For most of the Middle Ages there was no distinction between theology and science (scientia). Science was knowledge that was deduced from self-evident principles, and theology was knowledge that received its principles from God, the source of all principles. By the 14th century, however, scientific and theological thinking began to diverge. Roughly speaking, theologians began to argue that human knowledge was narrowly circumscribed. They often invoked the omnipotence of God in order to undercut the pretensions of human reason, and in place of rationalism in theology they promoted a kind of fideism (i.e., a philosophy based entirely on faith).

The Italian theologian Gregory of Rimini (died 1358) exemplified the development. Inspired by Ockham, Gregory argued that, whereas science concerns what is accessible to humans through natural means—i.e., through sensation and intelligence—theology deals with what is accessible only in a supernatural way. Thus, theology is not scientific. The role of theology is to explain the meaning of the Bible and the articles of faith and to deduce conclusions from them. Since the credibility of the Bible rests upon belief in divine revelation, theology lacks a rational foundation. Furthermore, since there is neither self-evident knowledge of God nor any natural experience of him, humans can have only an abstract understanding of what he is.

Ockham and Gregory did not intend their views to undermine theology. To the contrary, for them theology is in a sense more certain than science, because it is built upon principles that are guaranteed to be true by God, whereas the principles of science must be as fallible as their human creators. Unfortunately for theology, however, the prestige of science increased in the 16th century and skyrocketed in the 17th and 18th centuries. Modern thinkers preferred to reach their own conclusions by using reason and experience even if ultimately those conclusions did not have the authority of God to support them. As theologians lost confidence in reason, other thinkers, who had little or no commitment to Aristotelian thought, became its champions, thus furthering the development of modern science.

Faith and reason

Although modern philosophers as a group are usually thought to be purely secular thinkers, in fact nothing could be further from the truth. From the early 17th century until the middle of the 18th century, all the great philosophers incorporated substantial religious elements into their work. In his Meditations (1641), for example, René Descartes offered two distinct proofs of the existence of God and asserted that no one who does not have a rationally well-founded belief in God can have knowledge in the proper sense of the term. Benedict de Spinoza (1632–77) began his Ethics (1677) with a proof of God’s existence and then discussed at length its implications for understanding all reality. And George Berkeley (1685–1753) explained the apparent stability of the sensible world by appealing to God’s constant thought of it.

Among the reasons modern philosophers are mistakenly thought to be primarily secular thinkers is that many of their epistemological principles, including some that were designed to defend religion, were later interpreted as subverting the rationality of religious belief. The views of Thomas Hobbes (1588–1679) might briefly be considered in that connection. In contrast to the standard view of the Middle Ages that propositions of faith are rational, Hobbes argued that such propositions belong not to the intellect but to the will. The significance of religious propositions, in other words, lies not in what they say but in how they are used. To profess a religious proposition is not to assert a factual claim about the world, which may then be supported or refuted with reasons, but merely to give praise and honour to God and to obey the commands of lawful religious authorities. Indeed, one does not even need to understand the meanings of the words in the proposition in order for this function to be fulfilled; simply mouthing them would be sufficient.

In An Essay Concerning Human Understanding (1690), John Locke further eroded the intellectual status of religious propositions by making them subordinate to reason in several respects. First, reason can restrict the possible content of propositions allegedly revealed by God; in particular, no proposition of faith can be a contradiction. Furthermore, because no revelation can contain an idea not derived from sense experience, one should not believe St. Paul when he speaks of experiencing things as “eye hath not seen, nor ear heard, nor hath it entered into the heart of man to conceive.” Another respect in which reason takes precedence over faith is that knowledge based on immediate sense experience (what Locke called “intuitive knowledge”) is always more certain than any alleged revelation. Thus, people who see that someone is dead cannot have it revealed to them that that person is at that moment alive. Rational proofs in mathematics and science also cannot be controverted by divine revelation. The interior angles of a rectangle equal 360°, and no alleged revelation to the contrary is credible. In short, wrote Locke, “Nothing that is contrary to, and inconsistent with, the clear and self-evident dictates of reason, has a right to be urged or assented to as a matter of faith.”

What space, then, does faith occupy in the mansion of human beliefs? According to Locke, it shares a room with probable truths, which are propositions of which reason cannot be certain. There are two types of probable truth: that which concerns observable matters of fact and that which goes “beyond the discovery of our sense.” Religious propositions can belong to either category, as can empirical and scientific propositions. Thus, the propositions “Caesar crossed the Rubicon” and “Jesus walked on water” belong to the first category, because they make claims about events that would be observable if they occurred. On the other hand, propositions like “Heat is caused by the friction of imperceptibly small bodies” and “Angels exist” belong to the second category, because they concern entities that by definition cannot be objects of sense experience.

Although it might seem that Locke’s mixing of religious and scientific claims helped to secure a place for the former, in fact it did not, for Locke also held that “reason must judge” whether or not something is a revelation and, more generally, that “reason must be our last judge and guide in everything.” Although that maxim was intended to reconcile reason and revelation—indeed, Locke called reason “natural revelation” and revelation “natural reason enlarged by a new set of discoveries communicated by God”—over the course of the subsequent 200 years, reason repeatedly judged that alleged revelations had no scientific or intellectual standing.

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.

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.