C. S. Lewis uses the term “Model” to refer to the Medieval Model of the universe. This is an expansive term that covers the entire medieval cosmology, from the shape, size, and structure of the universe and the creatures that inhabit it, to the nature of the soul and the human temperament, and even the role of religion. This Model has its inconsistencies, and not all medieval people understood it in precisely the same way; even Lewis admits that he is generalizing. His main argument is that the Model is a “backcloth for the arts” (18), a comprehensive set of assumptions, beliefs, and preferences that informs virtually all aspects of medieval literature and many aspects of medieval life. Lewis stresses that anyone who wants to understand the literature of the medieval period must be intimately familiar with this model, because otherwise, one risks attempting to understand the medieval mind through an inappropriately modern lens.

Plenitude is one of the central tenets of the Medieval Model. It originates from Apuleius, though he took inspiration from Plato. The idea of Plenitude is that each realm of the universe must be occupied with its own lifeforms. The realm above the orbit of the moon, which most people would now understand to be the void of outer space, was thought to be the realm of the spheres, as described below.