The adiabatic enclosure is important because, according to one widely cited author, Herbert Callen, "An essential prerequisite for the measurability of energy is the existence of walls that do not permit the transfer of energy in the form of heat." In thermodynamics, it is customary to assume a priori the physical existence of adiabatic enclosures, though it is not customary to label this assumption separately as an axiom or numbered law.