The idea that “all those bodies which compose the mighty frame of the world, have not any subsistence without a mind—that their being is to be perceived or known” will hardly seem obvious to anyone unfamiliar with George Berkeley or with idealism. This startling statement has considerable shock value, but it is true to Berkeley’s bold metaphysical thesis that reality is mental or spiritual in nature.

The statement’s emphasis on perception reveals its author’s epistemological and methodological approach: empiricism. Although not all empiricists would accept Berkeley’s conclusions and not all metaphysical idealists would accept his method, none would deny his importance in the traditions of both empiricism and idealism. That his method and even his immaterialism have influenced some modern physicists and that his analytical technique is valued even by such antimetaphysicians as the logical positivists are proofs of the classical status Berkeley’s work has enjoyed.

His aims, however, were primarily those of a metaphysician and theologian; he wished to undermine skepticism and atheism by refuting materialism, to demonstrate God’s existence and immateriality, to show the immortality of the soul, and to clarify current scientific and philosophical confusions. The latter are due, he claimed, not to inherent defects in people’s mental faculties but to their use: “We have first raised a dust and then complain we cannot see.” Berkeley intended to settle this dust and to destroy materialism.

A chief cause of obscurity, Berkeley states, is the doctrine of abstract ideas, the theory that the mind can abstract from particular qualities a clearly conceived notion of what is common to them, but which itself is otherwise like none of them, or that the mind can separate in thought what cannot be separated in reality. An example of the first abstraction would be a notion of color that is neither red, blue, green, and so forth, or of extension that has neither size, shape, line, plane, nor surface; an illustration of the second would be an abstract idea of color or motion without extension.

Berkeley finds such abstraction psychologically impossible and challenges the reader to conceive such an idea as that of a triangle with all of the general and yet none of the specific characteristics of triangles. However, must Berkeley then deny the universality of ideas essential to rational demonstration such as geometrical proofs relevant to all triangles? No—ideas may be general without being abstract; one generalizes particular ideas by temporarily disregarding their unique features, while one’s demonstrations concern only features shared. However, this universality in function must not be mistaken for abstract conception; the latter is actually without content and unintelligible.

Berkeley claims that the confused belief in abstract ideas arises from language: The assumption is that general names signify precise abstract ideas indispensable to thinking and communication, but this is false. Attention should be paid not to words but to ideas themselves. Because ideas are perfectly transparent, being known directly, the verbal controversies and errors springing from abstraction can be avoided. Thus, Berkeley sets the stage for a far-reaching application of the foregoing conclusions to an analysis of the nature and existence of the objects of knowledge.

The objects of knowledge, Berkeley writes, are ideas of three kinds: sensations, ideas originating in the mind’s own passions and activities, and those of memory and imagination. He first deals with “sensible” objects. Through sight, one knows color; through sight and touch, one knows size and shape; through touch, hardness; and through smelling, odors. Certain constant collections of such ideas are considered one object or thing and accordingly named, such as “apple” or “tree.” However, obviously perceived ideas require a perceiver, and this is spirit or mind, not itself an idea. Careful examination shows that thoughts and ideas have no existence external to minds; hence “sensible” things or physical objects do not exist apart from their perception in minds—esse es percipi: For them to be is to be perceived.

The typical reaction to this conclusion is to accuse Berkeley of denying the reality of the physical world and even the evidence of his senses. However, Berkeley explains that when one says that a table exists, this means that someone sees and feels it or will do so on occasion. The very meaning of “existence” or “being” applied to perceptible objects is exhaustively described in terms drawn from perception—nothing else can meaningfully be said about them. To think that sensible objects or their alleged metaphysical substratum, matter, exist “without” (external to) the mind is to entertain an unintelligible abstraction and a clear contradiction. People commonly think that houses or mountains exist unperceived. However, what are these but objects of the senses? Is it not self-contradictory to think that sensations or ideas exist unperceived? When one imagines that one can think of unperceived objects, one is merely thinking of objects while forgetting the perceiver, but meanwhile one is perceiving or thinking of them. One cannot conceive the inconceivable.

However, the common belief that matter exists even when it is unperceived will not die easily, so Berkeley tries to anticipate every possible objection. One of the first arises from the “representative” theory of perception, which grants that ideas occur only in minds but holds that they represent or copy things outside minds. Berkeley’s most direct answer is that ideas can resemble nothing but other ideas. How could a color represent something uncolored, or a sound something inaudible?

