In the widely cited presentation of Callen, the notion of an adiabatic wall is introduced as a limit of a wall that is poorly conductive of heat. Although Callen does not here explicitly mention temperature, he considers the case of an experiment with melting ice, done on a summer's day, when, the reader may speculate, the temperature of the surrounds would be higher. Nevertheless, when it comes to a hard core definition, Callen does not use this introductory account. He eventually defines an adiabatic enclosure as does Carathéodory, that it passes energy only as work, and does not pass matter. Accordingly, he defines heat, therefore, as energy that is transferred across the boundary of a closed system other than by work.