epistemology John Lockephilosophy

As mentioned above (The origins of knowledge: Rationalism and empiricism), whereas rationalist philosophers such as Descartes held that the ultimate source of human knowledge is reason, empiricists such as Locke argued that it is experience. Rationalist accounts of knowledge also typically involved the claim that at least some kinds of ideas are “innate,” or present in the mind at (or even before) birth. For philosophers such as Descartes and Gottfried Wilhelm Leibniz (1646–1716), the hypothesis of innateness is required in order to explain how humans come to have ideas of certain kinds. These ideas include not only mathematical concepts such as numbers, which appear not to be derived from sense experience, but also, according to some thinkers, certain general metaphysical principles, such as “every event has a cause.”

Locke claimed that this line of argument has no force. He held that all ideas (except those that are “trifling”) can be explained in terms of experience. Instead of attacking the doctrine of innate ideas directly, however, his strategy was to refute it by showing that it is explanatorily otiose and hence dispensable.

There are two kinds of experience, according to Locke: observation of external objects—i.e., sensation—and observation of the internal operations of the mind. Locke called this latter kind of experience, for which there is no natural word in English, “reflection.” Some examples of reflection are perceiving, thinking, doubting, believing, reasoning, knowing, and willing.

As Locke uses the term, a “simple idea” is anything that is an “immediate object of perception” (i.e., an object as it is perceived by the mind) or anything that the mind “perceives in itself” through reflection. Simple ideas, whether they are ideas of perception or ideas of reflection, may be combined or repeated to produce “compound ideas,” as when the compound idea of an apple is produced by bringing together simple ideas of a certain colour, texture, odour, and figure. Abstract ideas are created when “ideas taken from particular beings become general representatives of all of the same kind.”

The “qualities” of an object are its powers to cause ideas in the mind. One consequence of this usage is that, in Locke’s epistemology, words designating the sensible properties of objects are systematically ambiguous. The word red, for example, can mean either the idea of red in the mind or the quality in an object that causes that idea. Locke distinguished between primary and secondary qualities, as Galileo did. According to Locke, primary qualities, but not secondary qualities, are represented in the mind as they exist in the object itself. The primary qualities of an object, in other words, resemble the ideas they cause in the mind. Examples of primary qualities include “solidity, extension, figure, motion, or rest, and number.” Secondary qualities are configurations or arrangements of primary qualities that cause sensible ideas such as sounds, colours, odours, and tastes. Thus, according to Locke’s view, the phenomenal redness of a fire engine is not in the fire engine itself, but its phenomenal solidity is. Similarly, the phenomenal sweet odour of a rose is not in the rose itself, but its phenomenal extension is.

In Book IV of the Essay Concerning Human Understanding, Locke defines 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 calls “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 says that “the knowledge of our own being we have by intuition.” Relying on the metaphor of light as Augustine and others had, Locke says 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 defines 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 mentions 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.