The abstraction ignores the linear throughput of matter and energy that must power the continuous motion of money, goods and services, and factors of production. Matter and energy enter the economy in the form of low entropy natural capital, such as solar energy, oil wells, fisheries, and mines. These materials and energy are used by households and firms a like to create products and wealth. After the material are used up the energy and matter leaves the economy in the form of high entropy waste that is no longer valuable to the economy. The natural materials that power the motion of the circular flow of the economy come from the environment, and the waste must be absorbed by the larger ecosystem in which the economy exists.