What makes you the same person over time? You have changed since childhood. Your body has grown. Your memories have faded. Your beliefs and desires have shifted. Yet you are still you. Personal identity is the continuity of a person's self over time. It is a question about what grounds that continuity. Is it the body, the brain, memory, or something else?

Locke argued that personal identity is grounded in continuity of consciousness, especially memory. If you can remember being a child, you are the same person. Reid objected that this account is circular, since memory presupposes personal identity. Hume argued that there is no enduring self, only a bundle of perceptions. Parfit argued that personal identity is not what matters. What matters is psychological connectedness and continuity, which can come in degrees. These debates matter for ethics and law. Advance directives, organ donation, and end-of-life decisions all depend on assumptions about personal identity. If you are not the same person you were ten years ago, can you be held responsible for what you did then?

Theories of personal identity

• Body. You are your body.
• Memory. You are the continuity of your memories.
• Psychological continuity. You are the continuity of your mental life.
• Bundle. There is no enduring self, only perceptions.

Personal identity is not just a philosophical puzzle. It is a practical question about responsibility, commitment, and care.