Part of the confusion round "energy as just another input" is that the conception of energy is mistakenly materialised. It is not a substance in itself. Talk of "pure energy" confuses people. Energy is an abstract concept describing the state of the universe and its components, with the relevant state characteristic being the potential for spontaneous change. Low energy states have less potential for further spontaneous change than high energy states, which have considerable potential for such change; and so called "pure energy" is a theoretical concept describing an unlimited potential, never actually observed by us in the current universe, and in fact unobservable since knowledge is only possible where there are energy gradients; in a state of radiation equilibrium nothing can be known.

Thus, energy is not an input at all, since it is not a substance, but a state of all inputs, and the state of such inputs, and their potential for change, is the heart of economic system theory, or should be.