Thomas Aquinas is perhaps the most influential thinker to shape ethical thought in the West. He synthesized Aristotle’sthought with Latin theology in his extensive Summa Theologiae. This work is remarkable is the way in addresses a fundamental difficulty in both Plato and Augustine’s views.
Plato and Augustine focus on the restraint of appetite or desire. By focusing on the negative—on how we should not be ruled by appetites—they do not explain positive motivation to act ethically. My reason tells me that it is right to return a lost wallet, but my desire motivates me to keep it. What motivates me to do right in this situation? To act against an immoral desire, I must have a contrary motivation or appetite to do good. If this is so, then it cannot be the case that all appetites or desires are irrational, excessive or bad. Borrowing from Aristotle, Aquinas argues that humans must have good desires that direct us to act morally.
Aquinas distinguishes acts of a man from human acts. Acts in the more general sense, that is, activities of humans but also found in some non-human agents, are not the same as human acts which proceed from knowledge and will. Here we see Thomas Aquinas’ definition of human nature as having both rationality and will. In the Thomistic view the human act is the pursuit of a known good. He defines this good as the soul’s pursuit of God.
Aristotle believed that the good is expressed when a man exercises reason as a political creature, seeking personal happiness in both private and public life. For Aristotle, this is what humans exist to do. Aquinas borrows Aristotle’s notion of the good as fulfilling our final end, but instead of viewing our final end as rational activity, claims instead that humanity’s final end is the contemplation of God (the “beatific vision”). Aquinas agrees with Augustine and Anslem that happiness, defined as spiritual flourishing, is found in knowledge of God. Unlike Augustine, Aquinas places less emphasis of divine revelation. Humans are capable of knowing God through their reason. Unlike Anselm, Aquinas believes that it is possible to reason oneself to faith, if one reasons logically.
Following Aristotle, Aquinas justifies his claim about human happiness using Nature. Happiness is found in the fulfillment of our natural function, which for Aquinas is self-preservation. Consequently, we naturally seek to preserve our soul and in accomplishing this natural function, we achieve true happiness. The ethical virtues are those activities and character traits that help us fulfill self-preservation. We are naturally inclined toward virtue out of our natural tendency to preserve ourselves.
Following Plato, Aquinas explains moral failure as a failure of knowledge. If we fail to be virtuous, it is not because we do not desire to be so, but because we are ignorant or confused about what virtue is. Rather than condemning all desire as sinful, as Augustine seems to do, Aquinas distinguishes correct and incorrect desire. He says that humans naturally desire and seek God. Virtue consists in training ourselves to successfully reach the goal to which we are naturally destined.
This distinctive feature of Aquinas’ thought has an important consequence for the relationship of philosophy and religion. It means that human nature is not hopelessly sinful or flawed. Because human beings are rational and desire God, they have the ability to act virtuously.
Thomas Aquinas’ work represents great intellectual rigor, complexity and subtlety. He is the most impressive of the medieval “schoolmen” and like all schoolmen he asked questions that had a theological bearing.
His Summa Theologiae was left unfinished at his death. There is some question as to whether he may have changed his mind on certain points before he died. It is reported that on December 6, 1273 he had a mystical experience while attending Mass and thereafter wrote nothing more. His explanation was: “All that I have written seems to me like straw compared to what has now been revealed to me.” He died four months later.