WHAT FOLLOWS BELOW offers a critical rereading of Pope Leo XIII's Aeterni patris (1879, hereafter AP) and assesses its attempt to propose Thomism as the philosophical and theological basis for Catholic engagement with modernity. In AP, Leo decried those of the new order which are well known to be dangerous to the peaceful order of things and to public safety (n. 29). In fact, Leo was as convinced as his much-maligned predecessor, Pius IX, that modern philosophies and secular notions of progress were corrosive and, ultimately, untenable. In proposing the teachings of St. Thomas Aquinas, Leo proposed an alternative philosophy; he made rival truth claims by offering Thomism as a coherent, rational, and true philosophy for the modern world, just as it had been, he believed, in the medieval period. From this grounding in Thomism, Leo offered other important statements on church-state relations and on the nature of human liberty. Doubtless, most historians and Catholic social activists today fondly remember Leo as the father of modern Catholic social thought--expressed most coherently and forcefully in Rerum novarum. This he was, to be sure, but it is important to point out that this foundational papal document can only be understood properly when set against Leo's more fundamental concern to draw the Church and society at large back to authentic first principles from which to engage with modernity. There is little question that Leo's legacy was, and is, a substantial one. Well into the latter half of the twentieth century, and still today, Catholic intellectuals, politicians, and laypeople struggle--and often fumble--with basic questions about the authentic first principles for being and acting in the modern era. We still debate over the ways Christians ought to engage with modern life, about the true meaning of human liberty, about the proper role for Christians in society and politics, and about the nature and purpose of government. I approach this rereading of AP as a historian interested in tracing the roots of modern Catholic social and political thought. More generally, I am interested in understanding the variegated ways in which Catholicism--understood in the fullest sense of its rich, complex ecclesial, social, and cultural dimensions--encountered modernity and especially modern thought in the late nineteenth and early twentieth centuries, and what effect this encounter had on Catholic thought and action vis-a-vis the public sphere. It is the historian's task to understand historical actors and texts on their own terms, to establish intent as well as effect, and to keep historical study from becoming too deeply entangled in contemporary debates. So it is with AP and its place in the history of modern Catholic thought. If it is the case, as John McGreevy notes, that the Thomistic revival of the late nineteenth and early twentieth centuries dominated Catholic thought in the twentieth century, then a reassessment of Leo's decisive intervention to bolster that revival--through AP and other decisions--seems the logical place to begin a study of Catholic political thought on modernity, liberalism, authoritarianism, and democracy. (1) What does the encyclical tell us about Leo's pontificate? What does it tell us about the signs of the times in which it was issued? What might a rereading of the encyclical at the start of the twenty-first century tell us about the state of Catholic culture and institutions in the first part of the twentieth century? Indeed, what is the enduring relevance of AP for our own time? Reading the Signs of the Times: Leo XIII and the Modern World Like his besieged predecessor Pius IX, Leo XIII worked ceaselessly to find answers to the singular challenge Catholicism faced at the close of the nineteenth century: to find a modus vivendi to allow the papacy and the Church to be in the modern world, but not of it. In doing so, he is widely acknowledged to have helped set the course for the Church's engagement with modernity for the century to follow, and arguably beyond. …