Determining the boundaries of responsibility is an essential part of human life. While we tend to associate responsibility with the ability to act with forethought, we also sometimes praise and blame people for things they apparently do without thinking at all, such as a mother's instantaneous action to save her child from danger. This book addresses this apparent contradiction with the help of Thomas Aquinas. The author examines five atypical cases of responsibility in Aquinas's corpus: sudden acts done from habit, negligent omissions, effects of one's actions, first movements of the passions, and immoral dreams. Through an examination of these five "borderline cases," we gain a clearer understanding of both the nature of responsibility and Aquinas's treatment of this theme. This book will be of interest to scholars of medieval philosophy and theology, as well as those interested in ethics and action theory.