Both rationalists and empiricists such as René Descartes and John Locke held that in describing one’s knowledge of the physical world, one must distinguish sensed qualities, which are mostly subjective, from others that are wholly objective. These philosophers argue that “primary” qualities such as figure, motion, spatial location, and shape are inherent in objects themselves and are perceived without distortion or addition by the observer. However, “secondary” qualities such as color, sound, and taste are so obviously variable that they must be contributed by the subject’s mind, though of course originally caused by action upon him of the primary qualities. Thus color qualities are subjective but caused by motion of light—color is “in the mind” but motion is “out there.” Because qualities must qualify something (it was assumed), the primary qualities subsist in matter, the reality of which they are the appearances. Thus primary qualities really do represent or copy the external world.

However, this theory is fallacious, Berkeley holds; if it admits that secondary qualities are in the mind, it must concede that primary qualities are also, because both types are inseparable actually and conceptually. Can one conceive of an extended, moving body that has no color or temperature? Also, the arguments that secondary qualities are subjective apply equally to the primary. Consider size; one’s estimate of size depends on the nature and position of one’s sense organs. Berkeley’s Three Dialogues Between Hylas and Philonous (1713) makes this point by noting that what will seem minute to a person may appear mountainous to a mite. Even number varies with point of view, as when a given length is considered as one, three, or thirty-six (yard, feet, or inches). However, finally, the copy theory leads to utter skepticism by insisting that ideas represent something wholly unlike ideas and by distinguishing between “mere appearance” and “reality,” for it thus posits an external world forever unknowable.

Still, belief in a material substratum or support of sensed qualities will persist. Yet matter cannot literally “support” qualities, since “support” is itself a spatial term and space is perceptual. Even if there were such a substance, the problem of knowledge would remain. Knowledge stems from either sense or reason; the former yields only immediate objects of perception, or ideas, as even the materialists hold. However, reason cannot bridge the gap between ideas and matter because it would then have to argue from what one knows—ideas—to something quite alien; and materialists themselves admit no logically necessary relationship between ideas and matter. Furthermore, it sometimes happens, and conceivably always could happen, that one entertains ideas when no external bodies are supposed present, as in the case of dreams. Finally—and here Berkeley broaches a problem Descartes could not solve—how could matter possibly act on spirit to produce ideas? The more one insists on their substantial differences, the less conceivable is causal interaction.

However, if one denies the reality of external bodies, will it not sound very odd to say that one eats and drinks ideas? Of course, agrees Berkeley; but his argument is about truth, not terminology. People may use common speech, even the term “matter” itself, as long as they refer only to the sensible world. If an opponent boasts his senses’ superiority to any argument whatever, Berkeley is only too glad to join him, for he denies nothing actually perceived. “It were a mistake to think that what is here said derogates in the least from the reality of things.” Berkeley intended to refute skepticism and atheism not by denying reality but by showing the impossibility of the materialistic account of it.

Yet if his theory is true, can it distinguish reality from illusion—for example, real from merely imaginary fire—since everything perceived consists only of ideas? If there is any doubt, Berkeley answers, put your hand in the real fire, and you will sense a pain lacking in the imaginary one—but can you suppose pain existing externally to a mind? Fantasy and illusion are differentiated from the real world by obvious differences in their ideas; those of the latter are more vivid, constant, and coherent; their regular, predictable order constitutes the laws of nature, and they are independent of people’s wills as imagination is not. In fact, this independence marks the one legitimate sense in which one speaks of “external objects”; sensed qualities are external to finite spirits’ wills but not to that of the eternal Spirit, God, of whose will they are a perceptible expression.

Critics point out that if the existence of things depends on perception, they will exist and cease to exist with the occurrence and cessation of perception, and therefore, this theory is absurd. Berkeley counters by asking whether the statement that a table continues to exist when everyone leaves the room means anything more than that if one were still there one would perceive it, or if one were to return, one would once again see it. >From the reliability of nature’s order, one can both reconstruct the past and predict the future, in neither of which are there present finite minds as perceivers, but this is wholly consistent with saying that objects and events are only what they are perceived to be (in past, present, or future). If no finite minds existed at all, whatever remained would nevertheless be perceived by the omniscient, eternal Spirit. Clearly, the strength of Berkeley’s arguments here lies in the difficulty of describing an existent known to no mind whatever.

