St. Thomas Aquinas (1224-1274) was a theologian, Aristotelian scholar, and philosopher. Called the Doctor Angelicus (the Angelic Doctor,) Aquinas is considered one the greatest Christian philosophers to have ever lived.

Much of St. Thomas's thought is an attempt to understand Christian orthodoxy in terms of Aristotelian philosophy. His five proofs for the existence of God take "as givens" some of Aristotle's assertions concerning being and the principles of being (the study of being and its principles is known as metaphysics within philosophy). Before analyzing further the first of Aquinas' Five Ways, let us examine some of the Aristotelian underpinnings at work within St. Thomas' philosophy.

The first proof for the existence of God is sometimes known as the cosmological argument. Aristotle, much like a natural scientist, believed that we could learn about our world and the very essence of things within our world through observation. As a marine biologist might observe and catalog certain marine life in an attempt to gain insight into that specific thing's existence, so too did Aristotle observe the physical world around him in order to gain insight into his world. The very term cosmological is a reflection of Aristotle's relying upon sense data and observation. The word logos suggests a study of something while the noun cosmos means order or the way things are. Thus, a cosmological argument for the existence of God will study the order of things or examine why things are the way they are in order to demonstrate the existence of God.

Aristotle and Aquinas also believed in the importance of the senses and sense data within the knowing process. Aquinas once wrote nothing in the mind that was not first in the senses. Those who place priority upon sense data within the knowing process are known as empiricists. Empirical data is that which can be sensed and typically tested. Unlike Anselm, who was a rationalist, Aquinas will not rely on non-empirical evidence (such as the definition of the term "God" or "perfection") to demonstrate God's existence. St. Thomas will observe the physical world around him and, moving from effect to cause, will try try to explain why things are the way they are. He will assert God as the ultimate Cause of all that is. For Aquinas, the assertion of God as prima causa (first cause) is not so much a blind religious belief but a philosophical and theoretical necessity. God as first cause is at the very heart of St. Thomas' Five Ways and his philosophy in general.

One last notion that is central to St. Thomas' Five Ways is the concept of potentiality and actuality. Aristotle observed that things/substances strive from an incomplete state to a complete state. Things will grow and tend to become as they exist. The more complete a thing is, the better an instance of that thing it is. We have idioms and expressions within our language that reflect this idea. For example, we might say that so-and-so has a lot of potential. We might say that someone is at the peak of their game or that someone is the best at what they do. We might say It just does not get any better than this if we are are having a very enjoyable time. Aristotle alludes to this commonly held intuition when he speaks of organisms moving from a state of potentiality to actuality. When Aquinas speaks of motion within the First Way (the cosmological argument) he is referencing the Aristotlean concepts of potentiality and actuality.