As anyone who has paid an electricity bill knows, energy is a very real physical quantity but unlike other expensive commodities such as books, records, and bottles of wine, it has a universal and all-pervading character. But what actually is energy? We shall attempt to answer this question by simply listing its defining properties. We must, however, be careful to list enough properties so as to capture the notion of energy as is actually used in fundamental physics, and also to distinguish it from other conserved quantities such as electric charge. We must, of course, not list too many properties, or properties that are too rigid, as this may lead to a trivial or nonexistent quantity. We do not want, for example, to end up with a quantity that turns out to be identically zero.

Energy is a measurable quantity possessed by all physical objects, which is always found to be strictly positive. Energy comes, of course, in many forms, ranging from pure radiative energy, which tends to be its most useful but most dangerous form, to its most benign form, which is safely locked away, like a genie in a lamp, in all inert matter. Even the paperweight in front of me contains energy (quite a considerable amount) but, fortunately for me, this is in its most benign form. I could, in principle, if I so wished, convert this energy into its radiative form by bringing my paper weight into contact with an otherwise identical one made of antimatter, and this would result in a spectacular firework display of pure radiative energy. (Such experiments are, however, not recommended, as they can cause quite a mess.)