When I was at primary school, so many years ago, we as students were taught that energy is simply a value for the ability to do work. wiki states: However the same wiki article also refers to energy delivered by way of "radiation" which is where it appears that energy has acquired a "substance" rather than being a mere value or potential to do work. The question for me is then how is it that science requires the need to grant energy substance? What is the reality of energy having a potential for mass rather than just mass having the potential for energy? We know that Mass has the potential to deliver energy but I wonder how we can state the opposite that energy has the potential to deliver mass? We believe that the Energy / mass equivalence, as suggested by A. Einstein, implies equality between mass and energy, but does it also imply symmetry or reciprocation [reversal] Explainer: We can observer objects of mass performing work but as far as I know we have not observed Energy creating mass. Simply put: Does: E = mc^2 equal mc^2 = E [or it's later derivatives and generations] So I question the context that the equivalance of energy and mass is used in. Care to discuss?