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The magnitude of the transformation taking place in the sixteenth and seventeenth centuries was not lost on any of its participants. Copernicus, Kepler, Descartes, Galileo and Newton all understood that what was at stake in the revolution under way was the fate of the Christian soul. Each of these men stood on the side of God and argued that the emerging cosmology supported a case for the divine. What all of them feared was a universe stripped of spirit. They believed in a Holy Spirit whose Love in-formed the world and in the immanent spirits of their fellow human beings; in ‘the new astronomy’ they saw the reflected glory of their Creator, whose presence in the material universe supported their faith in Christianity’s promise of the soul’s eternal salvation. As Johannes Kepler summed up the case: ‘For a long time I wanted to become a theologian…Now, however, behold how through my effort God is being celebrated in astronomy.’

The literally soul-destroying potential of the new cosmology hung like a cloud over the consciousness of seventeenth-century science and the source of this angst originated in concerns quite apart from its mechanistic tendencies. By the middle of the sixteenth century thoughtful minds had begun to discern that the idea of continuity between the terrestrial and celestial realms threatened the foundation of Christian faith as it had been construed for 1,500 years. By supplanting the geocentric finitude of medieval cosmology, the new science threatened to undo the metaphysical balance between body and soul on which Christian theology relied.

Contrary to accounts given in many popular science books, medieval cosmology was underpinned by a rigorous logic that attempted to encompass the totality of humans as physical, psychological and spiritual beings. Medieval scholars read the world in an iconic rather than a literalist sense; nature was a rebus in which everything visible to the eye represented multiple layers of meaning within a grand cosmic order. The physical world was the starting point for investigations that ultimately sought to comprehend a spiritual reality beyond the material plane and what is so beautiful here is that the metaphysical duality of body and soul was mirrored in the architecture of the cosmos.

As is well known, the medieval cosmos was finite, with the Earth at the centre surrounded by concentric spheres that carried the Sun, the Moon, the planets and stars revolving around us. Beyond the sphere of the stars was the final sphere of the universe proper, what the medievals, following the Greeks, called the primum mobile. Technically this constituted the limit of the universe – here, as Aristotle argued, space and time ended. Critically, because physical space was finite, medieval minds could imagine that ‘beyond’ the material world there was plenty of ‘room’ left for some other kind of space. On medieval cosmological diagrams we see it labelled the ‘Heavenly Empyrean’. What lay ‘beyond’ physical space was the spiritual space of God and the soul.

In the final stanzas of The Divine Comedy Dante enacts this transition. Having traversed the span of his universe from the depths of Hell in the centre of the Earth, up the purifying mountain of Purgatory and through the celestial layers, Dante pierces the shell of the primum mobile and bursts through the skin of the world to come face to face with God, ‘the Love that moves the Sun and the other stars’. For medieval thinkers this spiritual domain was the primary realm of the Real with the physical realm serving as a secondary and rather pale reflection. Just what it meant to have a ‘place’ outside physical space was a question that much exercised medieval minds – no scholar of the Middle Ages believed that Heaven lay literally beyond the stars. Yet whatever the philosophical difficulties, scholars of the time insisted that physical space was not the totality of reality but one half of a larger metaphysical whole.

This dualism of body and soul – matter and spirit – was mirrored in a dualism that was believed to exist between the terrestrial and celestial realms. Again, following the Greeks, medieval natural philosophy held that the two regions were qualitatively distinct regions: in the terrestrial realm things were made up from the four mundane elements, earth, air, fire and water; those in the celestial realm (stars, planets, comets and so on) were composed of a fifth element, or quintessence, also known as the æther. Everything in the terrestrial realm was subject to decay and death, those in the heavens were believed to be eternal, prone neither to decay nor change. Subtleties compounded, for the celestial realm was not itself homogeneous. Ascending from the surface of the Earth medieval cosmology posited that in each successive sphere things became more ethereal. In effect, celestial space exhibited a vector of grace: the closer one got to God, the more ‘pure’ the region was said to be. Within the scheme of medieval cosmology, celestial space thus served as a mediating zone between the purely material realm of the Earth and the purely spiritual realm of the Empyrean. To put it another way, the celestial heaven of the planets and stars stood as a metaphor for and pointer to the religious Heaven of God and the soul, and the whole of medieval thinking rejoiced in this analogy.

But what if terrestrial space and celestial space were not qualitatively distinct? What if the cosmos was a homogeneous domain? Just such an idea began to bubble into European consciousness during the fifteenth century forming the seeds of what would become, in the seventeenth, a full-blown reconfiguration of Western cosmological thinking.

The first person to express this vision in anything like its modern form was a cardinal of the Roman Catholic Church, Nicolas of Cusa, who completed in 1440 a masterpiece of scientific proleptics entitled On Learned Ignorance. The universe Cusa proposed had no crystal spheres and no hierarchy of planets; in one daring swoop he abolished the distinction between the ‘base’ Earth and the ‘ethereal’ heavens, positing that the stars and planets were also mundane material bodies. Cusa’s cosmos was infinite – ‘unbounded’ is the word he used – a space in which all regions were materially and spiritually on par. He even suggested that other stars were peopled by other physical beings, an idea that would not be broached again for 150 years. Cusa’s ideas were too radical for most of his contemporaries, but in the sixteenth century the tectonic plates of the Western psyche began to shift, resulting in the work of Copernicus and all that came after him.

