Identity

What makes something the same thing, either as one thing rather than two or as one thing over time.

Theme: Being and reality
Tradition: Cross-tradition
Term fixed: 1714

In plain terms

Two quite different questions share one word. The first is whether this and that are one thing or two. The second is whether the thing here now is the thing that was here before, after everything about it has changed.

The second is the one that causes trouble, because ordinary objects survive change and there is no obvious rule for how much. A repaired chair is the same chair. A chair rebuilt entirely from new wood is less clearly the same chair. Nobody can say where between those the line falls, and the suspicion is that there is no line to find.

Which raises the possibility that our confidence about persistence is a practice rather than a discovery - that we treat things as continuous because it is useful, and the world contains no fact that settles the hard cases.

As a philosopher would put it

Numerical identity is governed by Leibniz's Law: if a is identical to b, every property of one is a property of the other. It is transitive, symmetric and reflexive, and admits of no degrees - which is exactly why persistence is difficult, since ordinary objects clearly change.

The main responses are endurantism, on which a thing is wholly present at each moment and identity over time is primitive or grounded in continuity; and perdurantism, on which objects are extended in time as well as space, so that what persists is a series of temporal parts and the puzzles dissolve into questions about which parts belong to which whole.

The hardest cases are the fission ones. If a thing divides into two equally good continuers, identity - being one-one - cannot hold with both, and there is no ground for preferring either. Parfit's response is that identity is not what matters and that we should care about psychological continuity instead. The Buddhist tradition reaches a related conclusion from the other direction, denying that there is a persisting bearer at all.

Where you meet it

The Ise Grand Shrine in Japan has been rebuilt every twenty years since the seventh century. The 2013 rebuilding was the sixty-second. An identical shrine is constructed on the adjacent site with new timber and traditional joinery, the deity is transferred, and the old building is dismantled and its timber distributed to other shrines.

No original material remains, and none is meant to. What is transmitted is the technique: the rebuilding is how the joinery survives, since a craft not practised is a craft lost.

When Japan sought World Heritage listing, this ran into criteria written around material authenticity - the assumption that an ancient building is ancient fabric. The 1994 Nara Document on Authenticity was drafted partly in response, arguing that authenticity has to be judged within a culture's own terms. That is a committee of conservation professionals discovering that identity over time has no neutral answer, and writing it down.

What is actually disputed

Whether persistence has determinate answers.

On one view the hard cases are simply hard, and there is a fact about whether the rebuilt shrine is the same shrine that we may never know. On another the concept is vague, so there is no fact - and where a question about identity has no answer, the question was defective rather than difficult. A third position accepts that identity is conventional for composite objects while remaining strict for whatever the fundamental things turn out to be.

What is not in dispute is Leibniz's Law itself. What is in dispute is whether ordinary objects are the sort of thing it applies to cleanly, or whether they are patterns we track for reasons of our own, in which case persistence is a decision that feels like a discovery.

Commonly confused with

identity in the social sense Who someone takes themselves to be is a question about self-understanding. Numerical identity is about whether this is that, and the two questions can give opposite answers about the same person.
Essence c. 330 BC Essence asks what kind of thing something is. Identity asks whether this one is that one, which can be settled without settling the kind.

Defined against

Change Identity is what is supposed to survive change, and the two terms are defined by the strain between them

Philosophers

Gottfried Wilhelm Leibniz The law that identical things share every property
John Locke Separates the identity of a person from that of a man or a substance
David Hume Finds no impression of a persisting self, and treats identity as a habit
Nagarjuna Denies that there is any persisting bearer to be identical with
Thomas Reid The objection that Locke's account makes identity intransitive

Works

Monadology Identity given its formal treatment
An Essay Concerning Human Understanding Personal identity as continuity of consciousness
A Treatise of Human Nature The bundle, and identity as something we impose
Reasons and Persons The case that identity is not what matters
Mulamadhyamakakarika No persisting bearer at all

Schools

British Empiricism Where personal identity becomes continuity of consciousness
Scottish Common Sense School Reid's objection that the account is intransitive
Madhyamaka Buddhism No persisting bearer to be identical with
Process Philosophy Temporal parts rather than an enduring thing

Related concepts

Particulars c. 330 BC · Essence c. 330 BC · Becoming c. 500 BC · Change c. 500 BC · Personal identity 1694

The puzzle is ancient - Plutarch reports the ship of Theseus - but Leibniz gives identity its formal treatment, and the law that identical things share all properties is his.