This introductory chapter discusses the peculiar combination of the unfelt emergence and motions of strongly felt feelings, which appears all over eighteenth-century writing. The usages express something deep but inexplicit about how affect was understood in the eighteenth century, how feeling, passions, the emotions, and even perception itself were seen subtly to come into existence and move people. Instead of drawing attention to itself as an especially significant, well-defined concept or idea, a word like insensibly occurs almost in passing in the period's writing. But this unstudied casualness, far from rendering its meaning insignificant, holds a key to its power. A scarcely noticed but crucial and consistent set of gestures to an affect that cannot be felt: that is the terrain this book explores. Instead of indicating a mere lack of feeling—an affective blockage, impassivity, stupefaction—insensibly unfolding processes initiate and build strong feeling or make it possible.