When I was a student, I used to go at the end of the school year to the Yale Co-op to see what I could find to read over the summer. I had very little pocket money, but the bookstore would routinely sell its unwanted titles for ridiculously small sums. They were jumbled together in bins through which I would rummage until something caught my eye. On one of my forays, I was struck by an extremely odd paperback cover, a detail from a painting by the Surrealist Max Ernst. Under a crescent moon, high above the earth, two pairs of legs—the bodies were missing—were engaged in what appeared to be an act of celestial coition. The book, a prose translation of Lucretius’ two-thousand-year-old poem “On the Nature of Things” (“De Rerum Natura”), was marked down to ten cents, and I bought it as much for the cover as for the classical account of the material universe.

Lucretius anticipated the core scientific vision of modernity. Illustration by LUIS GRAÑENA

Ancient physics is not a particularly promising subject for vacation reading, but sometime over the summer I idly picked up the book. The Roman poet begins his work (in Martin Ferguson Smith’s careful rendering) with an ardent hymn to Venus:

First, goddess, the birds of the air, pierced to the heart with your powerful shafts, signal your entry. Next wild creatures and cattle bound over rich pastures and swim rushing rivers: so surely are they all captivated by your charm and eagerly follow your lead. Then you inject seductive love into the heart of every creature that lives in the seas and mountains and river torrents and bird-haunted thickets, implanting in it the passionate urge to reproduce its kind.

Startled by the intensity, I continued, past a prayer for peace, a tribute to the wisdom of the philosopher Epicurus, a resolute condemnation of superstitious fears, and into a lengthy exposition of philosophical first principles. I found the book thrilling.

Lucretius, who was born about a century before Christ, was emphatically not our contemporary. He thought that worms were spontaneously generated from wet soil, that earthquakes were the result of winds caught in underground caverns, that the sun circled the earth. But, at its heart, “On the Nature of Things” persuasively laid out what seemed to be a strikingly modern understanding of the world. Every page reflected a core scientific vision—a vision of atoms randomly moving in an infinite universe—imbued with a poet’s sense of wonder. Wonder did not depend on the dream of an afterlife; in Lucretius it welled up out of a recognition that we are made of the same matter as the stars and the oceans and all things else. And this recognition was the basis for the way he thought we should live—not in fear of the gods but in pursuit of pleasure, in avoidance of pain.

As it turned out, there was a line from this work to modernity, though not a direct one: nothing is ever so simple. There were innumerable forgettings, disappearances, recoveries, and dismissals. The poem was lost, apparently irrevocably, and then found. This retrieval, after many centuries, is something one is tempted to call a miracle. But the author of the poem in question did not believe in miracles. He thought that nothing could violate the laws of nature. He posited instead what he called a “swerve”—Lucretius’ principal word for it was clinamen—an unexpected, unpredictable movement of matter.

The poem’s rediscovery prompted such a swerve. The cultural shift of the Renaissance is notoriously difficult to define, but it was characterized, in part, by a decidedly Lucretian pursuit of beauty and pleasure. The pursuit shaped the dress and the etiquette of courtiers, the language of the liturgy, the design and decoration of everyday objects. It suffused Leonardo da Vinci’s scientific and technological explorations, Galileo’s vivid dialogues on astronomy, Francis Bacon’s ambitious research projects, and Richard Hooker’s theology. Even works that were seemingly unrelated to any aesthetic ambition—Machiavelli’s analysis of political strategy, Walter Raleigh’s description of Guiana, Robert Burton’s encyclopedic account of mental illness—were crafted in such a way as to produce pleasure. And this pursuit, with its denial of Christian asceticism, enabled people to turn away from a preoccupation with angels and demons and to focus instead on things in this world: to conduct experiments without worrying about infringing on God’s jealously guarded secrets, to question authorities and challenge received doctrines, to contemplate without terror the death of the soul.

The recovery of “On the Nature of Things” is a story of how the world swerved in a new direction. The agent of change was not a revolution, an implacable army at the gates, or landfall on an unknown continent. When it occurred, nearly six hundred years ago, the key event was muffled and almost invisible, tucked away behind walls in a remote place. A short, genial, cannily alert man in his late thirties reached out one day, took a very old manuscript off a shelf, and saw with excitement what he had discovered. That was all; but it was enough.

By that time, Lucretius’ ideas had been out of circulation for centuries. In the Roman Empire, the literacy rate was never high, and after the Sack of Rome, in 410 C.E., it began to plummet. It is possible for a whole culture to turn away from reading and writing. As the empire crumbled and Christianity became ascendant, as cities decayed, trade declined, and an anxious populace scanned the horizon for barbarian armies, the ancient system of education fell apart. What began as downsizing went on to wholesale abandonment. Schools closed, libraries and academies shut their doors, professional grammarians and teachers of rhetoric found themselves out of work, scribes were no longer given manuscripts to copy. There were more important things to worry about than the fate of books. Lucretius’ poem, so incompatible with any cult of the gods, was attacked, ridiculed, burned, or ignored, and, like Lucretius himself, eventually forgotten.

