Closed system

A closed system is a physical system that does not allow the transfer of matter in or out of the system. In the context of the earth, the best was to think of it is as a spaceship. We are onboard a physical, self-sustaining object floating through space powered by the sun. The sun's rays hit the earth and photosynthesis creates carbon by immobile plants and letting off waste oxygen. The carbon is ingested alongside oxygen to make the building blocks of mobile animals. These animals eat the seeds of the plants and mobilise them via their faeces. Everything on the spaceship works in these circles to ensure balance as there is no external sink of waste. One person's treasure is another person's waste!