On the “Inverted Cosmos”—From Aristotle to the Middle Ages

Long before he gained fame as one of the world’s most beloved writers of children’s fiction, C. S. Lewis earned his reputation as a scholar of medieval literature. He taught this subject first at Oxford University before moving to Cambridge in 1954 to assume the newly created Chair of Medieval and Renaissance Literature, a position he would hold for the rest of his career. One of the subjects Lewis lectured on at both storied schools was how people in the Middle Ages imagined the cosmos.

Like Kant, Lewis was filled with awe by the starry skies above. On occasion he would invite his students to walk with him at night and look up at those skies, and challenge them to cast aside their accustomed, modern way of viewing the heavens. “Whatever else a modern feels when he looks at the night sky,” he would tell them, “he certainly feels that he is looking out—like one looking out from the saloon entrance on to the dark Atlantic or from the lighted porch upon dark and lonely moors.”

But the cosmos didn’t appear that way to the poets and natural philosophers of the Middle Ages, Lewis went on to explain. If you succeeded in seeing with medieval eyes, he told them, “you would feel like one looking in.”

What could it mean to look out at the night sky, or any sky for that matter, and feel that we are looking in? To do this, we might begin by imagining that the surface of the earth on which we stand is not curved convexly, bending away from the soles of our feet, but rather concavely, like the inside of a bowl, such that, way out there, beyond any visible horizon, the earth’s surface slopes gently up to eventually encompass everything we see.

In this inverted cosmos, no matter where we stood on the surface of the earth, we could stare straight up and point toward the exact same point. But even if we could manage to twist our minds around this way, we would surely be tempted to wonder why anyone would concoct such a bizarre cosmic architecture. The reason medievals did so stemmed from a long-standing debate about what it meant for the cosmos to be somewhere at all.

University life in thirteenth-century Europe was dominated by a movement now known as Scholasticism. The Scholastics’ primary aim was to reconcile the teachings of Aristotle—whose works had arrived in Europe via the translations and commentaries of Arab philosophers—with their own culture’s dominant Christian theology. One of the ideas they adopted from Aristotle was his physical model of the heavens: imagine our earth as a giant marble encased in multiple, nesting, perfectly smooth layers of glass, each moving independently of the one below it and carrying the orbs we see above us in the sky—the sun, the moon, the planets, the blanket of stars.

The outermost of these spheres imparted its motion to the others without itself needing another sphere to contain it and give it motion. This primum mobile, or first mover, appealed mightily to Christians, just as it had to its Muslim importers—not least because it seemed to offer a physical embodiment of a popular and intuitive proof for the existence of God (who was, after all, a pretty good candidate for the ultimate cause for which no other cause is needed).

While God as the first mover made for a nifty trick, reconciling all of Aristotle’s teachings with Christian ideas was more of a stretch. A particularly thorny problem arose from Aristotle’s definition of what constitutes where something is. The place something occupies, he believed, is “the innermost, motionless surface of the containing body in direct contact with the contained body.” This was important because it helped explain the concept of motion: namely, an object’s change in relation to its place. But if the outermost container of the cosmos itself had no container, how could it be said to move? Even more nerve-racking, if the cosmos as a whole had nothing surrounding it, might one not infer that it is impossible for anyone, even God, to move it?

In 1277 the bishop of Paris, one Stephen Tempier, had an answer for the Scholastics. He issued a proclamation ordering the university professors to stop instructing God what He couldn’t do. The list of 219 Condemnations in the bishop’s decree covered an enormous array of topics and censored specific books, but a good chunk of them dealt with the irksome (for the ecclesiastical authorities) hubris of philosophers who thought they had the standing to decide on God’s limitations.

So God could, it seemed, make the cosmos move, even if there were nothing outside the cosmos in which to make it move. This then raised the head-scratching question of relative to what, exactly, the cosmos was moving. Luckily, the Scholastics had a number of thinkers to whom they could turn for help—specifically the Arab commentators and translators who had brought Aristotle’s teachings to Europe. The most influential of these was named Ibn Rushd, a twelfth-century polymath from the south of Spain whose interpretations of Aristotle were so widely read and respected that he became known as the Arab Aristotle.

As a young man Ibn Rushd had been called into the presence of the sultan, himself a learned man, who had shown great interest in discussing deep theological questions. After some pleasantries and general inquiries about his family and provenance, the sultan jumped right into the deep end and, to the young philosopher’s considerable alarm, asked him what people were saying about the heavens, specifically, “Are they eternal or created?”

