"For the last thirty years or so, there has been a search underway for a theory that can accommodate our intuitions in regard to moral duties to future generations. The object of this search has proved surprisingly elusive...The main problem has been to find an adequate population theory, that is, a theory about the moral value of states of affairs where the number of people, the quality of their lives, and their identities may vary. Since, arguably, any reasonable moral theory has to take these aspects of possible states of affairs into account when determining the normative status of actions, the study of population theory is of general import for moral theory."