The Ship of Theseus paradox asks: if you replace every plank of a ship, one at a time, is it still the same ship? This question about objects has a deeply personal version: what makes you the same person across time? Your body replaces most of its cells over seven years. Your beliefs, values, and personality change through experience. Yet you feel like a continuous self. What exactly is the "you" that persists?

Two main theories compete. Physical continuity theories hold that personal identity consists in bodily continuity — you are the same person as long as your body (especially your brain) is continuously connected. Psychological continuity theories hold that what matters is the continuous connection of memories, personality, intentions, and beliefs. Locke's original formulation held that memory was the key: if you remember doing something, it was you who did it. If you have no memory of an event, even if it was your body involved, it was not "you" in the relevant sense.

These questions are not merely academic. They matter practically: what obligations do you have to your future self? Should you be punished for crimes you have no memory of committing? If you radically change your values and personality over decades, is it fair that your younger self's contracts bind you? Personal identity intersects with moral responsibility, intergenerational ethics, and the meaning of promises made across time.