Of course, it's virtually impossible to describe or even summarize “the medieval worldview”, although Lewis occasionally pretends otherwise in his book. What he is really describing is presumably the high medieval worldview of literate people in England, France and northern Italy as seen through the prism of medieval works of literature and their echoes during the Renaissance. As behoves a series of college lectures, Lewis is occasionally somewhat rambling or frankly boring, but I suppose that goes with the territory!
A few points stand out. Lewis points out that virtually all educated people during the Middle Ages knew that the Earth was spherical, not flat. They also knew that our planet was very small and insignificant, compared to the vast distances in the surrounding space. While it's true that the medieval worldview was geocentric, ideologically Earth wasn't considered the most important part of the cosmos. On the contrary, Earth was the *least* important part. When Dante sees the “true” outline of the universe in “The Divine Comedy”, God's heaven is in the centre, with the material world being a circumference. The medieval literati also knew that the stars were larger than the Earth, and that the sun was larger than the Moon.
Lewis (perhaps taking a cue from his friend Owen Barfield) also believes that the Ptolemaic system was a mere model, not to be taken absolutely literary. It's function was to “save the appearances”, and as long as this could be accomplished by adding new epicycles, that's what happened. Copernicus could avoid the wrath of the Church by proposing his heliocentric system as another mathematical model. However, the medievals never realized that the universe was infinite. Lewis likens the medieval model of the cosmos to that of a cathedral. It may be enormous, but it's not infinite. Above all, it's meaningful, with God at the helm. Medieval man didn't have Pascal's fear of the dark, silent skies.
Other topics covered by “The Discarded Image” include the relationship between Christianity and philosophy, astrology, the Longaevi (otherwise known as fairies or elves), Neo-Platonism, and the Platonism of Chalcidius' important Latin translation of “Timaeus”. Lewis also discusses historicism, and makes the observation that the Hebrew scriptures weren't the only ones with a teleological sense of history. What about Roman epics, such as the Aeneid?
Lewis ends his lectures by pointing out that the medieval model was…well, wrong. However, he also says that perhaps no model is “right”, surely another possible influence from Barfield. “The Discarded Image” isn't for the general reader, but advanced students of the Middle Ages (or the history of science!) might want to sink their teeth into this one. Four stars. And hush…they say Aslan is on the move…