If a patient decides she doesn't want to live any longer, should she be allowed to die? Should she be allowed to kill herself? If a patient is in no position to decide - perhaps she's in a coma - then should somebody else be able to decide to kill her? Who? Is there a moral difference between killing and allowing someone to die? And is the role of the doctor always to prolong life? Peter Singer, of Princeton University, is one of the world's leading bio-ethicists, and has been reflecting on life and death issues for four decades.