 Moral requirements determine what it is to respond properly to the value of persons as rational agents. The distinctive value of human life lies in the human capacity to assess reasons and justifications. Therefore, appreciating the value of a person involves recognising her capacity to appreciate and act on reasons. The way to value this capacity is to treat persons in accord with principles they could not reasonably reject. In doing so, the agent is guided by a principle that can rightly be characterised as one that the person herself authorised that agent to be guided by, in thinking about the appropriate way to relate to her. Contractualism illuminates the compelling Kantian insight that we ought to treat persons never as mere means but always as ends in themselves. It interprets this as treating them according to principles they could not reasonably reject.