How far do we expect philosophers’ ideas to reflect or reveal their personalities? The theories of philologists, fungus experts and chemists, said Nietzsche, can be quite distinct from their lives and interests, but a philosophical theory, however abstruse and metaphysical, can never be impersonal. Spun out of the philosopher’s deepest drives, it is ineluctably ‘a confession on the part of its author and a kind of involuntary and unconscious memoir’.

Derek Parfit, the subject of this fascinating biography, illustrates Nietzsche’s point. Parfit’s character and way of life were enmeshed in his theories. He almost seemed more the creation of his ideas than their creator. Unworldly, lacking self-awareness and indifferent to discomfort, he envisaged remote, unlived futures as a way of conceptualising his theories. Thought experiments in which brains are split and transplanted were already familiar, but Parfit’s were the stuff of science fiction. In one, Parfit’s body and brain are digitally scanned, then destroyed; a blueprint is beamed to Mars, where an exact physical and psychological replica of Parfit is made, which insists that he is Parfit. In another, which challenges our ‘bias towards the future’, a man is in hospital gloomily awaiting a long, painful operation (of which he will remember nothing). When told that in fact he has already had it, his gloom is unabated. Surely the ordeal is just as long, painful and life-altering whether it is over or yet to come.

Parfit is celebrated for denying that we have a personal identity, at least in the way that it is typically conceived – as an unchanging entity that persists from life to death, quite distinct from our discontinuous physical and psychological qualities. If, he argued, enough of my character traits, beliefs,