A Treatise Concerning the Principles of Human Knowledge (commonly called the Principles of Human Knowledge, or simply the Treatise) is a 1710 work, in English, by Irish empiricist philosopher George Berkeley. This book largely seeks to refute the claims made by Berkeley's contemporary John Locke about the nature of human perception. While, like other empiricist philosophers, both Locke and Berkeley agreed that we are having experiences regardless of whether material objects exist or not, Berkeley sought to prove that the outside world (the world which causes the ideas one has within one's mind) is also composed solely of ideas. Berkeley did this by suggesting that "Ideas can only resemble Ideas" – the mental ideas that we possess can only resemble other ideas (not material objects) and thus the external world consists not of physical form, but rather of ideas. This world is (or, at least, was) given logic and regularity by some other force, which Berkeley concludes is God.
Content
Introduction Berkeley declared that his intention was to make an inquiry into the First Principles of Human Knowledge in order to discover the principles that have led to doubt, uncertainty, absurdity, and contradiction in philosophy. In order to prepare the reader, he discussed two topics that lead to errors. First, he claimed that the mind cannot conceive abstract ideas. We cannot have an idea of some abstract thing that is common to many particular ideas and therefore has, at the same time, many different predicates and no predicates. Second, Berkeley declared that words, such as names, do not signify abstract ideas. With regard to ideas, he asserted that we can only think of particular things that have been perceived. Names, he wrote, signify general ideas, not abstract ideas. General ideas represent any one of several particular ideas. Berkeley criticized Locke for saying that words signify general, but abstract, ideas. At the end of his Introduction, he advised the reader to let his words engender clear, particular ideas instead of trying to associate them with non-existent abstractions.
Part I The following is a summary of Part I (Part II was never published).
"To be" means "to be perceived" Berkeley began his treatise by asserting that existence is the state of being perceived by a perceiver. Human minds know ideas, not objects. The three kinds of ideas are those of sensation, thought, and imagination. When several ideas are associated together, they are thought to be ideas of one distinct thing, which is then signified by one name., § 1
Ideas are known and perceived by a knowing perceiver. This active perceiver is designated by the names mind, spirit, soul, or self. Ideas exist by virtue of a perceiver. The existence of an idea consists in being perceived., § 2
What is meant by the term "exist" when it is applied to a thing that is known through the senses? To say that something exists is to say that it is perceived by a perceiver (Esse is percipi)., § 3 This is the main principle of human knowledge.
External objects are things that are perceived through our senses. We perceive only our own sensations or ideas. Ideas and sensations cannot exist unperceived., § 4
To say that an object exists without being perceived is to attempt to abstract that which cannot be abstracted. We cannot separate or abstract objects and their qualities from our perception of them., § 5
If an object exists or is perceived, it must be perceived by me or some other perceiver. It is impossible to separate the being of a sensible thing from its existence as a perception of a perceiver., § 6
There can be no unthinking substance or substratum of ideas. Therefore, the perceiving mind or spirit is the only substance of ideas. Ideas inhere in or belong to a perceiver., § 7
Are there things that exist in an unthinking substance outside of the perceiver's mind? Can they be the originals that the ideas copy or resemble? An idea can only be like an idea, not something undetectable. It is impossible for us to conceive of a copy or resemblance unless it is between two ideas., § 8
Locke's primary and secondary qualities According to Locke, a thing's primary qualities, such as its extension, shape, motion, solidity, and number, exist unperceived, apart from any perceiver's mind, in an inert, senseless substance called matter. Berkeley opposed Locke's assertion. Qualities that are called primary are, according to Berkeley, ideas that exist in a perceiver's mind. These ideas can only be like other ideas. They cannot exist in an unperceiving, corporeal substance or matter., § 9
The primary qualities of figure, motion, etc., cannot be conceived as being separate from the secondary qualities, which are related to sensations. Therefore, primary qualities, like secondary qualities, exist only in the mind., § 10 The properties of primary qualities are relative and change according to the observer's perspective. The greatness and smallness of figure, the swiftness and slowness of motion, exist in the mind and depend on point of view or position., § 11
Number Number exists only in the mind. The same thing is described by different numbers according to the mind's viewpoint. An object can have an extension of one, three, and thirty six, according to its measurement in yards, feet, and inches. Number is relative and does not exist separately from a mind., § 12
Sensed qualities are mental Unity is merely an abstract idea., § 13 Primary qualities, such as figure, extension, and motion, are relative, as are secondary qualities such as red, bitter, and soft. They all depend on the observer's frame of reference, position, or point of view., § 14 Berkeley's "...method of arguing does not so much prove that there is no extension or colour in an outward object, as that we do not know by sense which is the true extension or colour of the object.", § 15 Idealism, here, is epistemological, not ontological. Berkeley declared that it is "...impossible that any colour or extension at all, or other sensible quality whatsoever, should exist in an unthinking subject without the mind, or in truth, that there should be any such thing as an outward object.", § 15 Any quality that depends on sensation for its existence requires that a sense organ and a mind is conscious of it. By "unthinking subject," he means "mindless matter" or "substance, substratum, or support that is not a thinking mind." By "without the mind," he means "not in the mind."
