clear idea of matter itself, they must at least explain how it relates to qualities. But the notion of “support” cannot be literal, like pillars holding up a building, and without a clear explanation it becomes an empty term. Berkeley’s rhetorical questions highlight the incoherence of treating matter as the foundation of perceptible qualities.

XVII. “Material Substance”

… There is no clear meaning in either of the two parts or strands that are supposed to make up the meaning of the words ‘material substance’ … Isn’t this a direct contradiction, and altogether inconceivable?

Berkeley examines how even the most precise philosophers define “material substance,” showing that their explanations rest on two ambiguous notions: (1) being in general and (2) supporting qualities. The first refers to the supposed “being” that remains when all perceptible qualities — color, shape, motion, etc. — are stripped away. Berkeley argues this is meaningless because “being” without any qualities cannot be perceived; it is an abstraction with no content. The second notion of supporting qualities echoes earlier claims that perceptible attributes must inhere in a substance. Yet Berkeley presses: what does “support” really mean? It cannot be literal, like pillars holding up a building, because we can perceive this kind of support. In the case of matter, “support” is merely a word without an idea attached to it — an abstract and empty term.

XVIII. How Do We Know?

… Our senses give us knowledge only of our sensations … They don’t inform us that outside the mind (that is, unperceived) there exist things that resemble the items that are perceived … Evidently, then, we aren’t compelled to suppose that there are external bodies as causes of our ideas.

Suppose, for the sake of argument, that material substances — solid, shaped, movable things — do exist outside the mind. Berkeley asks: how could we ever know this? Knowledge must arise either from sense perception or from reason. Our senses, however, only provide us with ideas within the mind; even materialists concede that sensations do not prove the existence of corresponding objects outside perception. If the knowledge of matter cannot come from sense, it must come from reasoning. Yet reason cannot establish it either, since there is no necessary or logical connection between ideas in the mind and supposed external bodies. Dreams and hallucinations demonstrate that ideas can occur without external causes, so it is always possible that our experiences arise independently of matter. Thus, there is no basis for believing in material substance.

XIX. Do Not Claim Matter’s Existence Is Probable

The materialists admit that they cannot understand how body can act upon spirit, or how it is possible for a body to imprint any idea in a mind; and that is tantamount to admitting that they don’t know how our ideas are produced.

By this point, many of Berkeley’s opponents might concede much of his argument yet still maintain that, even if matter is not necessary to explain our sensations, its existence is at least probable, since it offers a simpler account of experience. Given this, might material bodies be the cause our ideas? Berkeley rejects this reasoning. Even materialists, he notes, admit they cannot explain how a material thing could act upon a spirit — how something extended and inert could produce an idea in the mind. This ignorance makes belief in material substance entirely unfounded. To affirm that matter exists even though it cannot be sensed, proved, or explained is, for Berkeley, to imply that God has created countless material things that are completely useless — things that serve no purpose in producing or explaining our ideas and sensations.

XX. With or Without Matter

… A thinking being might, without the help of external bodies, be affected with the same series of sensations or ideas that you have … that consideration is enough, all on its own, to make any reasonable person suspect the strength of whatever arguments he may think he has for the existence of bodies outside the mind.

Berkeley argues that even if external material bodies did exist, it would be impossible to know it. Conversely, if they did not exist, our experiences could remain exactly the same. He imagines a thinking being without access to any external world who nonetheless experiences the same sensations and ideas we do. Such a being would have the same reasons to believe in matter as we do — though no matter existed at all. This thought experiment demonstrates that belief in material substance is unreliable.

XXI. Closing Critique

I could cite many errors and difficulties (not to mention impieties) have sprung from that doctrine … arguments about its bad consequences are unnecessary for confirming what has, I think, been well enough proved a priori regarding its intrinsic defects.

In this section, Berkeley concludes his critique of the belief in a mind-independent substance, calling it a fundamentally mistaken doctrine. He observes that the doctrine of matter has produced numerous philosophical confusions and religious errors. However, Berkeley chooses not to elaborate on these moral or theological consequences, noting that his case against matter has already been proven a priori. Having shown the intrinsic contradictions for material substance, he deems further discussion of its harmful effects unnecessary.

