As, by definition, non-renewable sources can only be used for a limited time. Thus, some future generation will find the source exhausted. The legitimate question is thus whether we can use these sources sustainable. A World Bank economist suggested rules to define the maximum rate of sustainable use of sources. (adapted from: Beyond the Limits)
For nonrenewable sources, the sustainable rate of use can be no greater than the rate at which a renewable resource, used sustainably, can be substituted for it. For example, an oil deposit would be used sustainably if part of the profits from it were systematically invested in solar collectors or in tree planting, so that when the oil is gone, an equivalent stream of renewable energy is still available.
For renewable sources the sustainable use can be no greater than the rate of regeneration.