Lesson 3 – What's Wrong With St. Thomas' First Proof for the Existence of God?

The weakest link in St. Thomas Aquinas’ first way is the premise that says: “If something is changed by something else, then there must be an unchanged cause of change—i.e., an unmoved mover.”

Why should we think that?

What’s the connection between something being changed by something else and the existence of an unchanged cause of change?

Course Reading

Summa Theologiae I, q. 2, a. 2 | St. Thomas Aquinas

Summa Theologiae I, q. 2, a. 3 | St. Thomas Aquinas

Excerpt from the Summa Theologiae I, q. 2, a. 3:

[W]hatever is in motion must be put in motion by another. If that by which it is put in motion be itself put in motion, then this also must needs be put in motion by another, and that by another again. But this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover; seeing that subsequent movers move only inasmuch as they are put in motion by the first mover; as the staff moves only because it is put in motion by the hand. Therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.

Course Listening

Proofs for God's Existence: Aquinas on Reason, Uncertainty, and Faith | Prof. Joshua Hochschild

The Mind's Ascent to God: Theology as a Science and as Wisdom | Fr. Dominic Legge, O.P.