After the death of Aristotle the next significant development in the history of epistemology was the rise of Skepticism, of which there were at least two kinds. The first, Academic Skepticism, arose in the Academy (the school founded by Plato) in the 3rd century bce and was propounded by the Greek philosopher Arcesilaus (c. 315–c. 240 bce), about whom Cicero (106–43 bce), Sextus Empiricus (flourished 3rd century ce), and Diogenes Laërtius (flourished 3rd century ce) provide information. The Academic Skeptics, who are sometimes called “dogmatic” Skeptics, argued that nothing could be known with certainty. That form of Skepticism seems susceptible to the objection, raised by the Stoic Antipater (flourished c. 135 bce) and others, that the view is self-contradictory. To know that knowledge is impossible is to know something. Hence, dogmatic Skepticism must be false.

Carneades (c. 213–129 bce), also a member of the Academy, developed a subtle reply to the charge. Academic Skepticism, he insisted, is not a theory about knowledge or the world but rather a kind of argumentative strategy. According to the strategy, the Skeptic does not try to prove that he knows nothing. Instead, he simply assumes that he knows nothing and defends that assumption against attack. The burden of proof, in other words, is on those who believe that knowledge is possible.

Carneades’ interpretation of Academic Skepticism renders it very similar to the other major kind, Pyrrhonism, which takes its name from Pyrrhon of Elis (c. 365–275 bce). Pyrrhonists, while not asserting or denying anything, attempted to show that one ought to suspend judgment and avoid making any knowledge claims at all, even the negative claim that nothing is known. The Pyrrhonist’s strategy was to show that for every proposition supported by some evidence, there is an opposite proposition supported by evidence that is equally good. Such arguments, which are designed to refute both sides of an issue, are known as “tropes.” The judgment that a tower is round when seen at a distance, for example, is contradicted by the judgment that the tower is square when seen up close. The judgment that Providence cares for all things, which is supported by the orderliness of the heavenly bodies, is contradicted by the judgment that many good people suffer misery and many bad people enjoy happiness. The judgment that apples have many properties—shape, colour, taste, and aroma—each of which affects a sense organ, is contradicted by the equally good possibility that apples have only one property that affects each sense organ differently.

What is at stake in such arguments is “the problem of the criterion”—i.e., the problem of determining a justifiable standard against which to measure the worth or validity of judgments, or claims to knowledge. According to the Pyrrhonists, every possible criterion is either groundless or inconclusive. Thus, suppose that something is offered as a criterion. The Pyrrhonist will ask what justification there is for it. If no justification is offered, then the criterion is groundless. If, on the other hand, a justification is produced, then the justification itself is either justified or it is not. If it is not justified, then again the criterion is groundless. If it is justified, then there must be some criterion that justifies it. But this is just what the dogmatist was supposed to have provided in the first place.

If the Pyrrhonist needed to make judgments in order to survive, he would be in trouble. In fact, however, there is a way of living that bypasses judgment. One can live quite nicely, according to Sextus, by following custom and accepting things as they appear. In doing so, one does not judge the correctness of anything but merely accepts appearances for what they are.

Ancient Pyrrhonism is not strictly an epistemology, since it has no theory of knowledge and is content to undermine the dogmatic epistemologies of others, especially Stoicism and Epicureanism. Pyrrho himself was said to have had ethical motives for attacking dogmatists: being reconciled to not knowing anything, Pyrrho thought, induced serenity (ataraxia).

St. Augustine

St. Augustine of Hippo (354–430) claimed that human knowledge would be impossible if God did not “illumine” the human mind and thereby allow it to see, grasp, or understand ideas. Ideas as Augustine construed them are—like Plato’s—timeless, immutable, and accessible only to the mind. They are indeed in some mysterious way a part of God and seen in God. Illumination, the other element of the theory, was for Augustine and his many followers, at least through the 14th century, a technical notion, built upon a visual metaphor inherited from Plotinus (205–270) and other Neoplatonic thinkers. According to that view, the human mind is like an eye that can see when and only when God, the source of light, illumines it. Varying his metaphor, Augustine sometimes says that the human mind “participates” in God and even, as in On the Teacher (389), that Christ illumines the mind by dwelling in it. It is important to emphasize that Augustine’s theory of illumination concerns all knowledge, not specifically mystical or spiritual knowledge.

