Derek Parfit

Parfit’s population-ethics puzzles examine choices involving different future people and different population sizes. The Repugnant Conclusion pressures total-welfare rankings, while the nonidentity problem pressures explanations of wrongness based entirely on making an identifiable person worse off. His method follows apparently plausible commitments to their difficult implications.

Parfit’s puzzles reveal tensions among principles that can each seem plausible in isolation. Total welfare can generate the Repugnant Conclusion, while choices affecting identity challenge familiar explanations of harm. The method asks what follows if we apply a commitment consistently across several possible outcomes. It thereby shifts attention from a confident first intuition to the relationships among judgments. A theory must explain why its principles work together and what it is prepared to revise.

Imagine a policy that produces a worse environment but also changes who is born. Then compare it with a future containing many more people whose lives are only mildly positive. The first challenges comparative harm; the second challenges aggregation.

These puzzles show why intuitive rejection is not yet a satisfactory theory. An alternative must explain related cases and preserve defensible consistency. They do not imply that actual people with difficult lives are worth less; the subject is how to rank possible outcomes and justify choices that shape future lives.

Build a chain of fictional populations rather than comparing only two extremes. At each step, record whether the change improves lives, adds lives, or changes who exists. Then examine whether your local rankings yield a surprising final ranking. This exercise clarifies the source of a contradiction and helps evaluate alternatives. It does not imply that actual lives should receive numerical grades: idealized quantities expose the logic of a principle, while practical decisions require far richer information.