XXII. Conceiving Matter Outside the Mind is Impossible

… Try to conceive it possible for a sound or shape or motion or colour to exist outside the mind, or unperceived … if you can so much as conceive it possible for one extended movable substance … to exist otherwise than in a mind perceiving it, I shall cheerfully give up my opposition to matter.

Berkeley concludes by inviting the reader to reflect inwardly on the truth of his argument. If one doubts his claims, he challenges them to try to imagine a color, a shape, a sound, or any perceptible quality existing outside of perception. He states that he will abandon his entire thesis if anyone can successfully conceive this. Yet he is confident that such an act is impossible, since to think of a perceptible quality is already to perceive it.

XXIII. Conceiving the Unconceived

You form in your mind certain ideas that you call ‘books’ and ‘trees’… but while you are doing this, you perceive or think of them!… to show that, you would have to conceive them existing unconceived or unthought-of, which is an obvious contradiction.

At this point, a reader might object that it is easy to think of a tree in a park or a book on a shelf without actually perceiving it, and therefore such objects could exist outside the mind. Berkeley replies that this confuses imagination with material existence. When you imagine a tree or a book, you are forming ideas of them within your own mind — you are perceiving them as ideas, not proving their existence independent of perception. To conceive of something as unconceived is contradictory, since doing so requires an act of conception.

XXIV. Another Summary…

So this is what I insist on: the phrase ‘the absolute existence of unthinking things’ has either no meaning or a self-contradictory one.

Berkeley takes aim at the phrase “the absolute existence of unthinking things,” arguing that such language is either meaningless or self-contradictory. Through reflection, one realizes the impossibility of conceiving a perceptible object that is, at the same time, unperceived. To assert the “absolute existence” of unthinking things is to speak without meaning, since these things can only be conceived as ideas within perception. The very attempt to think of them apart from perception contradicts itself.

XXV. Ideas Are Passive

Think about it a little and you’ll realize that passiveness and inertness are of the essence of an idea, so that an idea can’t do anything or be the cause (strictly speaking) of anything; nor can it resemble anything that is active…

Berkeley now transitions from his critique of matter to the question of what causes ideas. He begins by rejecting the notion that ideas themselves are active. When one reflects on ideas — whether of color, sound, or motion — one finds no power, agency, or causal capacity in them. Ideas do not “do” anything: one idea cannot produce another; for example, the idea of redness does not generate the idea of a shape. Because motion and extension are themselves ideas, it is impossible for material things or mechanical interactions to cause sensations. This reveals that ideas, being passive, must depend on something active for their existence and order.

XXVI. The Cause of Ideas: A Spirit

These ideas must have a cause — something they depend on, something that produces and changes them. It cannot be a corporeal or material substance, because I have shown that there is no such thing. We must therefore conclude that the cause of ideas is an incorporeal active substance — a spirit.

Following from his claim in section XXV that ideas are entirely passive, Berkeley concludes that there must exist something active that produces ideas. This active cause he calls a “spirit.” Because Berkeley has already disproved the existence of material or corporeal substances, this spirit cannot be material in nature. It must therefore be an “incorporeal active substance” — a mind or spirit that perceives and produces ideas.

XXVII. Spirit is Non-Perceptible

A spirit is an active being … it follows that no-one can form an idea of a soul or spirit … the nature of spirit (i.e. that which acts) is such that it cannot itself be perceived

Berkeley defines spirit as an active being that is simple and without parts. When spirit perceives ideas, it is called “understanding”; when it produces or manipulates them, it is called “will.” These are not parts of the spirit but powers belonging to it. Because ideas are passive, it is impossible for them to represent or conceptualize something active. Thus, spirit cannot itself be perceived — it is fundamentally non-perceptible. What we can perceive are only the effects of spirit: acts of willing, loving, hating, and similar operations of the mind that reveal its activity.

XXVIII. The Mind is Active

I need only to will, and straight away this or that idea arises in my mind; and by willing again I can obliterate it and bring on another … It is because the mind makes and unmakes ideas in this way that it can properly be called active.

The mind is known to be active through direct experience. By an act of will, one can summon, change, or erase ideas at will. This capacity to produce and vary ideas demonstrates that the mind — or spirit — is an active being. Because it possesses volition and will, the mind can be properly called an agent. For Berkeley, the notion of an “unthinking agent” is nonsensical, since agency necessarily requires the ability to will.

