Lesson 4 – How St. Thomas Aquinas' Second Way for Proving the Existence of God Works

God does exist. He is real. Moreover, his existence is something that human reason can discover (even without divine faith). 

Although it is not easy to prove that God exists by reason alone, it is possible—especially when we are aided by the wisdom of Saint Thomas Aquinas.

Course Reading

Selection from On the Principles of Nature, cc. 18-21 | St. Thomas Aquinas

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

Selection Physics, bk. 2, c. 3 | Aristotle

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

The second way is from the nature of the efficient cause. In the world of sense we find there is an order of efficient causes. There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible. Now in efficient causes it is not possible to go on to infinity, because in all efficient causes following in order, the first is the cause of the intermediate cause, and the intermediate is the cause of the ultimate cause, whether the intermediate cause be several, or only one. Now to take away the cause is to take away the effect. Therefore, if there be no first cause among efficient causes, there will be no ultimate, nor any intermediate cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false. Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God.

Course Listening

Arguments for God's Existence: Recent Work on Ancient Arguments | Prof. Robert Koons

A Thomistic Account of Truth | Prof. Timothy Pawl