I am certain, as judged by the thoroughly informed previous comments, that this subject is way over my head. Notwithstanding, please allow me these unqualified opinions. The definition that energy is the ability to do work, has a twofold meaning; kinetic and static. Firstly, kinetic energy is the measure of energy potential of moving objects, relative to its mass and speed, and other possible influencing factors. Secondly, static energy is the measure of energy potential available if the static "object" in question was relished from its stasis, and, under the influence of gravity, and/or other forces, shift into a kinetic state. As well, the energy of a kinetic or static "object" can be represent by the maximum potential energy, as delineated by the Einstein equation that defines the conversion of mass into energy. So what is energy, with respect to the above statements? Apparently an actual force(kinetic energy), a potential force(static energy), or, as defined in the Einstein conversion, a nuclear or atomic force(optimal and potential). The problem in defining energy, as I see it, is due to the commonly understood "definition" of energy, and how and when that understanding conflicts with formal definitions and problem solving methods; which often transcend our intuitive grasp. Clearly, the Einstein concept for converting mass into energy is a paradigm shift in our understanding of energy. This concept defines energy in terms of mass. If we accept the idea of potential energy, we must accept the Einstein conversion principle.The issue herein has much to do with semantics and formal ways of thinking and describing energy , and how that bodes with the ordinary understanding of energy.