3 Identity and Persistence
Compare theories of how objects and people remain numerically the same through change, and examine the puzzles that test those theories.
Identity through change
An entity can undergo changes and remain the same individual. A tree may grow, lose leaves, and develop a hollow trunk; a person may change in appearance, beliefs, and abilities. asks what makes an entity existing at one time identical to an entity existing at another time.
differs from . Two objects are numerically identical only when they are one and the same object; means that they share properties. A new phone can look nearly identical to an old phone without being the same phone. Conversely, one phone can gain scratches and a new battery while remaining numerically the same device.
Identity is governed by a strict principle: if an object at one time is numerically identical to an object at another time, they are one thing and share the same properties, even though their properties can differ at different times. Saying that a tree is taller now than last year describes one tree changing, not the existence of two trees.
Two accounts of
and offer different accounts of how ordinary objects persist through time. Both aim to explain how one object can have different properties at different times, but they disagree about what consists in.
: The object is wholly present at every time at which it exists. The tree today is the very same whole tree that existed last year, despite changes in some of its properties.
: The object persists by having different at different times. Its earlier and later stages are parts of a larger, time-extended object, somewhat as its branches are spatial parts.
These theories address in general. Neither theory, by itself, determines what makes a person persist.
Replacement and
Some changes seem compatible with an object continuing to exist, while others make its identity harder to determine. Replacing a car's tires usually leaves the car intact. Gradually replacing all the parts of a wooden boat raises a more difficult question: is the repaired boat still the original? If the discarded planks are later reassembled into another boat, which boat, if either, is the original?
These puzzles indicate that judgments about depend partly on the kind of thing under consideration and on which features count as essential to it. A statue might be identified with a particular shape of clay, or it might be treated as distinct from the clay that constitutes it. The same material can therefore appear to support different answers about what persists.
Accounts of personal identity
concerns what makes a person now the same person as someone in the past or future. It is distinct from questions about character or social identity: a person can undergo a major personality change without becoming a numerically different person.
Several influential accounts propose different criteria:
: A person persists as the same living human organism, through the organism's continuing life rather than through the retention of every cell.
: A person persists through an appropriately connected mental life, such as memories, intentions, beliefs, and character, linked across time. Ordinary forgetting need not break identity, because overlapping connections can support continuity between different stages of a life.
: A person persists because the same immaterial soul continues to exist, even if the body or mental life changes. This is a metaphysical proposal, not an empirically established criterion.
The accounts illuminate different aspects of the question, but bodily continuity, mental continuity, and can come apart in imagined cases. Those cases prompt the question of whether survival means remaining the same organism, retaining a connected mental life, or something else. No single answer is universally accepted.
The
Imagine that a person's memories and personality are copied into two new bodies. Both successors might be psychologically continuous with the original, but they cannot both be numerically identical to that person: if they were, they would have to be identical to each other. This is the .
The case suggests that may matter to survival without, by itself, being sufficient for strict identity. Philosophers disagree about how to resolve such cases. More generally, thought experiments involving replacement, memory, and branching clarify what each account of explains and where it faces difficulties.