However, is this really a plausible account of nature? Must not any scientific explanation of natural events presuppose causal efficacy resident either in matter itself or in primary qualities such as extension and motion? Berkeley answers in the negative; he has already shown that the notion of matter explains nothing at all because it is incomprehensible and the primary qualities are ideas. Ideas are inert or inactive, having no causal power; there is no idea of causation in addition to those of successive events. Yet one gains a notion of causality from one’s own volition; one finds that one can produce and manipulate some ideas at will. However, if action is the prerogative of spirit, and if finite minds could not possibly produce the vast and intricate system of ideas called nature, it follows that nature is the work of the infinite Spirit.

Suppose, however, that one grants both the existence of this Spirit and the extremely complicated mechanism of nature. To what purpose did God create such a powerless machine if he wished merely to communicate with finite minds? Why not do it directly? Berkeley meets this objection by observing that if anything were superfluous, it would be an unknowable, ineffectual corporeal substance; it is possible, on the other hand, to give a rationale for nature.

Its orderly mechanism, while not indispensable to God, is still instrumental to human learning and profit. Observing the conjunction of fire and heat, people learn not that the idea of fire causes the idea of heat, but that the former signifies that the latter will follow. Single ideas are like words, and the laws of nature like the grammar of a language; however, just as it is unwise to study only grammar and neglect meaning, so it is folly for science to concentrate only on mechanical laws and neglect the final causes (purposes) they express, those determined by God’s wisdom and goodness. This does not derogate from science, but redirects it to explication of phenomena as signs rather than as effects of physical causes. Thus the hypothesis of matter is unnecessary even to physics.

Why, then, is belief in matter so pervasive? Partly because people found that objects of sensation seemed to be independent of themselves and thus supposed that such ideas exist externally. Philosophers saw the error of this supposition, but in trying to correct it by positing the external existence of matter, they substituted another mistake, unaware of the internal contradictions involved. Furthermore, the operations of the eternal Spirit are so lawful that it was not imagined they were those of a free spirit rather than those of rigidly mechanical causes; and although they clearly point to his being, still there is no collection of sensed qualities making God visible or tangible as people are.

In the foregoing considerations, the existence of spirit has been assumed on the basis of only one argument—that because ideas are not self-subsistent and matter is a nonentity, ideas can exist only in a different substance, spirit. However, if they are inactive and one can thus have no ideas of spirit, how does one know that spirit exists? Berkeley says that people have a notion of spirit because they understand the terms describing it and its activities, a notion people get “by inward feeling or reflection.” Other spirits are known by reasoning from analogy with one’s own; one perceives their effects and infers other minds as causes. A spirit’s existence consists not in being perceived but in perceiving; it is “one simple, undivided, active being—as it perceives ideas it is called the understanding, and as it produces or otherwise operates about them it is called the will.” No more than a notion of matter can be abstracted from sensed qualities can the existence of spirit be abstracted from its cogitation.

An interesting consequence follows from this in conjunction with Berkeley’s analysis of time; time cannot be abstracted from the succession of ideas one experiences, and so the duration of a spirit depends on the ideas and activities occurring within it. Therefore, Berkeley concludes, the spirit always thinks, the notion of a literally thoughtless mind being unintelligible. He asserts, “Spirits and ideas are things so wholly different. . . . There is nothing alike or common in them.” Because spirits are indivisible, incorporeal, and unextended, it follows that they are not subject to the laws of nature and hence enjoy immortality.

Berkeley’s arguments for God’s existence have been given in part; the eternal Spirit must exist as the only sufficient cause of nature. When one considers the lawfulness, perfection, beauty, and design of the whole system, it is obvious that the characteristics of nature suggest the character of God. God’s existence is in a sense known more certainly than that of any other spirit because one can constantly perceive God’s effects, even in those ideas by which one communicates with other people. If one does not realize this fact fully, it is because one is “blinded with excess of light.”

However, granted the existence of God, Berkeley is still faced with the problem of evil. Why does God’s universe contain pain, monstrosities, sorrow, death? Is the cumbersome machinery of nature very obviously turned directly by the hand of God? Berkeley answers that natural events occur according to rules of the greatest simplicity and generality; without such regularity, there could be no human foresight. What seems like waste from the human viewpoint—countless blighted plants, little fish devoured by parents, and so forth—can be understood as necessary to the riches of God. The apparent defects of nature really augment its beauty, and seeming evil contributes to the good of the whole. Even the mixture of pain with pleasure is necessary for humanity’s guidance. Clear understanding of these truths instills that holy fear that is the chief motive to virtue, and indeed “consideration of God and our duty,” was Berkeley’s chief aim in writing the book.