Atomism, the theory that matter consists of tiny, indivisible atoms whose varied combinations form the different substances around us, existed in Europe for more than two thousand years before its modern popularity. Equally ancient are the scientific theories of vacuum, of multiple Earth-like worlds, and of creation from chaos, the theory that in the beginning atoms floating in the void clumped together randomly to form substances. On the Nature of Things (De Rerum Natura) by Titus Lucretius Carus (94-55/51 BCE ) is the most complete surviving record of these ancient atomic theories, and even tells us that Earth originally produced a wide variety of creatures, but that only those suited to their environments survived to the present.1 These doctrines were all taught by Epicurus (341-270 bce), and if his theories sound suspiciously like those of the twentieth century CE, one critical question is how these ideas were preserved and transmitted over the long period before their broad modern acceptance, particularly in the Renaissance.Few of Epicurus's writings survive,2 but in the late first century BCE a Roman follower, Lucretius, laid out his key doctrines in Latin verse in the six-book didactic poem De Rerum Natura. The poem, and the bulk of classical atomism with it, disappeared after the ninth century, but was rediscovered in 1417 by the Renaissance book-hunter Poggio Bracciolini (13801459). Humanists produced more than fifty manuscripts within a century and thirty print editions by 1600.3 Lucretius was taught in schools in France and Italy in the early sixteenth century, frequently enough for the Florentine regional Church council to ban teaching him in 15 17,4 and for Petrus Nannius (1500-1557) at Louvain to complain of the absence of a suitable classroom edition in 1543. 5 Despite this extensive circulation, and the comparatively broad appearance of Lucretian poetic themes in art and literature of the sixteenth century, atomism remained extremely rare in scientific circles until the seventeenth century, when Pierre Gassendi (1592-1655) hybridized Epicureanism, Skepticism, and Christianity.6 The question is how and why the text was used and multiplied so broadly while its core doctrines remained conspicuously absent from scientific discourse. I have approached this question through a systematic examination of marginalia in surviving Renaissance copies of the De Rerum Natura, a new technique which exposes how the reading practices of Renaissance humanists affected the transmission of ideas.The scholars we call humanists worked to restore classical civilization in the fifteenth and sixteenth centuries by creating a new educational system founded on the study of classical texts. Humanism was supposed to produce virtuous men, who would imbibe in childhood the loyalty, nobility, courage, and patriotism which made ancient Rome strong, and without which the modern world was wracked by corruption, petty ambition, and cowardly self-interest. The beauty of ancient rhetoric was supposed to arm authors and orators to inspire virtue in others, especially princes. This humanism did not value learning only for learning's sake but had a very practical agenda, to repair Europe through the education of its elite. As my findings demonstrate, the specific methods of reading taught by this humanist agenda, with its focus on moral concerns and repairing Europe along classical lines, preserved and circulated the radical content of classical texts, even while only a tiny sliver of the humanists responsible for this transmission were demonstrably interested in the radical content.7 Humanist apologists, most comparatively orthodox, sheltered these texts, and endured great dangers to do so, as the tense relationship between science and heresy flared in the Renaissance as never before.From 1417 to 1600, while Epicureanism remained conspicuously absent from discourse on physics and natural philosophy, it was conspicuously present in discourse on heresy and atheism. …