According to demographers, a generation is an age-cohort of people born, living, and dying within a few years of each other. Human generations are roughly defined categories, and the demarcations are not as distinct as they are in many other species. As the Scottish philosopher David Hume noted in the eighteenth century, generations of human beings are not like generations of butterflies, who come into existence, lay their eggs, and die at about the same time, with the next generation hatching thereafter. But distinctions can still be made, and future generations are all age-cohorts of human beings who have not yet been born.

The concept of future generations is central to environmental ethics and environmental policy, because the health and well-being—indeed the very existence—of human beings depends on how people living today care for the natural environment.

Proper stewardship of the environment affects not...