Philosophy Index

Saint Thomas Aquinas

Thomas Aquinas (1225–1274) was an Italian theologian, priest and philosoher. Aquinas’ philosophical systems were declared by Pope Leo XIII to be the official philosophies of the Roman Catholic Church. He is also seen as one of the main figures of scholasticism. In the Christian tradition, Saint Thomas’ feast day is celebrated on January 28, formerly it was March 7.

If Augustine’s project was to Christianize the works of Plato, it was Aquinas’s to Christianize the works of Aristotle. In addition to his own works, which incorporate Aristotelean ideals and reconcile them with Christian doctrine, Aquinas wrote several commentaries on the works of Aristotle.

Knowledge of God

In the scholastic tradition begun by Saint Anselm, Aquinas attempts to apply reason to the question of God’s existence. However, Aquinas refused to accept Anselm’s ontological proof, and insisted that only faith and revelation can go beyond the limits of reason and truly understand the nature of God.

Part of that revelation, however, exists in the world, Aquinas believed. He argues that to define God one must know God's essence, and our knowledge comes from experience. To prove God, we must start with information we have available. But can we derive knowledge of God from experience?

Aquinas believes he has five ways to do this.

  1. From movement: Movement requires a mover, but the sequence backwards cannot be infinite. Potentiality becomes actuality by something actual, and the first mover — that first actual thing, is what we know as God.
  2. From causality: Similar to the movement argument, is the argument that causality has the same property. Effects must have causes, which themselves must have causes, and therefore there must be a first cause, which we know as God.
  3. From the existence and nonexistence of things: Nothing we perceive is permanent — all things can exist and not exist. All things at one point did not exist, but since nothing can be created from nothing, there must be a first creator, and that is God.
  4. From natural goodness: In order to grade things, we need to grade them relative to something. For good to exist, there must be an ideal that exists also, which we know as God. (This is an Aristotlean idea.)
  5. From the order of the world: The world sees all things moving purposefully to certain ends. These things seem to be well-designed, and there must be a designer, and that is God.

Aquinas believed that these arguments for God, as derived from experience, also counter a common argument against God. Those skeptical of God's existence often ask how an infinitely good being could allow evil to occur in his creation. Aquinas argues that, since we can trace evil back to God in the way we can track all worldly design, movement and cause back to God, evil must be part of God's plan. It must serve a purpose for the overall good of the world.

Beyond His most basic characteristics, however, God can not truly be known through reason. Details about his nature, such as the Trinity and the life, death and ressurection of Christ, however, may only be known through revelation, which is presented through the church and through scripture.


Full Name: Thomas Aquinas
Born: 1225, Sicily
Died: March 7, 1274
Occupation: Roman Catholic Priest
Canonized: 1279 by Pope John XXII
Feast Day: January 28