In recent years historians of the Renaissance and Enlightenment have paid increasing attention to the influence of Epicureanism upon European thought. As a result, Lucretius, the Roman who expounded Epicurean philosophy in his epic poem De rerum natura, has come to assume a foundational role in accounts of the development of modern science and philosophy. This change has been reflected within the world of Blake criticism, where, as scholars have become more interested in Blake’s complex response to materialism, so the presence of Lucretius, as both a focus for Blake’s hostility and as a shaping influence on his mythology, has become a subject of detailed scholarly investigation. We now have two studies specifically devoted to Blake and Lucretius. Stephanie Codsi has considered how Blake’s hostility to Epicurean Deism could help to explain his depiction of absent fathers in Songs of Innocence and of Experience. Joshua Schouten de Jel, in a book-length study, develops a comprehensive account of the sources from which Blake could have learned about Lucretius and a detailed view of particular areas of his response (focused upon figures that he associated with Epicurean atheism, Francis Bacon and Isaac Newton, and on specific areas of thought, including epistemology and cosmology). Since these studies have shed much light on this area, I need to explain why we need another discussion of Blake and Lucretius. In focusing upon the grounds of Blake’s hostility it is easy to overlook or underestimate the ambivalence that haunts his understanding of error and of prophecy. In this essay I shall argue that Blake saw in Lucretius not only a materialistic cosmology that he felt compelled to attack, but also a form of prophecy that represented an alluring alternative to his own prophetic mission, one whose malign influence could embroil those who tried to contain or oppose it—including John Milton. The work that deals with this issue most directly is Blake’s creation myth, The Book of Los—a work that seems to be nobody’s favorite, and that can appear frustratingly obscure.