Personal identity
The philosophical question of what makes a person at one time the same person as at another time — whether through psychological continuity (memories, personality, beliefs), bodily continuity, or some deeper essence. Derek Parfit's Reasons and Persons argues that what we ordinarily call identity is not a deep further fact but a matter of degrees of psychological connectedness, and that this matters less than we think — a position that has consequences for ethics, mortality, and how we relate to our future selves.