Is there an universal and complete definition for energy? Or has energy already being defined by the Principle of Conservation of Energy? Energy cannot be created nor destroyed We know that energy can be transformed from one form to another, but a force has to be applied to change the form of energy. e.g. When a man lifts a brick, the chemical energy possessed by the man is transformed into potential energy, and the transformation is caused by the force applied on the brick exerted by the man. I am confused of which comes, because neither force or energy can exist beforehand of one another. (Energy transfoms to other forms because of force, but the existence force and its magnitude is also dependent on the amount of energy).