What is the theme in The Discarded Image: An Introduction to Medieval and Renaissance Literature by C. S. Lewis?

The main theme of the book is the principle of the triad. A central principle of the Medieval understanding of the world is one inherited from the Pagan writer Apuleius known as the Principle of the Triad. According to this principle, two very different things cannot directly affect one another; there must be some third thing which, so to speak, bridges the gap between them. The original application of this principle was to explain how God, a transcendent spiritual being, could interact with flesh-bound man. Apuleius reasoned that there must be some kind of intermediate being, similar in some ways both to God and humans, who could communicate the orders of God to man. Pseudo-Dionysius would incorporate this into his work and identify the angels as the spiritual mediators between men and God.