(This is my summary of a chapter in a book I often used in university classes: Twelve Theories of Human Nature, Oxford Univ. Press)
Our discussion of theories of human nature will now move from the ancient worlds to the 18th century with Immanuel Kant. Needless to say, a lot happened in the interim. In order to make the transition of almost 2000 years, here is a brief description of a few of the major thinkers during this long period including: Augustine, Aquinas, Luther, Calvin, Hobbes, Descartes, Spinoza, Hume, and Rousseau. [This is a good representative sample. Notable omissions include: Bacon, Leibniz, Locke, Berkeley, Voltaire, and Hegel.]
Augustine – St. Augustine was the most important link connecting the ancient world and the Christian medieval world. Augustine affected a philosophical synthesis of Plato, the Neo-Platonists and Christianity. [Plato’s idea of the good became divine illumination; Platonic forms became ideas in God’s mind, the cave became the story of the soul’s journey to god and more. Moreover, the idea of the Trinity owes much to a similar 3 fold distinction in the philosophy of the neo-Platonist Plotinus.] Despite the influence of Greek rationalism, Augustine said: “I believe in order to understand.” In other words, faith comes first and an exercise of will is necessary for belief since reason by itself always fall short.
Augustine also believed that humans are sinful and that nothing we do can save us—our salvation depends on God’s grace. Thus we are predestined to be saved or not, we cannot merit salvation. This was the source of his dispute with Pelagius who argued we can save ourselves by our free acts. This is still a vexing issue in Christian theology. Augustine particularly identified sin as sexual, an idea which has had a profound influence on Christianity to this day. He also believed in a “city of god,” the ideal destiny where God’s will is fulfilled. The relationship between the church and this final destiny is ambiguous. [I think the idea is that the church helps bring about this final destiny, although if you are only saved by grace it is hard to see how this can be—which was Martin Luther’s point.]
The Islamic Philosophers – There were a number of prominent Islamic philosophers from the 9th through 13th centuries. They had discovered and preserved the work of Aristotle, which had been lost to the West, and their translators introduced Aristotle in Spain in the 12th century. [This would eventually make its way to Aquinas at the University of Paris.] Islam, like Christianity, assumes the authority of their religion is based on divine revelation, but this also suggests that issues of the relationship between faith and reason are prominent. The most important of these thinkers were Avicenna, who believed that god (Allah) spoke through both the intellect and imagination of Muhammad; Al-Ghazali, a Sufi mystic who emphasized mysticism rather than philosophical arguments; and Averroes, who reasserted the primacy of reason in interpreting the Koran. Each of these philosophers had considerable influence on the West.
Thomas Aquinas – While many in the Catholic Church tried to ban Aristotle, Aquinas embraced his thought and tried to synthesize it with Christianity. “Though controversial in its time, it has since become Roman Catholic orthodoxy, backed by papal authority.” Aquinas placed a greater emphasis on reason than had Augustine. Like Aristotle, he believed that knowledge begins with the senses and that the intellect recognizes types or forms of things in order to categorize the world. This leads to a distinction between rational and revealed theology, the former using unaided reason to prove God’s existence, while the latter depends on the Bible and the Church.
Like Aristotle, he believed we have a rational soul or structure which includes perception, intellect, reason and free will. He identifies the idea of eudaimonia with knowledge and love of God. [Is it better to know God or love him was a famous medieval dispute.] The most important virtues are the theological virtues of faith, hope, and charity. Aquinas believed that immortality would involve the resurrection of the body, but tried to also maintain that the disembodied soul somehow survived between death and resurrection. Still, despite his emphasis on reason, the authority of the Church was paramount and like Augustine he was prepared to use force against dissent, arguing that heretics should be killed.