The quantitative notion of psychic energy is the most fundamental of Freud's ideas, on which the entire edifice of his thought is built. For him, the activity of the mind consists of nothing but the formation, transformation, storage, and discharge of quantities of psychic energy. Virtually all of his main ideas derive from or depend on this concept, including primary and secondary process, displacement, condensation, reversal, repression, bound versus free processes, defense, instincts, identification—the list could go on much further. All of these consist of processes, states, or structures of quantities of psychic energy. The basic unit of psychic energy is the Besetzung, defined as a quantity of psychic energy invested in an object. (This