Any process will over time be closer or further away from sustainability, and a process may or may not produce adjacent states that diminish or increase the sustainability potential of the system we are talking about. Understanding this inherent dynamic nature of sustainability, we realize the need to constantly assess our actions with respect to their contribution to sustainability.

For the operationalization of the ethical and political concept of sustainable development, we need a quantitative definition and we need a metric to measure progress towards sustainability. Likewise, it requires the means to predict consequences of action, so that we can make informed decisions. We can introduce a quantitative definition based on the life expectancy of the system and trajectories of certain factors in the system.