Meaning of material substance Matter is material substance. What does this mean? "Material substance" has two meanings: "being in general" and "support of accidents." (The word accident is used here to mean an unessential quality.) "Being in general" is incomprehensible because it is extremely abstract. To speak of supporting accidents such as extension, figure, and motion is to speak of being a substance, substratum, or support in an unusual, figurative, senseless manner. Sensible qualities, such as extension, figure, or motion, do not have an existence outside of a mind., § 17
Knowledge of external objects Comparing ontology with epistemology, Berkeley asked, "But, though it were possible that solid, figured, moveable substances may exist without the mind, corresponding to the ideas we have of bodies, yet how is it possible for us to know this?", § 18 Knowledge through our senses only gives us knowledge of our senses, not of any unperceived things. Knowledge through reason does not guarantee that there are, necessarily, unperceived objects. In dreams and frenzies, we have ideas that do not correspond to external objects. "...[T]he supposition of external bodies is not necessary for the producing our ideas...." Materialists do not know how bodies affect spirit. We cannot suppose that there is matter because we do not know how ideas occur in our minds., § 19 "In short, if there were external bodies, it is impossible we should ever come to know it....", § 20 Suppose that there were an intelligence that was not affected by external bodies. If that intelligence had orderly and vivid sensations and ideas, what reason would it have to believe that bodies external to the mind were exciting those sensations and ideas? None.
Berkeley's challenge Through reflection or introspection it is possible to attempt to know if a sound, shape, movement, or color can exist unperceived by a mind. Berkeley declared that he will surrender and admit the unperceived existence of material objects, even though this doctrine is unprovable and useless, if "...you can conceive it possible for one extended moveable substance or, in general, for any one idea, or anything like an idea, to exist otherwise than in a mind perceiving it....", § 22 In answer to Berkeley's summons, it might be said that it is easy to imagine objects that are not perceived by anyone. But, he asked, "...what is all this, I beseech you, more than framing in your mind certain ideas which you call books and trees, and at the same time omitting to frame the idea of any one that may perceive them? But do not you yourself perceive or think of them all the while?", § 23 The mind had merely forgotten to include itself as the imaginer of those imagined objects.
Absolute existence It is impossible to understand what is meant by the words absolute existence of sensible objects in themselves. To speak of perceived objects that are not perceived is to use words that have no meaning or to utter a contradiction., § 24
What causes ideas? Ideas exist only in a mind and have no power to cause any effects. Ideas of extension, figure, and motion cannot cause sensations. "To say, therefore, that these [sensations] are the effects of powers resulting from the configuration, number, motion, and size of corpuscles must certainly be false." , § 25 Some non–idea must produce the succession of ideas in our minds. Since the cause can't be another idea, it must be a substance. If there are no material substances, then it must be an immaterial substance. Such an incorporeal, active substance is called a Spirit., § 26 A Spirit is that which acts. A Spirit is one simple, undivided, active being., § 27 It cannot be perceived. Only its effects can be perceived. The two principal powers of Spirit are Understanding and Will. Understanding is a Spirit that perceives ideas. Will is a Spirit that operates with or produces ideas. The words will, soul, or spirit designate something that is active but cannot be represented by an idea. Berkeley claimed that a person's active mind can imaginatively generate ideas at will., § 28 Ideas that are sensually perceived, however, are not dependent on the observer's will. The ideas that are imprinted on the mind when observing the external world are not the result of willing. "There is therefore some other Will or Spirit that produces them.", § 29
Natural laws Ideas that are perceived through our senses are lively and distinct, unlike imagined ideas. Their orderly connection and coherence reflects the wisdom and benevolence of the mind that made them. The ideas of sense occur according to rules. We call these connections and associations laws of nature., § 30 Necessary connections are not discovered by us. We only observe settled laws of nature and use them to manage our affairs., § 31 Erroneously, we attribute power and agency to ideas of sense, which are mere secondary causes. Ideas, we think, can cause other ideas. The primary cause, the "Governing Spirit whose Will constitutes the laws of nature", § 32 is ignored.
Strong and faint ideas