Stephen Greenblatt’s book The Swerve: How the World Became Modern begins by recounting Greenblatt’s chance discovery, while he was still in college, of a translation of De rerum natura (On the Nature of Things), a poem by the Roman philosopher Lucretius. He immediately found the work fascinating, especially its focus on the fear of death, a fear that had dominated Greenblatt’s life until then. His mother had been terrified of dying, and her terror had shaped her young son’s life. Lucretius, however, taught that fearing death was foolish since the universe consisted of nothing but atoms, ceaselessly arranging and rearranging themselves in different patterns. For Lucretius, there is no God, no pattern, and no purpose, merely atoms in flux. Everything is ultimately subject to change—a fact that should make us appreciate whatever beauty we encounter and which should also make us unafraid of dying. No living thing escapes death, but nothing—neither pleasure nor pain—lies beyond it except further atomic change.

The impact of Lucretius’s ideas (Greenblatt began to believe) helped explain a period of human history—the “Renaissance”—that seemed especially devoted to the pursuit of beauty and pleasure. During the Renaissance, people began to move away from supernatural explanations and began, more and more, to see the universe as consisting of matter. In other words, they began to think like Lucretius. This change of thought was due in great measure to the Italian scholar Poggio Bracciolini (1380-1459), a frequent secretary to various popes. In 1417, Bracciolini discovered, in an obscure German monastery, an extremely rare surviving copy of De rerum natura. The book, widely known and highly influential during the later classical period, had been lost for roughly a thousand years. Bracciolini’s discovery of the text suddenly renewed its life and revived its potential influence.

The Renaissance, of course, had been well under way by the time Poggio made his find. Poggio himself, with his love of ancient Greek and Roman texts and his eagerness to seek them out, might accurately be called a kind of “Renaissance man,” like his great predecessor Francesco Petrarca, or “Petrarch” (1304-1374). But the rediscovery of De rerum natura, at least according to Greenblatt, pushed the Renaissance even more toward secularism and a focus on the present material world. Lucretius’s poem helped give new attention and new attractiveness to the palpable beauties and pleasures of the here-and-now as opposed to the consolations or terrors of a mysterious hereafter.

Lucretius’s ideas had been strongly affected by those of Epicurus, an even more ancient thinker. Born in Greece, Epicurus had done much to formulate and advocate atomism, which did away with gods, superstitions, the supernatural, and the fear of death, replacing them instead (at least according to his followers) with rational thought, peace of mind, and the highest of all pleasures—the pleasures of the intellect. Epicureanism was not merely a scientific view but an entire philosophy of life. Although few of Epicurus’s own words survive, his ideas had an enormous impact, inspiring both fervent belief and equally fervent opposition. Opponents associated such thinking with mere hedonism and material self-indulgence; advocates and defenders (such as Lucretius in De rerum natura) emphasized the liberating potential of Epicurean thought.

According to Epicurus and Lucretius, atoms, and the space between them, have always existed. No creator made them. The objects formed of atoms (including people) may dissolve, but atoms do not. They simply recombine into different objects. The ways atoms...

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