In Book IV of An Essay Concerning Human Understanding (1689), Locke defined knowledge as “the perception of the connexion of and agreement, or disagreement and repugnancy of any of our ideas.” Knowledge so defined admits of three degrees, according to Locke. The first is what he called “intuitive knowledge,” in which the mind “perceives the agreement or disagreement of two ideas immediately by themselves, without the intervention of any other.” Although Locke’s first examples of intuitive knowledge are analytic propositions such as “white is not black,” “a circle is not a triangle,” and “three are more than two,” later he said that “the knowledge of our own being we have by intuition.” Relying on the metaphor of light as Augustine and others had, Locke said of this knowledge that “the mind is presently filled with the clear light of it. It is on this intuition that depends all the certainty and evidence of all our knowledge.”
The second degree of knowledge obtains when “the mind perceives the agreement or disagreement of…ideas, but not immediately.” In these cases, some mediating idea makes it possible to see the connection between two other ideas. In a demonstration (or proof), for example, the connection between any premise and the conclusion is mediated by other premises and by the laws of logic. Demonstrative knowledge, although certain, is not as certain as intuitive knowledge, according to Locke, because it requires effort and attention to go through the steps needed to recognize the certainty of the conclusion.
A third degree of knowledge, “sensitive knowledge,” is roughly the same as what Duns Scotus called “intuitive cognition”—namely, the perception of “the particular existence of finite beings without us.” Unlike intuitive cognition, however, Locke’s sensitive knowledge is not the most certain kind of knowledge it is possible to have. For him, it is less certain than intuitive or demonstrative knowledge.
Next in certainty to knowledge is probability, which Locke defined as the appearance of agreement or disagreement of ideas with each other. Like knowledge, probability admits of degrees, the highest of which attaches to propositions endorsed by the general consent of all people in all ages. Locke may have had in mind the virtually general consent of his contemporaries in the proposition that God exists, but he also explicitly mentioned beliefs about causal relations.
The next highest degree of probability belongs to propositions that hold not universally but for the most part, such as “people prefer their own private advantage to the public good.” This sort of proposition is typically derived from history. A still lower degree of probability attaches to claims about specific facts—for example, that a man named Julius Caesar lived a long time ago. Problems arise when testimonies conflict, as they often do, but there is no simple rule or set of rules that determines how one ought to resolve such controversies.
Probability can concern not only objects of possible sense experience, as most of the foregoing examples do, but also things that are outside the sensible realm, such as angels, devils, magnetism, and molecules. George Berkeley
The next great figure in the development of empiricist epistemology was George Berkeley (1685–1753). In his major work, A Treatise Concerning the Principles of Human Knowledge (1710), Berkeley asserted that nothing exists except ideas and spirits (minds or souls). He distinguished three kinds of ideas: those that come from sense experience correspond to Locke’s simple ideas of perception; those that come from “attending to the passions and operations of the mind” correspond to Locke’s ideas of reflection; and those that come from compounding, dividing, or otherwise representing ideas correspond to Locke’s compound ideas. By spirit Berkeley meant “one simple, undivided, active being.” The activity of spirits consists of both understanding and willing: understanding is spirit perceiving ideas, and will is spirit producing ideas.
George Berkeley, detail of an oil painting by John Smibert, c. 1732; in the National Portrait Gallery, London.Courtesy of The National Portrait Gallery, London
For Berkeley, ostensibly physical objects like tables and chairs are really nothing more than collections of sensible ideas. Since no idea can exist outside a mind, it follows that tables and chairs, as well all the other furniture of the physical world, exist only insofar as they are in the mind of someone—i.e., only insofar as they are perceived. For any nonthinking being, esse est percipi (“to be is to be perceived”).
The clichéd question of whether a tree falling in an uninhabited forest makes a sound was inspired by Berkeley’s philosophy, though he never considered it in those terms. He did, however, consider the implicit objection and gave various answers to it. He sometimes said that a table in an unperceived room would be perceived if someone were there. That conditional response, however, is inadequate. Granted that the table would exist if it were perceived, does it exist when it is not perceived? Berkeley’s more pertinent answer was that even when no human is perceiving a table or other such object, God is, and it is God’s thinking that keeps the otherwise unperceived object in existence.
Although that doctrine initially strikes most people as strange, Berkeley claimed that he was merely describing the commonsense view of reality. To say that colours, sounds, trees, dogs, and tables are ideas is not to say that they do not really exist. It is merely to say what they really are. Moreover, to say that animals and pieces of furniture are ideas is not to say that they are diaphanous, gossamer, and evanescent. Opacity, density, and permanence are also ideas that partially constitute those objects.
Berkeley supported his main thesis with a syllogistic argument: physical things—such as trees, dogs, and houses—are things perceived by sense; things perceived by sense are ideas; therefore, physical things are ideas. If one objects that the second premise of the syllogism is false—people sense things, not ideas—Berkeley would reply that there are no sensations without ideas and that it makes no sense to speak of some additional thing that ideas are supposed to represent or resemble. Unlike Locke, Berkeley did not believe that there is anything “behind” or “underlying” ideas in a world external to the mind. Indeed, Berkeley claimed that no clear idea can be attached to that notion.
One consequence of Berkeley’s view is that Locke’s distinction between primary and secondary qualities is spurious. Extension, figure, motion, rest, and solidity are as much ideas as green, loud, and bitter are; there is nothing special about the former kind of idea. Furthermore, matter, as philosophers conceive it, does not exist. Indeed, it is contradictory, for matter is supposedly unsensed extension, figure, and motion, but since extension, figure, and motion are ideas, they must be sensed.
Berkeley’s doctrine that things unperceived by human beings continue to exist in the thought of God was not novel. It was part of the traditional belief of Christian philosophers from Augustine through Aquinas and at least to Descartes that God not only creates all things but also keeps them in existence by thinking of them. According to that view, if God were ever to stop thinking of a creature, it would immediately be annihilated. David Hume