Before he articulated the theory in his mature years, soon after his conversion to Christianity, Augustine was concerned to refute the Skepticism of the Academy. In Against the Academicians (386) he claimed that, if nothing else, humans know disjunctive tautologies such as “Either there is one world or there is not one world” and “Either the world is finite or it is infinite.” Humans also know many propositions that begin with the phrase “It appears to me that,” such as “It appears to me that what I perceive is made up of earth and sky, or what appears to be earth and sky.” Furthermore, humans know logical (or what Augustine calls “dialectical”) propositions—for example, “If there are four elements in the world, there are not five,” “If there is one sun, there are not two,” “One and the same soul cannot die and still be immortal,” and “Man cannot at the same time be happy and unhappy.”

Many other refutations of Skepticism occur in Augustine’s later works, notably On the Free Choice of the Will (389–395), On the Trinity (399/400–416/421), and The City of God (413–426/427). In the last, Augustine proposes other examples of things about which people can be absolutely certain. Again in explicit refutation of the Skeptics of the Academy, he argues that if a person is deceived, then it is certain that he exists. Expressing the point in the first person, as René Descartes (1596–1650) did some 1,200 years later, Augustine says, “If I am deceived, then I exist” (Si fallor, sum). A variation on that line of reasoning appears in On the Trinity, in which he argues that if he is deceived, he is at least certain that he is alive.

Augustine also points out that since he knows, he knows that he knows, and he notes that this can be reiterated an infinite number of times: if I know that I know that I am alive, then I know that I know that I know that I am alive. In 20th-century epistemic logic, that thesis was codified as the axiom “If A knows that p, then A knows that A knows that p.” In The City of God, Augustine claims that he knows that he loves: “For neither am I deceived in this, that I love, since in those things which I love I am not deceived.” With Skepticism thus refuted, Augustine simply denies that he has ever been able to doubt what he has learned through his sensations or even through the testimony of most people.

One thousand years passed before Skepticism recovered from Augustine’s criticisms, but then it arose like the phoenix of Egyptian mythology. Meanwhile, Augustine’s Platonic epistemology dominated the Middle Ages until the mid-13th century, when St. Albertus Magnus (1200–80) and his student St. Thomas Aquinas (1224/25–1274) developed an alternative to Augustinian illuminationism.

St. Anselm of Canterbury

The phrase that St. Anselm of Canterbury (c. 1033–1109) used to describe his philosophy—namely, “faith seeking reason” (fides quaerens intellectum)—well characterizes medieval philosophy as a whole. All the great medieval philosophers—Christian, Jewish, and Islamic alike—were also theologians. Virtually every object of interest was related to their belief in God, and virtually every solution to every problem, including the problem of knowledge, contained God as an essential part. Indeed, Anselm himself equated truth and intelligibility with God. As he noted at the beginning of his Proslogion (1077–78), however, there is a tension between the view that God is truth and intelligibility and the fact that humans have no perception of God. How can there be knowledge of God, he asks, when all knowledge comes through the senses and God, being immaterial, cannot be sensed? His answer is to distinguish between knowing something by being acquainted with it through sensation and knowing something through a description. Knowledge by description is possible using concepts formed on the basis of sensation. Thus, all knowledge of God depends upon the description that he is “the thing than which a greater cannot be conceived.” From that premise Anselm infers, in his ontological argument for the existence of God, that humans can know that there exists a God that is all-powerful, all-knowing, all-just, all-merciful, and immaterial. Eight hundred years later the British philosopher Bertrand Russell would develop an epistemological theory based on a similar distinction between knowledge by acquaintance and knowledge by description, though he would have vigorously denied that the distinction could be used to show that God exists.