Adopting Aristotelian principles and concepts, Aquinas attempts to explain the origin, operation, and purpose of the entire universe and the role that everything in the universe plays in the attainment of that purpose. Aquinas never doubts the truth of the tenets of his faith. Rather, he employs techniques of argument that he learned in the disputatio to state, defend, and elaborate those tenets. The grandiose scope of Summa Theologica derives from Aquinas’s belief that a very significant portion of theology can be expressed and codified in a comprehensive and rational system.

Read more about the influence of the disputatio on Thomas Aquinas’s work.

Aquinas writes not only as a philosopher who is intellectually interested in the pursuit of truth, he writes primarily as a Catholic who is convinced that the salvation of humanity itself is at stake. This conviction propels him toward a rational exegesis of topics, the truth of which are ultimately derived and founded on divine revelation. When a specific topic allows, Aquinas uses philosophical concepts and vocabulary to examine that topic. The primary topics admitting of such philosophical examination are the existence of God, the nature and limits of human knowledge, and the purpose of man. For most other topics, Aquinas articulates a decidedly Catholic position on issues of Christian interest, such as the Holy Trinity, original sin, and the like.

At first glance, it might seem astonishing and even counterintuitive that Aquinas reframes much of Catholic theology in terms of Aristotle’s pre-Christian philosophy. The pursuit of philosophy traditionally requires one to enter into debates with an open mind and to identify and re-examine one’s own core assumptions about a given issue, yet Aquinas enlists Aristotle not for his aid in the unbiased critical examination of the tenets of Catholic belief but rather for the explication and defense of those tenets.

At the same time, though, Aquinas’s enlistment of Aristotle reveals Aquinas to be a remarkably fair, open minded, and indeed tolerant medieval thinker. He apparently believes that the fruits of the exercise of reason are not necessarily corrupt if the thinker is a non-Christian. This suggests that Aquinas believes that every human being, regardless of his or her beliefs, shares in humanity through the possession and use of reason. In this, Aquinas again reveals his indebtedness and allegiance to Aristotle, who had maintained that reason is the essential quality of humanity: it is that without which man cannot be man.