In a sustainable society, nature is not subject to systematically increasing:

• concentrations of substances extracted from the Earth's crust
• concentrations of substances produced by society
• degradation by physical means

and, in that society...

• people are not subject to conditions that systematically undermine their capacity to meet their needs.

Why these four system conditions?

Sustainability is fundamentally about maintaining life on earth and the ecosystems required to support it. Thus, addressing human needs is a basic element of creating a sustainable society. Therefore, meeting human needs worldwide is one of the four TNS System Conditions.

The other three system conditions focus on interactions between humans and the planet and are based on an understanding that contemporary life is fundamentally supported by natural processes,
such as the capturing of energy from the sun by photosynthetic organisms and the purification of air and water.

These processes are essential to maintaining human life. However, as a society we are systematically altering the ecosystem structures and functions that provide life-supporting services.

Based on this understanding, The Natural Step System Conditions are supported by the knowledge that ecosystem functions and processes are altered when:

• Society mines and disperses materials at a faster rate than they are redeposited back into the Earth's crust (examples of these materials are oil, coal, and metals such as lead);
• Society produces substances faster than they can be broken down by natural processes, if they can be broken down at all (examples of such substances include dioxins, DDT, and PCBs);
• Society extracts resources at a faster rate than they are replenished (for example, overharvesting trees or fish), or by other forms of ecosystem manipulation (for example, paving over fertile land or causing soil erosion).

By considering these three ways in which human life-supporting structures and functions are being altered, TNS has defined three basic principles for maintaining essential ecological processes.

Additionally, TNS recognises that social and economic dynamics fundamentally drive the actions that lead to ecosystem changes.

Therefore, the fourth System Condition focuses on socio-economic
dynamics in terms of the importance of meeting human needs worldwide as an integral and essential part of sustainability.