4. Conclusion

The first principle, which stipulates the conservation of energy and the possibility of converting one type of energy into another, is also a principle of quantity. In addition, the second principle associates a quality with this quantity. This quality is measured by the intensive variable corresponding to the energy in question. The higher the value, the more interesting the energy. It is then the conservation or otherwise of the energy-related extensities that determines the number of energy reservoirs needed to operate a converter. If extensities are conservative, a converter can't operate with less than four reservoirs, two for the motor and two for the generator. The particularity of thermal energy, for which the extensive variable, entropy, can be created but not destroyed, lies in the need for two thermostats for a heat engine, possibly also two thermostats for a heat generator, but...