The idea of pleasure and beauty that the work advanced was forgotten with it. Theology provided an explanation for the chaos of the Dark Ages: human beings were by nature corrupt. Inheritors of the sin of Adam and Eve, they richly deserved every miserable catastrophe that befell them. God cared about human beings, just as a father cared about his wayward children, and the sign of that care was anger. It was only through pain and punishment that a small number could find the narrow gate to salvation. A hatred of pleasure-seeking, a vision of God’s providential rage, and an obsession with the afterlife: these were death knells of everything Lucretius represented.

By chance, copies of “On the Nature of Things” somehow made it into a few monastery libraries, places that had buried, seemingly forever, the principled pursuit of pleasure. By chance, a monk laboring in a scriptorium somewhere or other in the ninth century copied the poem before it moldered away. And, by chance, this copy escaped fire and flood and the teeth of time for some five hundred years until, one day in 1417, it came into the hands of a man who proudly called himself Poggius Florentinus, Poggio the Florentine.

Poggio was, among other things, famous for the elegance of his script and for writing the best-known jokebook of its age, a chronicle of cynical tricksters, bawdy friars, unfaithful wives, and foolish husbands. He had served a succession of Roman Pontiffs as a scriptor—that is, a writer of official documents in the Papal bureaucracy—and, through adroitness and cunning, he had risen to the coveted position of Apostolic Secretary. He had access, as the very word “secretary” suggests, to the Pope’s secrets. But above all he was a book hunter, perhaps the greatest of his kind.

Italians had been obsessed with book hunting ever since the poet and scholar Petrarch brought glory on himself around 1330 by piecing together Livy’s monumental “History of Rome” and finding forgotten masterpieces by Cicero and Propertius. Petrarch’s achievement had inspired others to seek out lost classics that had been lying unread, often for centuries. The recovered texts were copied, edited, commented upon, and eagerly exchanged, conferring distinction on those who had found them and forming the basis for what became known as the “study of the humanities.” The “humanists,” as those who were devoted to this study were called, knew from carefully poring over the texts that had survived from classical Rome that many once famous books or parts of books were still missing.

As a humanist, Poggio had quite a few accomplishments. He uncovered an epic poem on the struggle between Rome and Carthage; the works of an ancient literary critic who had flourished during Nero’s reign and had written notes and glosses on classical authors; another critic who quoted extensively from lost epics written in imitation of Homer; a grammarian who wrote a treatise on spelling; a large fragment of a hitherto unknown history of the Roman Empire written by a high-ranking officer in the imperial Army, Ammianus Marcellinus. His salvaging of the complete text of the rhetorician Quintilian changed the curriculum of law schools and universities throughout Europe, and his discovery of Vitruvius’ treatise on architecture transformed the way buildings were designed. But it was in January, 1417, when Poggio found himself in a monastery library, that he made his greatest discovery. He put his hands on a long poem whose author he may have recalled seeing mentioned in other ancient works: “T. LUCRETI CARI DE RERUM NATURA.”

“On the Nature of Things,” by Titus Lucretius Carus, is not an easy read. Totalling seventy-four hundred lines, it is written in hexameters, the standard unrhymed six-beat lines in which Latin poets like Virgil and Ovid, imitating Homer’s Greek, cast their epic poetry. Divided into six untitled books, the poem yokes together moments of intense lyrical beauty; philosophical meditations on religion, pleasure, and death; and scientific theories of the physical world, the evolution of human societies, the perils and joys of sex, and the nature of disease. The language is often knotty and difficult, the syntax complex, and the over-all intellectual ambition astoundingly high.

The stuff of the universe, Lucretius proposed, is an infinite number of atoms moving randomly through space, like dust motes in a sunbeam, colliding, hooking together, forming complex structures, breaking apart again, in a ceaseless process of creation and destruction. There is no escape from this process. When we look up at the night sky and marvel at the numberless stars, we are not seeing the handiwork of the gods or a crystalline sphere. We are seeing the same material world of which we are a part and from whose elements we are made. There is no master plan, no divine architect, no intelligent design. Nature restlessly experiments, and we are simply one among the innumerable results: “We are all sprung from celestial seed; all have that same father, from whom our fostering mother earth receives liquid drops of water, and then teeming brings forth bright corn and luxuriant trees and the race of mankind, brings forth all the generations of wild beasts, providing food with which all nourish their bodies and lead a sweet life and beget their offspring.”

All things, including the species to which we belong, have evolved over vast stretches of time. The evolution is random, though in the case of living organisms it involves a principle of natural selection. That is, species that are suited to survive and to reproduce successfully endure, at least for a time; those which are not so well suited die off quickly. Other species existed and vanished before we came onto the scene; our kind, too, will vanish one day. Nothing—from our own species to the sun—lasts forever. Only the atoms are immortal.

In a universe so constituted, Lucretius argued, it is absurd to think that the earth and its inhabitants occupy a central place, or that the world was purpose-built to accommodate human beings: “The child, like a sailor cast forth by the cruel waves, lies naked upon the ground, speechless, in need of every kind of vital support, as soon as nature has spilt him forth with throes from his mother’s womb into the regions of light.” There is no reason to set humans apart from other animals, no hope of bribing or appeasing the gods, no place for religious fanaticism, no call for ascetic self-denial, no justification for dreams of limitless power or perfect security, no rationale for wars of conquest or self-aggrandizement, no possibility of triumphing over nature. Instead, he wrote, human beings should conquer their fears, accept the fact that they themselves and all the things they encounter are transitory, and embrace the beauty and the pleasure of the world.