Philosophy of Mind

Two main factors shaped medieval thinking about the mind or soul. The first is religious doctrine. The idea that God freely created the world from nothing is absent from ancient Greek philosophy, but more or less definitive of medieval philosophy in all three monotheistic traditions: Christian, Islamic, and Jewish. In the Western or Christian tradition, it was expressed in terms of providence, the idea that creation is a product of God's wisdom and goodness, and that this is manifested in the orderly structure of the universe all the way down to its smallest details. Needless to say, it would have struck an ancient Greek philosopher as absurd that something could be made from nothing, or that a divinity—especially an omnipotent divinity—would care what happens to beings less powerful than it. But such doctrines changed the way the mind was understood, granting pride of place to the human soul and human modes of cognition. Since humans are made in God's image (Gen. 1:26), their own nature must in some way reflect the divine.

The second factor is simply physical access to ancient texts, which became more and more difficult in the West until direct knowledge of most Greek sources was lost for nearly six centuries. Philosophical psychology was especially hard hit, as none of the works mentioned above was available after the sixth century, and eventually only Aristotle's De anima was recovered in a form that could have any direct influence. This meant that medieval thinkers had to learn about ancient theories indirectly, via textbook summaries and discussions by early Church fathers who were trained in or otherwise influenced by pagan schools of philosophy. Platonic, Epicurean, Stoic, and Neo-Platonist doctrines went underground, as it were, and sometimes came to be defended by philosophers who were unaware of their true origins.

Augustine is the most important medieval philosopher in the sense that his teachings set the agenda in Western thought for the next millennium, including the kinds of questions that were asked about the soul. For Augustine, the human mind is the foremost expression of the truth of Genesis 1:26, and the doctrine of the Trinity provides the mode of resemblance. Just as God is three persons (Father, Son, and Spirit) in one being, so the mind is three aspects or activities in one substance: "Since then these three, memory, understanding, will, are not three lives but one life, nor three minds but one mind, it follows certainly that neither are they three substances but one substance" (De trinitate X, 11.18). It is possible that Descartes was influenced by Augustine's use of the Latin term mens or "mind" here (anima was used for the souls of living things more generally), except that Augustinian mens always has a dense layer of Neo-Platonic and Christian associations attached to it that would have certainly made Descartes cringe. Augustine also thought that because it is immediately present to itself, the mind knows itself and that in knowing itself, it knows God as well. There are remarkable similarities between Augustine's argument that a man who knows he is alive cannot be deceived about this fact (from De trinitate XV, 12.21), and Descartes's more famous anti-skeptical argument, the "I think; therefore, I am," of his Meditations II.

Some seven centuries after Augustine, philosophical psychology was transformed again by the reintroduction of Aristotle's De anima and the commentary tradition that surrounded it. Philosophers and theologians struggled to assimilate this new authority with Christian teaching on the soul, which by now had acquired its own authority in Augustine. St. Thomas Aquinas (c. 1224–1274) was perhaps the most successful at synthesizing pagan and Christian philosophical learning, especially in his magisterial Summa theologiae, a beautifully ordered compendium of theological teachings prepared for Dominican novices. The first part contains a series of fifteen "Questions" on human nature in which he defends the Aristotelian account of the soul as the first principle of the human body, and explains the soul's various powers and modes of operation. But he parts company with Aristotle on the question of the human soul's immortality (recall that Aristotle was willing to treat only the active part of the capacity of thinking as immortal). To allow for disembodied existence, Aquinas argues that the soul is a special kind of form because it is also a substance, and that it can therefore continue to exist after the death of the body. In fact, he claims that "a separated soul is in a way more free to use the intellect, insofar as the weight and distraction of the body keeps it from the pure operation of intellect" (Summa theologiae Ia, q.89, a.3). Much of his account of disembodied thinking is indebted to Augustine and Christian Neoplatonism. But Aquinas also subscribed to the Christian doctrine of the resurrection, according to which everyone's separated souls will be reunited with their (glorified and incorruptible) bodies at the Last Judgment. This forces him into the awkward position of arguing that despite its capacity to exist on its own in a purer and presumably higher state, it is somehow more natural for the soul to be united to the body.

After Aquinas, philosophers tended to be less optimistic about the prospects of uniting Athens and Jerusalem. Theories about the nature of the soul were trimmed almost to the vanishing point in favor of discussions of what the soul does, on the grounds that only the latter is naturally or empirically evident to us. Thus, John Buridan (1300–1358) argues that there is no philosophical knowledge of the soul, if by that one means the soul's essential nature, although one can know its faculties and operations. The notion that the human soul, something that is by definition immaterial and unextended, could inhere in a divisible and extended body, amazed him—he declared it "a miracle" (mirabile) (Questions on Aristotle's De anima II.9). That is, he believed it, but he did not regard it as knowledge. In this, of course, Buridan is well on the way to modernity, and to the modern distinction between faith and reason.