This volume explores scholars’ use of Lucretius’ Epicurean didactic poem De Rerum Natura from its rediscovery in 1417 to 1600, focusing on the challenges its atomistic physics posed to Christian patterns of thought. In a period when atheism was often considered a sign of madness, the sudden availability of a sophisticated system that explained natural phenomena in non-theistic ways, and that argued powerfully against the immortality of the soul, the afterlife, and a creator God, threatened to supply the one weapon unbelief had lacked in the Middle Ages: good answers. At the same time, humanist scholars who idealized ancient Rome were eager to study a poem whose language and structure so often anticipated their beloved Aeneid. This book uncovers humanist methods for reconciling Christian and pagan philosophy, and shows how atomism and ideas of emergent order and natural selection, so critical to our current thinking, became situated in Europe’s intellectual landscape at the beginning of the scientific transformations of the seventeenth century. It employs a new quantitative method for analyzing marginalia in manuscripts and printed books, whose results expose how changes in scholarly reading practices over the course of the sixteenth century, fostered by the growth of printing, controlled the circulation of texts and gradually expanded Europe’s receptivity to radical science, setting the stage for the scientific revolution.