Reasons, identity, and future people › Identity and resemblance answer different questions

Numerical identity means being one and the same individual. Qualitative similarity means sharing features. Two people can look alike without being the same person, while one person can change greatly and remain numerically identical. Parfit uses this distinction to prevent the phrase “the same person” from doing several jobs at once.

The distinction becomes useful in thought experiments involving replicas. Imagine a device that records a person's physical and psychological organization and creates a matching person elsewhere. We can stipulate very close resemblance while still asking whether the resulting individual is numerically identical to the original. Establishing that the replica would remember a childhood does not settle that question merely by repeating the word same.

These are philosophical cases, not descriptions of available technology. In Parfit's discussion, they separate bodily persistence from psychological continuation. A thought experiment asks what we would say if these came apart. Its conclusion depends on the case being coherent and on the relevance of the judgments it elicits.

Parfit's reductionism says that personal identity consists in more particular facts, rather than in the persistence of a separately existing subject over and above them. Physical and psychological accounts can both take a reductionist form. Reductionism therefore should not be defined simply as the claim that memories alone determine who someone is.

The view also does not say that people are imaginary. An institution can exist through facts about people and their activities without being a separate additional substance. That comparison helps explain the form of reductionism; it does not prove that persons are institutions. Parfit's substantive argument concerns which account fits the facts about persons and the possible cases.