Sustainability- just what is sustainable?

The topic 'sustainable living' is topical, but what does it mean. The question arises, just what is sustainable?

Simplistically, all mining of natural resources is unsustainable.

Each natural resource that is mined is finite and thus must one day be exhausted if we simply keep digging from the ground. But will we ever run out?

Through landfills etc., it is clear the minerals we mine do usually eventually go back into the ground in some way. However, what was extracted from the natural resource originally is effectively 'used up' once and forever unless we have a realistic and sufficiently cost and energy efficient way of extracting the material back from landfill.

Of course, if we did have cost and energy efficient ways of recycling minerals that we have previously used, surely there would no longer be a need for mining? Well in reality we would still need some mining if the per/person requirement of a given resource increased, and also as the number of people increase. Now of course we cannot have infinite people. The earth has a finite amount of each element so you cannot produce infinite people on a finite Earth. One day population must stabilize.
So this leaves increases in the amount of a given element per person. Again this should be finite, so one day we should be able to treat mining natural resources as a special exception and simply get virtually all resources from recycling.

Until that day, we will not have reached 'sustainable living'. A zero 'carbon footprint' alone simply is not enough. It comes down to everything we dig out of the ground, not just oil and gas.