What Is Personal Identity?

Direct Answer

Personal identity is the philosophical question of what makes a person the same person over time — what it is that persists from birth to death and grounds our being one and the same individual. The question is not "what makes you the person you are" in the sense of your character or personality, but "what makes the person here today the very same person as the child in that old photograph." It asks about the conditions under which a person continues to exist.

The classical answers divide into three families. The bodily criterion says that personal identity consists in the continuity of a single living body. The memory theory, associated with John Locke, says that a person is the same as long as it can remember its own past experiences — consciousness, as Locke put it, "accompanies thinking" and makes a person the same to itself. The psychological continuity view, developed in its modern form by Derek Parfit, says that identity consists in overlapping chains of memory, belief, intention, and character: you are the same person as your past self because your present mental life grows out of it.

Each view faces famous difficulties. The bodily criterion fails if we imagine gradual replacement of all body parts: the person whose body is gradually rebuilt seems to persist. The memory theory struggles with forgetfulness and childhood amnesia — if you remember nothing from your first years, does that mean you are not the same person? Parfit's response to such problems was radical: he argued that strict identity is simply not what matters. What matters is psychological continuity and connectedness, which are matters of degree. Identity is all-or-nothing; survival by continuity can be more or less. When a continuity relation holds strongly enough, we should care about the future self the way we care about our own future — even if strict identity, in some technical sense, fails.

Historical Context

The modern debate begins with Locke, who in the Essay Concerning Human Understanding (1690) distinguished the person from the human being and located personal identity in consciousness and memory. His view was criticized almost immediately: Bishop Butler objected that memory presupposes identity (you can only remember what you yourself experienced), and Thomas Reid offered the "brave officer" objection — a soldier who remembers his childhood exploits is the same person as the child, but an old general who has forgotten those exploits is not, by the memory criterion, the same person as the soldier, while he is the same as the child: contradiction.

Hume took the critique further, arguing in the Treatise that when he looks inward he finds only perceptions, never a self, so the self is a "bundle" of perceptions held together by resemblance and causation. Kant responded that the unity of consciousness is a condition of experience, even if we cannot prove it belongs to a persisting substance. In the twentieth century the debate was transformed by Parfit's Reasons and Persons (1984), which used thought experiments about brain division and teleportation to show that identity can be indeterminate — and that, since it can, it cannot be what ultimately matters to us about survival.

Philosophical Significance

Personal identity matters because our deepest practices presuppose it. Moral responsibility requires that the person who is punished is the same person who committed the crime. Prudence requires that I care about the welfare of the future self as my own. Promises, contracts, and obligations all presuppose that people persist through time. The philosophy of personal identity asks whether these practices are grounded in a fact about the world — a fact about what persistence consists in — or whether they rest on conventions and attitudes we are free to revise. It also connects to the ethics of future selves: if psychological continuity holds to degrees, then the way I treat my distant future self is more like the way I treat another person, and questions about whether to save for retirement, or whether to respect an advance directive, take on a different shape.

The question also intersects with consciousness and the self. If the self is, as William James suggested, a stream of experiences, then personal identity is about the unity and continuity of that stream, not about a hidden mental substance. Eastern traditions such as Buddhism and nonduality have long denied the existence of any permanent self, and contemporary philosophers of mind have found these views increasingly congenial.

Examples & Thought Experiments

The most famous thought experiments concern division. Suppose your brain is split and each half is transplanted into a new body. Both resulting people remember being you and have your character. Which one is you? Neither, if identity is a one-one relation — and yet both have what seems to matter in survival. Parfit concluded that identity cannot be what matters: if two people can both have everything we care about in survival, then survival by continuity can outrun identity.

The teletransporter makes the same point: you step into a machine that destroys your body while transmitting your blueprint to Mars, where a new body is built. The person who steps out remembers being you and is psychologically continuous with you. Is it you, or a copy? If identity is what matters, you have died. If continuity is what matters, you have survived. Parfit argued that the second answer is right, and that this should change how we think about death: what matters is not whether "I" continue, but whether there is someone whose mental life is continuous with mine.

Related puzzles include the ship of Theseus, which asks when an object whose parts are all gradually replaced remains the same object, and the brain in a vat, which asks how much of our world could be false while our mental lives continue. Both press the same question: what kind of continuity, if any, constitutes persistence?

Related Concepts

• The self and Exploring the Self
• Identity and the ship of Theseus
• Death and survival
• Free will and moral responsibility
• Consciousness and memory
• Reasons and Persons — the modern classic

Further Learning

• Stanford Encyclopedia of Philosophy, Personal Identity
• Stanford Encyclopedia of Philosophy, Personal Identity and Ethics
• Derek Parfit, Reasons and Persons (Oxford: Clarendon Press, 1984)