It is _the_ distinction of modern psychology that it has established the wish (craving, need, desire, libido) as the moving force in any psychic process. The position of the wish in psychology as the force within and behind the instinct may be compared to that of energy in physics, when it was elevated to a central position in the explanation of physical processes in the nineteenth century. The concept of the _charged_ wish has illuminated all the hidden recesses and rendered audible all the subdued murmurings of the mind. The truly novel in the content of the idea is the recognition of the fact that the wish is charged. Now it could never be charged in a vacuum. That means that a wish could never be born in the brain alone. For the brain has no power to charge itself with energy--it can only store and transmit. If a wish is potential energy that must be transformed into kinetic, it must have a source. That source is the vegetative system. Without the vegetative system, the great complex of viscera in the abdomen and chest, blood and its vessels, endocrines, muscles and nerves, the brain would remain but an intricate cold storage plant of memories, associations of past experiences. It would need no change and initiate no effort. But when the wish enters upon the scene, it is as if a dead storage battery has been refreshed with new current. Enriched with billions of electrons there is a stir and a movement, dynamic mind. But the dynamo is the more ancient possession of the animal, the vegetative apparatus. In short, what must always be remembered is that a wish is never cerebral, but always sub-cerebral, visceral, in its origins. The sub-cerebral makes the cerebral. Activities in the nervous system below the brain and especially the vegetative system, force upon it its function of the active verb. It has to be, to do, and to suffer, and then to manipulate the environment to satiate the insatiable viscera, insatiable because the local chemistry is continually raising the tension of one or the other of them. A physics of human behaviour becomes possible with the aid of these concepts of endocrine regulation of intravisceral pressure, and intervisceral equilibrium, an intramuscular pressure and an intermuscular equilibrium, with the brain as the shifting fulcrum of the system.