Despite the strong religious elements in the thought of modern philosophers, especially those writing before the middle of the 18th century, the vast majority of contemporary epistemologists have been interested only in the purely secular aspects of their work. Accordingly, those aspects will predominate in the following discussion. Epistemology and modern science
The Polish astronomer Nicolaus Copernicus (1473–1543) argued in On the Revolutions of the Celestial Spheres (1543) that Earth revolves around the Sun. His theory was epistemologically shocking for at least two reasons. First, it directly contravened the way in which humans experienced their relation to the Sun, and in doing so it made ordinary nonscientific reasoning about the world seem unreliable—indeed, like a kind of superstition. Second, it contradicted the account presented in several books of the Bible, most importantly the story in Genesis of the structure of the cosmos, according to which Earth is at the centre of creation. If Copernicus were right, then the Bible could no longer be treated as a reliable source of scientific knowledge.
Many of the discoveries of the Italian astronomer Galileo Galilei (1564–1642) were equally unsettling. His telescope seemed to reveal that unaided human vision gives false, or at least seriously incomplete, information about the nature of celestial bodies. In addition, his mathematical descriptions of physical phenomena indicated that much of sense experience of these phenomena contributes nothing to knowledge of them.
Another counterintuitive theory of Galileo was his distinction between the “primary” and the “secondary” qualities of an object. Whereas primary qualities—such as figure, quantity, and motion—are genuine properties of things and are knowable by mathematics, secondary qualities—such as colour, odour, taste, and sound—exist only in human consciousness and are not part of the objects to which they are normally attributed. René Descartes
Both the rise of modern science and the rediscovery of skepticism were important influences on René Descartes. Although he believed that certain knowledge was possible and that modern science would one day enable humans to become the masters of nature, he also thought that skepticism presented a legitimate challenge that needed an answer, one that only he could provide.
Descartes, RenéRené Descartes.National Library of Medicine, Bethesda, Maryland
The challenge of skepticism, as Descartes saw it, is vividly described in his Meditations (1641). He considered the possibility that an “evil genius” with extraordinary powers has deceived him to such an extent that all his beliefs are false. But it is not possible, Descartes contended, that all his beliefs are false, for if he has false beliefs, he is thinking, and if he is thinking, then he exists. Therefore, his belief that he exists cannot be false, as long as he is thinking. This line of argument is summarized in the formula cogito, ergo sum (“I think, therefore I am”).
Descartes distinguished two sources of knowledge: intuition and deduction. Intuition is an unmediated mental “seeing,” or direct apprehension. Descartes’s intuition of his own thinking guarantees that his belief that he is thinking is true. Although his formula might suggest that his belief that he exists is guaranteed by deduction rather than intuition (because it contains the term therefore), in the Objections and Replies (1642) he stated explicitly that the certainty of this belief also is based upon intuition.
If one could know only that one thinks and that one exists, human knowledge would be depressingly meager. Accordingly, Descartes attempted to broaden the limits of knowledge by proving to his own satisfaction that God exists, that the standard for knowing something is having a “clear and distinct” idea of it, that mind is more easily known than body, that the essence of matter is extension, and, finally, that most of his former beliefs are true.
Unfortunately for Descartes, few people were convinced by these arguments. One major problem with them has come to be known as the “Cartesian circle.” Descartes’s argument to show that his knowledge extends beyond his own existence depends upon the claim that whatever he perceives “clearly and distinctly” is true. That claim in turn is supported by his proof of the existence of God, together with the assertion that God, because he is not a deceiver, would not cause Descartes to be deceived in what he clearly and distinctly perceives. But because the criterion of clear and distinct perception presupposes the existence of God, Descartes cannot rely upon it in order to guarantee that he has not been deceived (i.e., that he did not make a mistake) in the course of proving that God exists. Therefore, he does not know that his proof is cogent. But if he does not know that, then he cannot use the criterion of clear and distinct perception to show that he knows more than that he exists. John Locke
Whereas rationalist philosophers such as Descartes held that the ultimate source of human knowledge is reason, empiricists such as John Locke argued that the source is experience (see Rationalism and empiricism). 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. Such 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, JohnJohn Locke.© Everett Historical/Shutterstock.com
Locke claimed that that 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 the 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 used 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 that 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.