As Caputo explains it, the Cartesian description of God as causa sui entails an important re-definition of cause. The sort of redefinition is important. Modernity prides itself on its embrace of movement and dynamism, and portrays the pre-modern world as insufferably static. The change in the meaning of cause tells a different story.

For medievals, Caputo says, “a cause communicates something to the effect that the effect of itself does not have (like being or motion), so nothing can be the cause of itself.” This is dynamic in two ways:

First, because the cause actually passes something to the effect; second, because the effect, once affected, contains something more than it had before. A billiard ball that has been caused to move is no longer simply a billiard ball; its being is enhanced by the fact that it is in motion; it has become a moving-billiard-ball.

For Descartes, there had to be “as much reality in the cause as in the effect.” Since reason says that everything must be caused, God must be caused; and since God is infinite, He requires an infinite cause: “The cause must be sufficient to produce the effect, and the uniquely infinite God is the one and only sufficient cause of God.”

By comparison with the medieval idea, this is a static notion of cause. Descartes’s view doesn’t include the transitive moment found in the medieval conception; nothing passes over from the cause to effect. More importantly, the modern notion suggests a world in equilibrium, with causes and effects in perfectly balanced proportion.

As CS Lewis made plain, the dynamism of the medieval worldview is evident everywhere: not only in medieval notions of causation but in the ceaseless dance of Dante’s Paradise, the cosmic eschatological movement of return to God in Thomas’s theology, the singing of the planetary spheres.