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. Medieval philosophy 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.
St. Anselm (centre), terra-cotta altarpiece by Luca della Robbia; in the Museo Diocesano, Empoli, ItalyAlinari/Art Resource, New York
St. Thomas Aquinas
With the translation into Latin of Aristotle’s On the Soul in the early 13th century, the Platonic and Augustinian epistemology that dominated the early Middle Ages was gradually displaced. Following Aristotle, Aquinas recognized different kinds of knowledge. Sensory knowledge arises from sensing particular things. Because it has individual things as its object and is shared with brute animals, however, sensory knowledge is a lower form of awareness than scientific knowledge, which is characterized by generality. To say that scientific knowledge is characteristically general is not to diminish the importance of specificity: scientific knowledge also should be rich in detail, and God’s knowledge is the most detailed of all. The detail, however, must be essential to the kind of thing being studied and not peculiar to certain instances of it. Aquinas thought that, though the highest knowledge humans can possess is knowledge of God, knowledge of physical objects is better suited to human capabilities. Only that kind of knowledge will be considered here.
The Triumph of St. Thomas Aquinas, fresco by Andrea da Firenze, c. 1365; in the Spanish Chapel of the church of Santa Maria Novella, Florence.SCALA/Art Resource, New York
Aquinas’s discussion of knowledge in the Summa theologiae is an elaboration on the thought of Aristotle. Aquinas claims that knowledge is obtained when the active intellect abstracts a concept from an image received from the senses. In one account of that process, abstraction is the act of isolating from an image of a particular object the elements that are essential to its being an object of that kind. From the image of a dog, for example, the intellect abstracts the ideas of being alive, being capable of reproduction and movement, and whatever else might be essential to being a dog. Those ideas are distinguished from ideas of properties that are peculiar to particular dogs, such as the property of being owned by Smith or the property of weighing 20 pounds.