Aquinas is a theologian who employs philosophy in an attempt to provide, insofar as possible, a rational explanation of doctrines that are revealed knowledge, or matters of faith. Although the Summa Theologica is in some respects a work of philosophy, its primary purpose is as a work of theology. This distinction was important to Aquinas and his fellow Scholastics, who held that theology and philosophy proceed according to different paths. Theology concerns itself with knowledge that has been revealed by God and that man must accept on faith. Philosophy, at least as defined by Aristotle, is concerned with knowledge that man acquires through sensory experience and the use of the natural light of reason. In other words, philosophy attempts to arrive at general principles through a consideration of that which is perceived by the senses and then rationally evaluated. While some subjects, such as knowledge of the existence of God, are common to theology and philosophy, theology also encompasses subjects that reason cannot fathom, such as the mystery of the Holy Trinity.

Following Aristotle’s famous dictum that “all men by nature desire to know,” Aquinas holds that people naturally seek knowledge of that which is their true goal and happiness, that is, the vision of God. While reason and philosophy have their respective roles in the acquisition of knowledge, they are inherently limited in their ability to apprehend all truths. Rather, philosophical knowledge is a subset of theological knowledge: all theologians are philosophers, but not all philosophers are theologians. The fact that theological knowledge is based on revealed truth and faith rather than on sensory experience and the exercise of reason does not mean that theological knowledge is in any way inferior to philosophical knowledge. On the contrary, theological knowledge is superior to philosophical knowledge not only insofar as it deals with issues of the utmost importance but also insofar as it alone can actually afford us complete knowledge of those issues.