There are many books claiming to explain the Enlightenment, but most assume that it was a thing. J. C. D. Clark shows what it actually was, namely a historiographical concept. Currently 'the Enlightenment' is a term widely accepted across popular culture and in a variety of academic disciplines, notably history, philosophy, political theory, political science, literary studies, and theology; Clark calls for a fundamental reconsideration in each.

The Enlightenment: An Idea and Its History provides a critical historical analysis of the Enlightenment in England, Scotland, France, Germany, and the United States from c. 1650 to the present. It argues that the degree of commonality between social and intellectual movements in each--and, more broadly, between the five societies--has been overstated for polemical purposes. Clark shows that the concept of 'the Enlightenment' was not widely adopted in those societies until the mid-twentieth century; indeed, that it was unknown in the eighteenth. Without the concept, people at the time were unable to act in ways that would have created the Enlightenment as a coherent movement. Since the conventional account has held that the Enlightenment was a phenomenon, the idea could be used as a component of what has been called a 'civil religion': a summing up of the myths of origin, aims, and essential values of a society from which dissent is not permitted. An appreciation that it was instead a historiographical concept undermines, in turn, the idea that there was any great transition to what came to be called 'modernity'.