Overview

Identity is the concept of “sameness.” It asks: What makes an object the same object over time, even if it changes? What makes you the same person you were as a baby?

Core Idea

The core idea is persistence through change. If I replace every part of a car, is it still the same car? Identity deals with the criteria for numerical sameness.

Formal Definition

  • Qualitative Identity: Two things are exactly alike (e.g., two identical twins).
  • Numerical Identity: They are literally the one and the same thing (e.g., Clark Kent is Superman).
  • Leibniz’s Law: If $x$ and $y$ are identical, they share all properties.

Intuition

  • Ship of Theseus: If you replace every plank of a ship one by one, is it the same ship? What if you build a new ship from the old planks? Which one is the “real” ship?
  • Teleporter Problem: If a machine scans you, destroys you, and rebuilds an exact copy on Mars, is the copy you? Or did you die?

Examples

  • Personal Identity: What defines “you”? Your body? Your memory? Your soul? (Locke argued for memory).
  • 4Dism (Perdurantism): Objects are 4-dimensional “worms” in spacetime. The “you” right now is just a temporal slice of the whole object.

Common Misconceptions

  • Misconception: Identity is simple.
    • Correction: It gets very messy with vague objects (clouds) or fission cases (splitting an amoeba).
  • Misconception: Change is impossible if identity holds.
    • Correction: Identity is about how things persist despite change.
  • Ontology: The study of being.
  • Time: Identity over time is the central puzzle.
  • Essentialism: The view that objects have essential properties that define their identity.

Applications

  • Law: Can you punish a criminal for a crime committed 20 years ago? (Assumes personal identity persists).
  • Computer Science: Object identity in programming (two variables pointing to the same memory address vs. two equal values).

Criticism and Limitations

  • Bundle Theory: Hume argued there is no underlying “self” or substance, just a bundle of properties and perceptions.

Further Reading

  • Reasons and Persons by Derek Parfit
  • Identity and Necessity by Saul Kripke