A definition of energy

On reading Lisa Crystal’s review (PHYSICS TODAY, April 2011, page 61) of Jennifer Coopersmith’s book Energy, the Subtle Concept: The Discovery of Feynman’s Blocks from Leibniz to Einstein, I remembered having shared Crystal’s desire to learn what energy is—that is, energy without such qualifications as mechanical, chemical, nuclear, solar, and so forth. I finally found the answer, from Max Planck: “The energy of a system is, therefore, sometimes briefly denoted as the faculty to produce external effects.”¹ 

That definition suggests to me a philosophical answer, which Crystal says is lacking in Coopersmith’s book. I would say that the energy of a system is a measure of its presence in the universe.