Abstract: Energy is the most fundamental concept in physics, but the question“What is energy”remains unanswered. Here I point out that Energy is the unique universal attribute characterizing isolated systems. It is an abstract, trivial mathematical quantity devoid of any physical essence, and it is positive definite. Interacting subsystems together constitute a grand isolated system, and the quantification of the interactions therein follows the metaphysical principle of causa aequat effectu. The law of energy conservation has thus twofold meanings, one is that the energy for an isolated system is fixed, it is simply the confirmation of isolatedness of a system; the other refers to the statement that the total energy change for all the interacting partners within an isolated system should be zero, and the energies here concerned are concrete terms of actualized energy, which are the elements of the abstract concept of energy characterizing isolated systems. The primary method of research in physics is“to isolate”, to isolate the system of concern, and consequently to isolate the problems to be studied. The fundamental theories of physics combine exact conservation and extremism, i. e., under the presumption that energy is fixed, a physical quantity at the other stratum of dimension, that’s action for mechanics as well as entropy for thermodynamics, strives for extremum. The various terms of energy and the concept of entropy are introduced via the principle of equivalence. The current work may help clarify many incorrect perceptions over the concept of energy and the law of energy conservation.