It should be clearly grasped that the energy lying behind our primitive instincts, whether it be repressed, displaced or sublimated, is a very real force, comparable with the physical energy which we are accustomed to deal with in everyday life. _And this energy must find some outlet for its discharge._ Thus,[2]“We know as regards physical energy that there are not several kinds of energy, but merely several manifestations of it, and that it may be changed from one form of manifestation to another, but that still the sum total of the original energy remains without addition or loss.” Thus there is a given amount of energy stored in a ton of coal. This energy can manifest itself as _heat_ in the furnace and boiler. By means of an engine we can change the manifestation into that of _motion_, then with a dynamo to _electricity_; the electricity we can again change into _light_, or back again into _heat_ or _motion_. There is _one_ energy, but by suitable means we can turn it to different uses, and give different manifestations of it. Owing, however, to the imperfection of the boiler, machinery, etc., we never transform the _whole_ of our energy into another form. In transforming heat into electricity, there is always some heat wasted; it is not destroyed, but it remains as heat for a time, and is absorbed by surrounding objects. A complete transference of energy does not take place, and the less efficient the machinery the less is the transference. Now evidence tends to show a considerable similarity between psychic and physical energy. In all probability there is only one ultimate psychic energy which, like physical energy, can be directed into different channels. Thus, the energy of erotic desire can be directed to a large extent into the energy of desire for music, religion, science, or sport; or the energy of the desire for sport may be changed into the energy of the desire for mental exercise, such as chess, mathematics, or science. For example, an individual feels “restless,” he then desires to play tennis; the afternoon is wet: he plays chess instead. His psychic energy has been diverted from one channel into another with its accompanying excitement and satisfaction of desire: with its final feeling of fatigue and repletion. Psychic energy, like physical energy, can never be entirely diverted from one channel to another. There is always some, often a large quantity, which is not altered in character. The amount of this depends largely on the person concerned, just as the amount of physical energy, changed from one form to another depends on the efficiency of the engine or machinery.