Thomistic Philosophy: Reality, Existence, and God
The Creation of the World and Thomistic Philosophy
Thomistic theory adopts concepts from the Aristotelian theory of reality. However, a central problem in Christian philosophy arises from the religious belief that the world was created by God *ex nihilo* (from nothing). The world, therefore, transforms from something eternal and without origin to something created in time, a result of God’s free will. St. Thomas Aquinas concludes that in creatures, one can distinguish between *essence* and *existence*.
Created Substances: Essence and Existence
Essence and existence are two fundamental principles in the Thomistic system. They explain the structure of created substances and establish a radical differentiation between them and God. St. Thomas Aquinas added to the Aristotelian distinction of matter and form. He proposed that:
- Essence refers to *what* a thing is.
- Existence is understood as the fact *that* something is.
This constitutes a profound change to the Aristotelian system. For Aristotle, form and essence are eternal. For St. Thomas, the essence of created beings includes both matter (first) and form, and their distinct existence is not eternal. Essence is in potency to existence; it *can* exist or not exist. Therefore, essences are contingent, not necessary. Existence is characterized as an act of the essence.
The Limits of Human Understanding of God
St. Thomas Aquinas opposed Christian theologians who defended the thesis that God’s existence is self-evident. Among these was St. Anselm with his ontological argument, an *a priori* argument based solely on the idea of God. The argument implies that one cannot deny the existence of God if one understands the idea of God. St. Thomas does not accept this demonstration because human understanding cannot prove the existence of God; such an idea cannot transition from logical justification (thought) to ontological reality. St. Thomas’s viewpoint is that God’s existence is self-evident *in itself*, but it is not so for our reason, which has a limited capacity, while divine reality is infinite. God’s existence is problematic for us, but we can come to know it through its effects.
Five *A Posteriori* Proofs for the Existence of God
These proofs are valid only *after the fact*. Therefore, we can know the cause, not its essence, but as it is manifested in its effects. St. Thomas presents five ways or channels that allow human reason to access knowledge of God’s existence from sensory experience. They all use the principle of causality. The five ways share the same argumentative structure:
- Starting Point: All ways begin with data supplied by sensory experience that cannot be denied. It is something observable.
- Second Step: Application of the principle of causality to the starting point. The facts we observe postulate their respective causes.
- Third Step: Corollary of the principle of causality: It is impossible to have an infinite series of causes subordinate to one another; it is necessary to assume a First Cause.
- Conclusion: This First Cause is recognized as God, or the existence of God causes the effects observed in the sensory world.