This article is a study to explore the theory of passion in Thomas Aquinas’s Moral Theology from a pedagogical perspective. Aquinas’s discussion on the passions is an advantageous position in examining how the philosophical discussion on the passions that started in ancient times developed in the medieval Christian context. As a matter of fact, on the one hand, his discussion can be seen as representing the tradition of medieval Christianity as it is based on the Bible and the thoughts of the Church Fathers, and on the other hand, it can be seen as reflecting discussion of ancient philosophy as it is based on Aristotle’s discussions of metaphysics and ethics. 
 To this end, first, the view on human nature was confirmed through the doctrine of original sin regarding the origin of sins as the context in which passion was dealt with in medieval Christianity. Next, the study attempted to examine the practical impact of the theoretical discussion on passion through the doctrine of capital sins regarding the aspects of sins on the moral life of medieval people, especially through the ascetic life of the monastic system. According to Aquinas, original sin corresponds to the sin of human nature, and it is not simply due to the disorderly action of a single element called passion. The privation of original justice, whereby the will was made subject to God, is the formal element in original sin; while every other disorder of the soul’s powers, is a kind of material element in respect of original sin. On the other hand, cardinal sins correspond to the sin of an individual personality, and it can be defined as a disorder that appears in relation to the good pursued by certain areas of human nature. These doctrines can be seen not only as an understanding and explanation of human existence, but also as directing educational practice regarding moral life, and rather, they can be seen as being justified by educational practice. Passion can become materials for sin and vice but at the same time, it can also become materials for ethical virtues through its interaction with reason. We are given educational tasks to continuously ethically explore the meaning of goodness and to form habits of virtue.
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