Thermodynamics' first main principle is a law of nature that cannot be proved, but is accepted without reservation. It says that energy can neither be created nor destroyed and from that it follows that the total energy in a closed system is constant. Thermodynamics' second main principle says that heat can never of "its own effort" be transferred from one source to a hotter source. This means that energy can only be available for work if it can be converted from a higher to a lower temperature level. Therefore in, for example, a heat engine the conversion of a quantity of heat to mechanical work can only take place if a part of this quantity of heat is simultaneously led off without being converted to work.