Why was such a shift occurring? After all, the medieval world picture had held stable as a philosophical construct for more than a thousand years. The telescope had not yet been invented, astronomical observations were not qualitatively better, the Ptolemaic model continued to yield reasonable results. Cosmological technologies were not perceived to be failing. So what was going on? Astronomy wasn’t undergoing a crisis, nonetheless underlying conceptions of how reality might be were beginning to change. We know this primarily not from what scientifically minded thinkers were saying but from what painters were doing. Long before the rise of science a new Western attitude to space was apparent in the realm of art, and to understand the cosmological transformation wrought by Galileo and Newton in the seventeenth century it is instructive to turn first to the frescoes of Giotto. Here we can see explicitly the spiritual stakes that were coming into play as Europeans began to feel their way out of a medieval world.

Along with medieval philosophy, medieval art focused on the numinous realm of the soul. Art also was iconic, aiming to represent the spiritual order beyond the material world. One way of conveying that order was through scale; thus Christ would be the largest figure in a painting, with angels next in size, followed by saints and martyrs, then ordinary human beings. Backgrounds too were iconic; gold and azure represented Heaven, whose value was viscerally present in exorbitantly expensive gold leaf and lapis lazuli pigments. Depth was almost absent from these images. But in the late twelfth century representation began to undergo a subtle transformation with a gradual interest in three-dimensionality starting to emerge. This new style reached a crescendo with Giotto’s work in the Arena Chapel in Padua in which he depicted a sequence of near-life-sized images recounting the life of Christ. What is immediately startling about these Christ Cycle frescoes is their sense of physical presence. Figures look solid and are anchored to the ground as if compelled by gravity. We are clearly no longer in Heaven but on Earth. Everyone appears at the same scale: Christ and humans and angels. Flat blue and gold backgrounds are replaced by attempts at genuine landscapes; there are mountains, trees and carefully observed studies of animals. Buildings seem to be leaping out of the surface. True, they are not entirely convincing, but one feels here that the artist is striving to convey three-dimensional space.

All this was in keeping with a revivified interest in the natural world. After the hiatus of the early Middle Ages, scholars had begun to recover the science and mathematics of ancient Greece, and during the thirteenth century the study of nature underwent a renaissance. With his careful attention to empirical detail, Giotto reflected this novel scientific bent. For artists and their patrons (many of whom were leaders of the Catholic Church) the observations of the outer eye were becoming more interesting than the revelations of the inner eye. In short, visual attention was shifting towards the material realm.

Paradoxically, this refocusing from spirit to matter was given credence by a novel theological development and it is here that science and religion intersect in a uniquely Western way. As Europeans recovered the heritage of the Greeks one thinker they increasingly encountered was Pythagoras, a mathematician and mystic who had dreamed the dream that would become modern physics. In the fifth century BCE Pythagoras posited that the structure of the world was determined by mathematics: ‘All is number’, he famously declared. A small band of medieval thinkers took Pythagorean precepts and transformed them into a Christian context, giving rise to the then-novel idea that God had created the material world according to mathematical rules.

Among God’s primary tools was Euclidean geometry and in 1267 the Franciscan friar Roger Bacon argued in a treatise to Pope Clement IV that artists ought to follow their Creator and construct images accordingly with geometric relationships. Bacon called the new style ‘geometric figuring’ and he proposed that the Church encourage painters to adopt it as a matter of principle. Artists who did so would not just be rendering Creation truthfully, Bacon said, they could also serve a powerful propaganda purpose, for according to him the techniques of three-dimensional verisimilitude were so psychologically powerful that viewers beholding such images would believe they were actually witnessing the scenes depicted. They would believe they were really seeing, for example, Christ raising Lazarus in front of them. To put this into current parlance, Bacon was suggesting that ‘geometric figuring’ acted as a kind of virtual reality and, as he saw it, this medieval VR would have the power to convert unbelievers to the Christian faith.

From the fourteenth through sixteenth centuries artists elaborated Bacon’s vision with ever-greater finesse, a movement that culminated in the formalisms of ‘linear perspective’. The consequences of this representational revolution reached far beyond the painted surfaces of the churches from which it began. Art historian Samuel Edgerton has argued that ‘geometric figuring’ retrained European minds to see space in a Euclidean sense and that in this respect Renaissance artists from Giotto through Raphael paved the way for the physicists who came after them. Edgerton’s thesis helps make sense of a historical conundrum, for following Aristotle most Western thinkers pointedly rejected a Euclidean view of space. As physicist and science historian Max Jammer has stressed, such a view of space was not ‘thought reasonable until the seventeenth century’. No other culture we know of has conceived of its cosmic scheme in this mathematical manner, and even in the West most learned people rebelled against the idea for several hundred years. Perspectival painting served to introduce the concept as a visceral experience, subverting intellectual objections by giving viewers a powerful psychological illusion that the painted scenes they were looking at were actually there.

By the mid-sixteenth century, educated Europeans were coming to believe that the space around them here on Earth was a Euclidean realm. But that raised an uneasy question: How far out does this space extend? Does it extend to the Moon? To Mars? To the Sun and stars? Though not articulated in quite this form during the Renaissance, the question assumed immense importance because it challenged the medieval distinction between the terrestrial and celestial realms. If Euclidean space proceeds beyond the Earth then that suggests that similar laws and similar things should be found in both regions.

The unification of the two domains was of course cemented by Isaac Newton and in some ways it remains his most profound legacy. Newton showed that the same force of gravity that makes an apple fall to the ground also operates to keep the Moon revolving around the Earth and the planets orbiting the Sun. Newton’s law (a Pythagorean triumph if ever there was one) demonstrated an essential continuity, for if gravity operates between celestial bodies then they too must be mundane matter like the pebble that rolls down a hill. Moreover, once astronomers abandoned the medieval distinction between earthly and celestial space, there was no longer any reason to imagine a limit to the physical world. Why should physical space not go on for ever? By the end of the eighteenth century, that view had become scientific orthodoxy.