Let us build on these two ideas that the moral gravity of an action affecting the environment is determined by the irreversibility of its effects and by the spatial scale of those effects. We can do so by introducing a decision space defined by two continua, each of which ranks possible outcomes of a policy or action—one ranks the impacts according to how long natural processes will require to "heal" negative alterations to the environment, and the other ranks the spatial scale of the impact. These two scales are combined in Figure 2. Decisions with quickly reversible impacts and decisions affecting small scales probably do not raise questions of intergenerational moral importance. They fall in the northeast, the southeast, or the southwest quadrants of our decision space; they can be decided on normal, individualistic criteria of economic efficiency, balanced, we hope, by considerations of interpersonal equity. Ecological economists and environmental managers should, according to this analysis, categorize environmental problems according to the irreversibility and scale of the risks involved as a first step in any problem analysis, because this categorization determines the horizon of concern involved in the decision.