Derek Parfit
Derek Parfit (1942–2017) was an English philosopher whose work reshaped debates about personal identity, rationality, ethics, and reasons.
Personal identity: The question of what makes a person the same individual across time. Parfit challenged the idea that identity itself is what matters in survival.
Teletransportation paradox: A thought experiment in which a machine copies a person elsewhere and destroys the original, raising questions about survival and identity. Parfit uses this case to test whether psychological continuity can matter without identity.
Reasons and Persons: Derek Parfit’s 1984 book on rationality, personal identity, and obligations to future people. This book brought together his influential arguments on identity, self-interest, and population ethics.
Egoism: The view that an agent ought to pursue their own interests or that self-interest fundamentally motivates action. Parfit argues that rationality does not require giving special priority to one’s own future welfare.
Psychological continuity: Continuity linking mental states across time through memory, intention, character, and related psychological connections. Parfit uses overlapping psychological connections to explain survival without requiring strict identity.
Branching identity: A problem arising when one person’s psychological continuity appears to continue into two future people. The branching case exposes why identity cannot be the relation that explains survival in every case.
On What Matters: Derek Parfit’s two-volume work on reasons, morality, and the relation among major ethical theories. It develops his attempt to show how consequentialism, Kantian ethics, and contractualism can converge.
Animalism: The view that each human person is fundamentally identical to a living human animal. This account grounds persistence in biological continuity, unlike Parfit’s psychological approach.
Reductionism: The view that facts about a subject are wholly determined by more basic facts, rather than by an additional underlying fact. His account treats personal identity as reducible to physical and psychological facts.
Prisoner's dilemma: A strategic game in which individually rational choices can leave participants worse off than mutual cooperation. Parfit uses it to examine whether rationality requires pursuing only one’s own interests.