The Summa theologiae is his culminating systematic treatise. The cosmological argument appears here right at the beginning as the initial and defining statement. It anchors a complex understanding of God and his actions in a simple rational exercise that demonstrates that the reality of God’s existence is the only way to make sense of our world at all. It is fifteen centuries after Aristotle when Thomas picks up his argument again. Here is what has come to be known as “the Five Ways.” The existence