Personal identity over time asks what makes you at age 30 the same person as you at age 5, given that nearly everything has changed. Locke held that personal identity consists in continuity of consciousness; Hume denied there is any enduring self at all. The puzzle becomes acute in fission cases: if your brain is split and each half implanted in a new body, producing two people each with your memories, which is you? Parfit argued these cases show that personal identity is not what matters in survival — psychological continuity without identity is enough for what we care about. This connects to questions about moral responsibility, compensation across time, and the ethics of death.
Read Locke's Essay Book 2 Ch. 27, then Parfit's Reasons and Persons Part III. Work through the fission thought experiment carefully: can there be two equally valid heirs to your personal identity?
You know from your introduction to metaphysics that identity is a deep puzzle: what makes a thing the same thing over time, given that things change? For persons, this question has a peculiar urgency. Physical matter is replaced; beliefs, memories, and personality transform; even the continuity of consciousness is interrupted every night. You studying this text are numerically the same person as the child who first learned to read — but what does "numerically the same" mean here, and what grounds that identity?
Locke's influential answer centers on memory and consciousness. Personal identity consists in the continuity of psychological states — especially memory. You are the same person as your childhood self because you remember (or can in principle trace a chain of memories back to) your childhood experiences. This is the psychological continuity theory in its original form. It has intuitive appeal: we feel that what matters about our survival is that our mental life continues, not that our particular body atoms persist. If your brain were transplanted into a new body, most people's intuition is that *you* would go with the brain.
Hume challenged even the existence of a persisting self. When he introspected, he found only a bundle of perceptions — feelings, images, sensations — with no underlying "self" that possesses them. The bundle theory treats the self as a series of mental states connected by relations of resemblance and causation, without any additional substrate. This is a startling view: there may be no fact of the matter about personal identity beyond the existence of appropriate psychological connections.
The fission thought experiment puts maximum pressure on the psychological theory. Suppose your brain hemispheres are surgically separated, each is transplanted into a different empty body, and each resulting person wakes up with your memories, personality, and values. Which one is you? Both have equal psychological continuity with you. Both can't be you (they're not identical to each other). So neither can be you — yet each has everything we thought guaranteed personal identity. Parfit's radical conclusion is that this shows personal identity is not what matters. What matters for survival is psychological continuity, which comes in degrees and can branch. You shouldn't fear a fission procedure on behalf of "yourself" — two people with your psychology will survive. Whether they are *you* is a further question that may be indeterminate, but indeterminacy about identity doesn't mean anything of value is lost. This reorientation has deep implications for ethics: if identity isn't what matters, the boundaries between persons may be less morally significant than we assume.
Topics in reflective domains aren't scored by quiz answers. Read, reflect, and mark when you've thought it through.