In his encyclical letter Laudato Si’, Pope Francis appeals “for a new dialogue about how we are shaping the future of our planet (14).” 1 He sees the “need for a conversation which includes everyone, since the environmental challenge we are undergoing, and its human roots, concern and affect us all (ibid.).” In developing his argument, which is intended to persuade a broad audience, he draws on multiple authoritative sources but especially on the social teachings of the Catholic Church, which are predicated on strong metaphysical assumptions that are not universally shared. Constructivism in ethics is the position that universal principles for guiding action in a world of multiple and diverse audiences and conflicting cultural viewpoints cannot be established by metaphysical arguments or religious doctrines, or discovered (empirically) in the world. Rather, action-guiding principles for a cosmopolitan audience must be constructed on the basis of plausible assumptions about agents and conditions of action that do not appeal to unvindicated ideals or particularities. Yet, constructivist ethical analyses of environmental issues can arrive at prescriptions for action that are quite compatible with those derived from religious beliefs and theological arguments. This chapter reviews a number of essential features of Francis’s argument that will enable its conclusions to be compared with those of one form of cosmopolitan constructivism. Next the main lines of Onora O’Neill’s constructive approach to practical reasoning are sketched. Then the chapter explores how assessments and prescriptions about human relationships with the natural environment reached through the latter approach compare with those of Francis’s encyclical letter.