Be Sober and Reasonable deals with the theological and medical critique of "enthusiasm" in the seventeenth and early eighteenth centuries, and with the relationship between enthusiasm and the new natural philosophy in that period. "Enthusiasm" at that time was a label ascribed to various individuals and groups who claimed to have direct divine inspiration - prophets, millenarists, alchemists, but also experimental philosophers, and even philosophers like Descartes.

The book attempts to combine the perspectives of intellectual history, Church history, history of medicine, and history of science, in analyzing the various reactions to enthusiasm. The central thesis of the book is that the reaction to enthusiasm, especially in the Protestant world, may provide one important key to the origins of the Enlightenment, and to the processes of secularization of European consciousness.