The lone person can object to a principle permitting you to save the five on the same grounds — that principle leaves the lone person to die

Suppose you decide to save the lone swimmer on the second rock. Intuitively, this seems wrong. Surely you should have saved five people instead of one. The challenge for contractualism is to explain why what you did is wrong. Utilitarians have a straightforward answer, based on aggregation. You should save the five people instead of the one simply because five deaths is a worse result than one death. This case is tricky for contractualism because it rejects aggregation. The five people will each