Aquinas adopts Aristotle’s doctrine of the Four Causes and couches much of his theology and philosophy in its terms. 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.