In another thread when this Stephen Hawking quote was cited, “What is it that breathes fire into the equations and makes a universe for them to describe," it reminded me of a question that has been on my mind a lot lately. It seems to highlight the apparent absurdity of representing matter as energy, but then in a different context describing energy as a mere abstraction which has no reality beyond how it helps explain the behavior of matter. As science writer Paul Davies explained, “Energy is . . . an imaginary, abstract concept which nevertheless has become so much a part of our everyday vocabulary that we imbue it with concrete existence.” Energy, whatever it is, moves things, heats things, gets work done . . . So how can what is capable of all that, and which is absolutely essential to matter, be only a concept? After all, the formula isn’t C=mc^2 (C standing for concept). Is there really nothing actual to what supposedly constitutes matter other than it’s just an explanatory convenience? Should one deduce that since we are only able describe how things happen, but don’t know what it is that is driving it or what the physical is made up of, that the physical behaves but there's nothing substantial comprising it? My question is, are we missing something essential in our accounts of the physical universe? Is there something most basic which energy is (or is a manifestation of) which is so fine/unstructured/subtle we can’t observe it directly and so must content ourselves with describing what it does? Or if you reject that, can you resolve what appears to be the contradiction in representing energy as only a concept, yet simultaneously having it be the most essential (or only) ingredient of matter?