The interaction between mankind and the natural world.
Is our interaction currently sustainable?
Do we take out only as much as we put in?
We take out products of nature (oil, coal, wood, water etc.) "natural resources" and use them for fuel or to make things.
How much of these do we have left if they are irreplaceable and are we replacing at the right rate for the things that are replaceable?
Definition: Sustainability: "a means of configuring civilization and human activity so that society, its members and its economies are able to meet their needs and express their greatest potential in the present, while preserving biodiversity and natural ecosystems, planning and acting for the ability to maintain these ideals for future generations"
Definition of Civilization: "An advanced state of intellectual, cultural, and material development in human society, marked by progress in the arts and sciences, the extensive use of record-keeping, including writing, and the appearance of complex political and social institutions."