IB Philosophy 1.1.2 Identity Over Time Key Definitions

Numerical identity is the philosophical claim that a person or object at one time is literally one and the same individual as that person or object at another time, not merely similar to it. This is contrasted with qualitative identity, which only requires sharing the same properties.

Psychological continuity, associated with John Locke, is the view that personal identity consists in overlapping chains of memory, consciousness, and psychological connection linking earlier and later stages of a person's life, prioritising mental over physical continuity as the criterion of personal identity.

Qualitative identity is the relation between two things that share the same properties or characteristics without being the same individual, contrasted with numerical identity. For example, two identical twins are qualitatively identical but numerically distinct, being two separate persons.

The body theory of personal identity holds that a person persists over time in virtue of the continuity of their physical body, so that what makes you the same person tomorrow is the ongoing existence of the same biological organism. The view faces challenges from thought experiments involving brain transplants, where our intuitions about identity seem to track the brain rather than the body as a whole.

The soul theory of personal identity holds that what persists through change is an immaterial, non-physical soul that constitutes the true self and survives bodily death. While the view provides a straightforward account of post-mortem survival, it faces the objection that an unobservable soul offers no empirical criterion for re-identifying the same person across time.