XXIX. Source of Sensory Ideas Beyond One’s Will

When in broad daylight I open my eyes, it isn’t in my power to choose whether or not I shall see anything, or to choose what particular objects I shall see … my will is not responsible for the ideas that come to me through any of my senses. So there must be some other will — some other spirit — that produces them.

Berkeley distinguishes between ideas that arise from one’s own will and those that come through sensory perception. While imaginative ideas can be summoned or changed at will, sensory ideas are imposed upon the mind without choice. When one opens their eyes, for instance, one cannot decide whether to see sunlight or determine what specific objects will appear. This lack of control applies to all the senses. Because sensory ideas do not depend on personal will, Berkeley concludes that they must be caused by another spirit. This transition leads him toward identifying that higher spirit — the ultimate source of sensory experience — as God.

XXX. The Order of Nature = God’s Will

The ideas of sense are stronger, livelier, and clearer than those of the imagination … they come in a regular series, and are inter-related in admirable ways that show us the wisdom and benevolence of the series’ author … the phrase ‘the laws of nature’ names the set rules or established methods whereby the mind we depend on — God — arouses in us the ideas of sense.

The ideas we receive through sensory perception are far stronger and more vivid than those produced by our own will. For example, one may imagine being burned by a fire, yet the actual sensation of being burned is far more forceful and immediate. This distinction shows that sensory ideas are not random creations of the mind but appear in a steady and orderly sequence. Berkeley argues that this order reflects the laws of nature — fixed patterns designed by God. Through experience, we learn these divine laws by observing how certain ideas accompany or follow others.

XXXI. The Laws of Nature are Practical

That food nourishes, sleep refreshes, and fire warms us … we know all this not by discovering any necessary connection between our ideas but only by observing the settled laws of nature.

Without knowledge of the laws of nature, Berkeley argues, there would be no distinction between a grown adult and an infant. Survival depends on understanding certain natural regularities — knowing what to eat, when to rest, how to avoid harm, etc. Sensory perceptions follow specific patterns through which we learn to maximize pleasure and minimize pain. This understanding does not arise from ideas joining or coupling within the mind but from observing the fixed and intelligible order established by God.

XXXII. Do Not Mistake Divine Order For Material Causation

When we perceive that certain ideas of sense are constantly followed by other ideas … we immediately attribute power and agency to the ideas themselves, and make one the cause of another — than which nothing can be more absurd and unintelligible.

The uniformity and regularity of sensory experience are not proof of material causation but of God’s divine order. Paradoxically, however, this very consistency leads people to believe the opposite — to think that one sensory idea causes another. Because we observe certain perceptions regularly follow one another, we begin to assume that ideas themselves possess power or agency. Berkeley argues that this is mistaken: ideas are passive and cannot act. When we see that the sun’s rays are followed by the feeling of heat, we wrongly attribute causal force to the ideas themselves. In truth, these orderly patterns solely arise from God’s will.

XXXIII. Divine Ideas in the Mind

The ideas imprinted on the senses by the author of nature are called ‘real things’ … but still they are ideas, and certainly no idea — whether faint or strong — can exist otherwise than in a mind perceiving it.

In this final section, Berkeley distinguishes between two kinds of ideas: (1) those imprinted on the senses by the “author of nature” — God — which we call “real things,” and (2) those formed by imagination, which are less structured and orderly. Although sensory ideas appear more vivid, coherent, and stable than imagined ones, they are still ideas and exist only in the mind. The distinction between them lies not in where they exist, but in how they exist: sensory ideas are caused by God’s will, while imagined ideas arise from one’s own. Thus, all ideas — whether sensory or imagined — exist solely as perceptions within the mind.

Conclusion

This blog post analyzed the first 33 sections of Berkeley’s most famous work on empiricism. His theory of immaterialism — a view denying the existence of matter and affirming that only minds and ideas exist — remains a deeply influential contribution to philosophy. While sensory perceptions may feel stronger and more vivid than imagined ideas, Berkeley reminds us that they are still ideas, existing only in the mind. Whether his argument convinces or not, it is undeniable that Berkeley stands as one of the most brilliant and provocative thinkers in the history of philosophy.