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

Another theme of the book is The Principle of Plenitude. The Medieval understanding of the composition of the world was informed by a principle from the Pagan writer Apuleius known as the Principle of Plenitude. According to this Plenitude, every region of the universe is inhabited with its own type of beings, even the remote regions of space. There is some relation between this principle and the principle of the triad, for it is supposed that God would create all the gradations of beings between himself and the lowest form of life, man.