In almost every instance, the means which humans have thus far devised to provide their food, clothing, shelter, transportation, and luxuries are dependent on a one-way flow of energy, materials, and living substance from nature toward human society. Today, most of the materials which humans do return to nature are true waste, in the sense that they are either so degraded or concentrated that they are toxic or that they will not biodegrade in a reasonable length of time and simply pile up, hoarding their stored energy rather than releasing it back into the environment for reuse