The desirability and the difficulty of retrieving modern scientific concepts in classical scientific authors is discussed with particular reference to a recently published re-evaluation of Sadi Carnot's memoir. Evidence is presented to support the interpretation of Carnot provided by his nineteenth-century successors and current in modern texts: Carnot's use of the material theory of heat led him to misconstrue the foundations of thermodynamics, but the misconstruction, discoverable only in retrospect, is irrelevant to his stature as a profound and original investigator.