What was the Enlightenment? This question has been endlessly debated. In , historian Matthew Kadane advances the bold claim that the Enlightenment is best defined through what it set out to accomplish, which was nothing short of rethinking the meaning of human nature. Kadane argues that this project centered around the doctrine of original sin and, ultimately, its rejection, signaling the radical notion that an inherently flawed nature can be overcome by human means. Kadane explores this and other wide-ranging themes through the story of a previously unknown figure, Pentecost Barker, an eighteenth-century purser and wine merchant. By examining Barker’s personal diary and extensive correspondence with a Unitarian minister, Kadane tracks the transformation of Barker’s consciousness from a Puritan to an Enlightenment outlook, revealing through one man’s journey the large-scale shifts in self-understanding whose philosophical reverberations have shaped debates on human nature for centuries.