What makes you you? Not your properties or characteristics—those change constantly. The question cuts deeper: what makes the person reading this sentence numerically identical to the person who will remember reading it tomorrow? This is the persistence question, and it turns out to be surprisingly difficult to answer.

The philosophical stakes are immediate. If you committed a crime ten years ago, what justifies punishing you now? If you're saving for retirement, why should you sacrifice present pleasure for some future person's benefit? These practical questions presuppose answers to the metaphysical puzzle. The wrong theory of personal identity could undermine our entire framework of moral responsibility and prudential concern.

Three major accounts compete for our allegiance: psychological continuity theories tracing identity through mental connections, biological theories grounding identity in organismic persistence, and narrative theories emphasizing the role of self-understanding. Each captures something important about our persistence through time, yet each faces serious objections that reveal the problem's genuine difficulty.

John Locke initiated modern debate on personal identity with a deceptively simple insight: identity follows consciousness, not substance. His famous example of the prince and the cobbler—where the prince's memories and psychological states transfer into the cobbler's body—suggests that the resulting person is the prince, despite biological discontinuity. What matters for identity is the psychological connection between temporal stages.

Contemporary neo-Lockeans have refined this into the psychological continuity theory. Derek Parfit and Sydney Shoemaker argue that personal identity consists in overlapping chains of psychological connections: memories, intentions, beliefs, desires, and character traits. The crucial move is allowing indirect connections. You needn't remember your fifth birthday to be identical to that child; it suffices that there's a continuous chain of overlapping psychological states connecting you.

This theory handles ordinary cases elegantly but generates puzzling verdicts in extraordinary ones. Consider teleportation: a device scans your body, destroys it, and creates an exact physical and psychological duplicate elsewhere. The psychological continuity theorist must say you survive—the duplicate has all your memories, intentions, and character traits. Yet many find this counterintuitive. Worse, consider fission: what if the device creates two duplicates? Both have equal psychological continuity with you. Identity is transitive (if A=B and B=C, then A=C), so both cannot be you without being each other.

Parfit's radical response was to deny that identity is what matters. What we care about in survival—the continuation of our projects, memories, relationships—is preserved in psychological continuity regardless of strict identity. Perhaps fission is survival, just better than ordinary survival because you continue twice over. This reconceptualization challenges deep assumptions about self-concern but follows logically from the psychological approach.

Takeaway

Psychological continuity theories force us to distinguish between identity (a strict logical relation) and what matters in survival (the continuation of our mental life)—and these might come apart in ways that transform how we think about death and persistence.

Animalism offers a strikingly different answer: you are a human animal, and your persistence conditions are those of that animal. Eric Olson, the theory's most prominent defender, argues that psychological theories face a devastating objection. There's certainly a human animal sitting where you're sitting, thinking your thoughts. If you're not identical to that animal, then there are two thinkers present—you and the animal. But this is absurd; surely there's only one thinker here.

The animalist solution is elegant: you are that animal. Your identity through time consists in biological continuity—the continued functioning of the same living organism. This explains why you were once a fetus (which lacked psychological continuity with you) and would persist through total amnesia or permanent vegetative state. The psychological approach struggles with these cases, seemingly implying that you weren't the fetus or wouldn't survive into the vegetative state.

Animalism handles teleportation and fission differently. In teleportation, the original organism is destroyed; a new organism is created. You die, even if a psychological duplicate persists. In fission, if both resulting organisms have biological continuity with you (perhaps through some impossible scenario), the animalist faces the same puzzle as the psychologist. More realistically, biological fission seems conceptually impossible for organisms like us.

Yet animalism faces its own challenges. Consider cerebrum transplantation: your cerebrum is transplanted into another body. Intuitively, you go with your brain—you wake up in the new body with your memories and personality. But the original organism, the animal that was you, continues in the old body (perhaps in a vegetative state). Animalism must say you stay with the old body, and someone else wakes up with your memories. Many find this verdict deeply counterintuitive.

Takeaway

Animalism grounds identity in biological fact rather than psychological connection, but the divergence between 'the animal you are' and 'the psychological life you care about' raises questions about whether biological identity captures what actually matters to us.

These abstract theories generate concrete disagreements about moral responsibility, prudential concern, and institutional practice. Consider punishment: if psychological continuity constitutes identity, then punishing the severely amnesiac for crimes committed before their memory loss is punishing the wrong person. If animalism is correct, the amnesiac remains responsible—biological continuity suffices for identity, regardless of psychological disconnection.

The stakes extend to advance directives and medical decision-making. Should we honor the wishes of someone's past self when their present self, suffering from dementia, expresses contrary preferences? Psychological discontinuity theories suggest the present patient may be a different person than the one who wrote the directive. Animalism suggests they're the same person, but this doesn't obviously settle whose preferences should govern—perhaps present preferences always trump past ones.

Consider also prudential concern for future selves. Why sacrifice present pleasure for future benefit? The standard answer: because it's your future, and self-interest rationally extends through time. But if psychological continuity constitutes identity, your connection to your far-future self may be weak—your values, memories, and personality will have changed substantially. Parfit argued this should reduce our degree of concern: self-interest should be time-discounted based on psychological connectedness.

These practical implications reveal something important: our intuitions about cases like punishment and advance directives might themselves be evidence about the correct theory of personal identity. If a theory generates verdicts we cannot accept upon reflection, this counts against it. Yet our intuitions might also be confused or parochial. The dialectic between metaphysical theory and practical judgment runs both directions, neither fully determining the other.

Takeaway

Different metaphysical theories of personal identity yield different verdicts about punishment, medical ethics, and rational self-concern—making the abstract question of persistence conditions surprisingly urgent for how we structure institutions and lives.

The persistence question resists easy resolution because each major theory captures genuine insights while facing serious objections. Psychological theories respect our sense that mental life matters most to us; biological theories honor the embodied, animal nature of our existence; practical cases pull us in both directions depending on context and framing.

Perhaps the deepest lesson concerns the relationship between metaphysics and practical reasoning. We want our theories of personal identity to vindicate ordinary practices of responsibility, concern, and planning. But perhaps those practices rest on confused assumptions that careful analysis should correct rather than preserve.

What makes you the same person tomorrow may have no single answer—or an answer that transforms rather than confirms how you understand your persistence through time.