The Earth is a closed loop with regards to matter, but is open to a constant flow of energy. The matter on the Earth's surface may be rearranged, but the total of matter on the Earth is constant (exceptions being the conversion of matter to energy in nuclear reactions, falling to Earth and the loss of some matter to space in the form of rockets and space ships). There is a constant though-flow of energy from the sun.

A tree has a defined structure and plenty of biological and economic value. Compare the tree to a pile of sawdust, which has a similar concentration, but less structure - and compare this to the smoke and heat produced by burning the sawdust. Once the process of burning has happened, there is very little value in the dispersed end products.

Net production implies taking the entire process into account, such as in life cycle analysis, which takes into account the source of materials and the sink (where both the products and by-products will eventually end up). Industrial processes create material structure in their products, but the process of production often causes a great amount of loss of structure and concentration of matter. For example a spoon has more structure and concentration than iron ore, but in the entire process of producing that spoon, a large amount of matter has been dispersed, in the mining process, the burning of fuel for smelting and manufacture, etc.

Fossil fuels, metals and other minerals must not be extracted at a faster rate than their re-deposit and reintegration into the earth's crust. We must learn how to utilize the sun's energy to meet most of our energy needs, and learn how to change a wasteful use of metals and minerals from the earth's crust into a cyclic use, where materials can be re-used indefinitely. In terms of design, this implies making sure that different materials can easily be separated (design for disassembly) and reused, maintaining their purity and material quality.

Human-made substances must not be produced faster than they can be broken down and re-integrated in the cycles of the biosphere or deposited in the Earth's crust. This refers both to the accumulation of persistent synthetic compounds and of materials which can be assimilated by the biosphere, but which are currently being produced at a rate exceeding this capacity of re-absorption.

The productive surfaces of the biosphere must not be diminished in quality or quantity, and we must not harvest more from the biosphere than can be recreated and renewed. Sun driven processes are essentially the only net producers of concentration and structure in matter, and thus are the foundation of all economic value. The cycling of materials and the creation of structure in the biosphere can be seen as a quality cycle. Any societal activity that systematically reduces the physical basis for this cycle is inherently unsustainable. The role of diversity within this cycle is best expressed as a function of interrelationships, on which cycling of matter and the building of quality is dependent.

This is the core of the design challenge of sustainability; designing ways to increase quality of life and economic vitality in an ecologically sustainable way.