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.
Aquinas adopts Aristotle’s doctrine of the Four Causes and couches much of his theology and philosophy in its terms. (See Chapter 2, Aristotle, Physics, p. 47.) The Four Causes are (1) material cause, (2) formal cause, (3) efficient cause, and (4) final cause. The material cause, as its name implies, pertains to matter or the “stuff” of the world. Matter is potentiality, that is, that which something can become. The formal cause is the form or pattern that governs a particular thing, or the genus to which it belongs. The formal cause can also be called a thing’s essence. For example, the formal cause of a particular human being is his or her humanity, the essence of what it means to be human. God is the only creature embodying pure actuality and pure being, and God is thus the only pure formal cause. The efficient cause is what we normally understand by the word cause and indicates something that has an effect. The final cause is the goal or purpose toward which a thing is oriented.
Each of these causes is given a special application in Aquinas’s thought. The concept of material cause is crucial to his view of how humans gain knowledge of the external world and also appears in his proofs for the existence of God. The concept of formal cause is essential to his theory of knowledge and the nature of man but also defines his conception of God, whom Aquinas sees as complete actuality and thus without potential. The concept of efficient cause predictably appears in his theory of knowledge about the physical world but also explains human action, which is directed by the will. The concept of final cause explains the nature of the will itself, which naturally strives to achieve its goal of beholding the Divine Essence.
Aquinas revolutionized a thousand years of Christian tradition by rejecting Plato in favor of Aristotle. Plato maintained that ultimate reality consists of essence, whereas Aristotle maintained that existence is primary. For Plato, the world around us that we perceive with our senses contains nothing except impermanent, ever-changing objects. Plato reasoned that for our observations of the world to count as true knowledge and not just as anecdotal evidence, our minds need to make a conceptual leap from individual instances of things to general ideas. He concluded that there must be something permanent that lies behind and unites individual existences, and he referred to this something as “essence.” According to Plato, existence, or the everyday world of objects such as tables, chairs, and dogs, is inherently inferior to essence. Early church thinkers saw in Plato’s ideas a parallel to their own division of the universe into the inherently imperfect, corrupt world of matter and everyday existence and the perfect and heavenly world of spirit.
Aquinas follows Aristotle in concluding that Plato’s theory is deficient, in part because it is unable to account for the origin of existence and in part because it is unacceptably dismissive of existence. Holy Scripture states that after each of the six days of Creation, God saw that the fruit of his day’s work was “good” or even “very good.” Furthermore, when Moses asks God how he should refer to him, God responds, “I am that I am,” thereby equating himself with being. In other words, God is pure existence or Being itself. Aquinas argues that man’s purpose consists exactly in developing himself toward Being, not in attempting to escape Being. In the traditional church view prior to Aquinas, the difference between God and his creatures was one of kind, as existence was something that in itself separated us from God. In Aquinas’s view, the difference between God and his creatures is one of degree, and we are separate from God insofar as we do not have as much existence as God. Prior to Aquinas, traditional church thought maintained that existence was the chief impediment to the realization of our spiritual destiny. Aquinas held that our spiritual destiny consists precisely in the enhancement of our existence.