A finite mass of material which cannot be restored after use, such as natural gas. (S. Barr and P. Chaplin (2004) argue that cultural heritage is a non-renewable resource.) Each non-renewable resource has a theoretical depletion time—a ratio of its stock and rate of use. Non-renewable resources may be sustained by recycling, which Jenkins (2000) Econ. Ecol. 34, 3 claims tends to be built into traditional cultures.