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 this 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, and not specifically mystical or spiritual knowledge.
Before he articulated this 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, they know logical (or what he 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 of these, 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 this line of reasoning appears in On the Trinity, where 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, this 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 (1224/25–74) developed an alternative to Augustinian illuminationism.