From a permaculture point of view, sustainability is protecting biodiversity while generating a yield from a biological system for as long as possible.
There may be human interaction to these biological systems but kept to a minimum. These interactions are triggers to keep the system going.
Think about an ancient forest. This forest is surviving for centuries with no human interaction. Its biodiversity is so wide that there are certain patterns and loops created within it. Birds eat bugs and leave fertilizer on the floor. Micro-organisms break down this fertilizer and feed trees. Trees grow tall and they shed leaves in winter. Leaves became nesting areas for bugs. Birds eat bugs…
If a system is designed by mankind, it will diminish as time passes and will disappear. This is the application of the second law of thermodynamics. If this system is a mechanical system, it is even worse. It is near impossible to create a mechanical system where there will be patterns and loops to regenerate itself again to produce the same outcome. I am hopeful though.
A forest is a biological system established by nature and fed by thousands of mini-systems in it and evolved by the conditions it is subject to. Things in it come to life, die and reborn again. Laws of thermodynamics are applicable to the forest too but forest regenerated itself using its own mini-systems within it. Its scale is large enough that certain patterns and loops are developed that is ticking like a Swiss watch, time and again, century after century. It is a large, complicated, and seems like chaos yet survives and produces the same outcomes. Nature used the laws of thermodynamics to its own benefit.
Our comprehension is limited to our experiences. Our brain is also limited in remembering and recalling the data. Establishing complex systems is hard but not impossible. It is like writing the perfect algorithm; once you set up it will work and produce the outcome desired.
If we design a system for a long term yield which will surpass our lifetime, it needs to be triggered at certain intervals to boost its productivity. This can only be done by educating the new generation.
A truly sustainable system designed by mankind is possible only watching and replicating what nature is doing in a large enough scale that promotes the pattern and loops to develop.