The Third Way: Another Take On The Argument | Aquinas 101

Lesson 8 – The Third Way: Another Take On The Argument

Thomas Aquinas’ Third Way to prove the existence of God is unique to the Summa theologiae, and it contains certain intricacies that are difficult to interpret, but Saint Thomas runs substantially the same argument in a much simpler form in another one of his great works called the Summa contra gentiles. Let’s take a look now at the alternative version of the argument from possible being.

Course Reading

Excerpt from Aquinas: A Beginner's Guide | Edward Feser

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

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

The third way is taken from possibility and necessity, and runs thus. We find in nature things that are possible to be and not to be, since they are found to be generated, and to corrupt, and consequently, they are possible to be and not to be. But it is impossible for these always to exist, for that which is possible not to be at some time is not. Therefore, if everything is possible not to be, then at one time there could have been nothing in existence. Now if this were true, even now there would be nothing in existence, because that which does not exist only begins to exist by something already existing. Therefore, if at one time nothing was in existence, it would have been impossible for anything to have begun to exist; and thus even now nothing would be in existence---which is absurd. Therefore, not all beings are merely possible, but there must exist something the existence of which is necessary. But every necessary thing either has its necessity caused by another, or not. Now it is impossible to go on to infinity in necessary things which have their necessity caused by another, as has been already proved in regard to efficient causes. Therefore we cannot but postulate the existence of some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity. This all men speak of as God.