Not surprisingly, Ibn Rushd had given this some thought. Indeed, his answer would ultimately leave its enduring imprint on Christian as well as Islamic theology. Aristotle had believed that the earth and the system of crystalline spheres encapsulating it had existed for all time. Monotheists like Ibn Rushd and his sultan, however, believed that God had created the cosmos, which seemed to suggest that it had a beginning in time. Ibn Rushd’s response to this apparent contradiction was, in essence, to have his cake and eat it, too. The cosmos was both eternal and created by God.

Crucially, this solution had consequences for the place of the cosmos as well. Aristotle’s giant system of nested crystalline spheres had nothing around it. But nothing couldn’t really exist—nature, as the philosopher had taught, abhors a vacuum. So, what would you find if you managed to work your way to that outermost layer and tried to stick your hand out beyond it?

Ibn Rushd’s approach solved this tricky question, too. For just as the cosmos could be eternal—and hence have no beginning or edge in time—and yet still have been created by God, the cosmos could exist in space with nothing outside it—and hence have no edge in space—and yet still be something objectively movable for God. The cosmos could be said to be moving because its place was not determined by an external physical container; rather, it was determined by the very center it revolved around. The cosmos was contained by its own central point.

Ibn Rushd’s model, while not fully adopted by the theologians, had remarkable staying power. It was the model C.  S. Lewis had in mind when he tried to reorient his students to medieval ways of viewing the heavens, although Lewis had his version not from Ibn Rushd but from its adaptation by Dante. As he wrote in The Discarded Image, “A few astonishing lines from the Paradiso…stamp this [image] on the mind forever….The universe is thus, when our minds are sufficiently freed from the senses, turned inside out.”

The Italian poet’s Divine Comedy recounts in first person the mystical journey of his alter ego through the gates of hell down through its nine circles to the lowest point of creation, literally the nether parts of Lucifer, buried upside down in ice. From the nadir of creation that is Satan’s scrotum, the poet describes ascending to the surface of the earth, up the mountain of purgatory, and from there begins, in the third book, his ascent to the pinnacle of paradise. Improbably, bewilderingly, Dante managed to inscribe in his journey the model of a cosmos whose center is its own container.

To pass from the earth to the center point of the heavens, one must come to a point where one’s orientation flips, where what was up becomes down. More essentially, if even harder to put into words, one’s outward movement must also flip and become an inward movement—such that no matter what direction one chooses to gaze, one is looking at the same central point.

In a key passage in Paradiso, Dante manages to do both. Arriving at the upper reaches of the sky, he describes a point where “our atmosphere…sprinkles snowflakes downward with its frozen mists,” and yet now, instead of the snow falling downward, he sees “the upper air adorned, snowflaking upward with triumphant mists that for a while had stayed with us there.” There in the heights, among the snowflakes suspended in indecision about which way to fall, the pilgrim looks down as the sun races by below his feet. He turns helplessly to his beloved, who urges him to step “onto the swiftest heaven…whose nearest and most exalted parts are all so uniform, I cannot tell which Beatrice selected as my place.”

Indeed, it does not and cannot matter where Dante steps onto the primum mobile, the swiftest heaven, for in the architecture he has created for this journey, all paths outward converge on the same point:

The nature of the universe, which holds
the center still and moves all else around it,
begins here as if from its turning-post.
And this heaven has no other where than this:
the mind of God, in which are kindled both
the love that turns it and the force it rains.
Only one circle’s light and love enclose it,
as it encloses the rest…and that precinct
only He who girds it understands.
No other heaven measures this sphere’s motion
but it serves as the measure for the rest.

Gazing up and outward, Dante sees the enclosing circles of heaven around him become progressively smaller and more intense in light and joy until, ultimately, his eyes find that one central point, infinitely small and infinitely bright, that, paradoxically, encloses all of existence in its embrace.

It was to this eternal solace—this “amazing and angelic temple that has as boundaries only love and light,” this self-contained cosmos that knows no outside in space or before or after in time—that Borges’s mind fled as he contemplated the bad infinity of words on pages, pages in books, and books on shelf after dusty shelf lining the municipal library whose outdated catalogs it was now his job to maintain.

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Excerpted from The Rigor of Angels by William Egginton. Copyright © 2023 by William Egginton. Excerpted by permission of Pantheon, a division of Penguin Random House LLC. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.