“The hardest aspect of this idea is the word Kraft. Directly translated, it means force in English. In the context Helmholtz uses it, it’s perhaps better translated as energy, but energy is not straightforward to define. Even today, most of us are aware that energy resides in gasoline and food and that it flows into our homes as electricity or gas, but we lack an intuitive grasp for why all these disparate phenomena fall under the umbrella term energy. This problem also confused many nineteenth-century scientists. Helmholtz gave energy clarity by relating it to the impossibility of perpetual motion””
Einstein's Fridge: How the Difference Between Hot and Cold Explains the Universe