This paper explores the thought of St. Thomas Aquinas on the concept of evil. It will focus almost exclusively on his own writings, rather than on commentaries by others, and will trace how his concept of evil is related to other important concepts in his philosophical system.
Aquinas's concept of evil, like his entire philosophical system, already stood at the end of a long and profound philosophical tradition composed of several strands. The Christian strand led directly back to St. Augustine and then back to the Church Fathers and heresiologists of the second and third centuries. One pagan strand led back through the Neoplatonists to the writings of Plato himself, and the other pagan strand, newly restored to European awareness by the impact of Moslem scholarship, led back to Aristotle. It is not easy nor particularly useful to attempt to systematically distinguish these strands within Aquinas's writings. However, it is intellectually clear that they existed, and it is occasionally obvious which one Aquinas is dealing with because of some internal reference or allusion in his writing.
In dealing with the concept of evil, Aquinas naturally had to build upon the thought of St. Augustine, who established the theological position on evil which has been central to all Christian theology (except for that of recent Fundamentalists, who will be mentioned later) for the last 1500 years. Augustine's thoughts on evil were shaped by the fact that he began his adult religious life as a Manichaean and only later converted to orthodox Christianity. Hence, in order to understand Augustine's view, one must look at least briefly at the concept of evil within the Manichaean religion and within Zoroastrianism, from which Mani derived his beliefs about the nature of evil.
Zarathushtra Spitama proposed, in the seventh century B.C., one of the world's first great theological syntheses:
(1) that reality consists of an unending war betwe...