Theories of Personal Identity

1. What Kind of Identity?

Before we dive in, we must clarify what we mean by "identity." Philosophers distinguish between two types. To get a quick and clear sense of this distinction, please watch this short video excerpt.

As the video explains, the key distinction is between:

• Qualitative Identity: Two things are qualitatively identical if they share all the same properties. A new car of the same model and color as your old one is qualitatively identical to it.
• Numerical Identity: Two things are numerically identical if they are one and the same thing. You, now, and you, five years ago, are not qualitatively identical (you have different properties), but you are, presumably, numerically identical.

The problem of personal identity is a problem of numerical identity over time. What makes a person at time $t_1$ and a person at time $t_2$ one and the same individual?

This puzzle has been debated since antiquity, famously captured in the thought experiment of the Ship of Theseus.

This problem becomes especially acute when applied to ourselves. Your body replaces almost all of its cells over the years, and your psychological states—beliefs, desires, memories—are in constant flux. What, if anything, remains the same? Let's examine the main contenders for a criterion of personal identity.

2. The Bodily Criterion: Are You Your Body?

One straightforward answer is that you are your body, or more specifically, a biological organism. This view, often called Animalism, posits that what makes you the same person over time is the spatiotemporal continuity of your body as a living organism. This aligns well with a substance-based ontology where the enduring substance is the human animal.

Arguments for the Bodily Criterion:

• It easily accounts for our persistence through periods of unconsciousness, such as dreamless sleep or coma, where psychological continuity might be broken.
• It aligns with the common intuition that we were once fetuses and that a person in a persistent vegetative state is still the same person, even with the loss of higher mental functions.

However, this view faces a powerful challenge, vividly illustrated by the brain-swap thought experiment.

The Brownson case (a brain swap) pumps the intuition that your identity follows your brain precisely because your brain is the seat of your psychological life. If a person with Brown's brain and Robinson's body wakes up with all of Brown's memories, beliefs, and personality traits, most people would say that person is Brown. This suggests that bodily continuity is not sufficient for personal identity, and perhaps psychological continuity is what truly matters.

3. The Psychological Criterion: Are You Your Mind?

The primary rival to the bodily criterion is the Psychological Criterion, which proposes that our identity over time consists in the continuity of our mental lives. The historical root of this view is John Locke, who argued that personal identity is a matter of consciousness, particularly memory.

Modern versions of this theory are more sophisticated. They don't rely solely on direct memory but on the broader concept of psychological continuity, which is understood as a series of overlapping causal connections among all of your mental states (beliefs, desires, intentions, memories, personality, etc.).

This view aligns well with a process-based ontology. Just as a barchan dune's identity lies not in a fixed collection of sand grains but in the persistent, self-organizing process of aeolian transport, the psychological view sees a person's identity in the continuous, causally-linked flow of mental life.

To understand how this theory works in more detail, let's turn to a more in-depth philosophical lecture.

As the video explains, the psychological criterion accounts for gradual change by requiring only an overlapping chain of connections, like the links in a chain. The person you are at 80 is connected to the person you were at 5, not because you share many direct memories, but because there is an unbroken chain of psychological connections linking each day to the next. The notion of causal dependence ensures that these psychological states are inherited properly, not just coincidentally similar.

To further solidify your understanding of how philosophers have refined this view to address classic objections, please read the following section from the Stanford Encyclopedia of Philosophy.

4. Challenges to the Psychological Criterion

While intuitively appealing, the psychological criterion faces several severe challenges, many of which arise from bizarre but logically possible thought experiments designed to test the limits of the theory.

The Fission Problem

The most famous and difficult problem is that of fission. Imagine your brain's two hemispheres are separated and transplanted into two different, identical bodies. Both resulting people, Lefty and Righty, would be fully psychologically continuous with you.

According to the psychological criterion, you should be identical to anyone who is psychologically continuous with you. This leads to a contradiction:

1. You are identical to Lefty.
2. You are identical to Righty.
3. Lefty and Righty are not identical to each other.

This violates the transitivity of identity ($A=B$ and $A=C$ implies $B=C$). One thing cannot be identical to two distinct things.

The next segment of the Kane B video provides an excellent, in-depth analysis of the fission problem, including its connection to real-world studies of split-brain patients, which should be of particular interest given your background.

How can the psychological theorist respond?

• The Non-Branching View: One could add a clause: you are identical to a future person if and only if they are psychologically continuous with you and no other person is. This means that in the fission case, you cease to exist, and two new people are created. This has the strange implication that you can survive losing half your brain, but you die if both halves are successfully preserved. This led philosopher Derek Parfit to argue that perhaps identity is not what matters for survival; what matters is the continuity relation itself (Relation R), whether it branches or not.
• The Multiple-Occupancy View: Another, more radical, response is to claim that there were two people (you-Lefty and you-Righty) sharing your body all along, who only become distinguishable after the fission. This view is often tied to a "four-dimensionalist" or "perdurantist" metaphysics, which we will explore in a future lesson.

The "Too Many Thinkers" Problem

A second major objection, which brings us back to animalism, is the "thinking animal" or "too-many-thinkers" problem. If the psychological view is correct, you are not a biological organism. But there is a human organism sitting in your chair, and it seems to be thinking. If so, there are two distinct, thinking beings where we thought there was only one: you (the "person" with psychological persistence conditions) and the organism (the "animal" with bodily persistence conditions). This seems absurd and raises an epistemic problem: how could you possibly know which one you are?

These powerful objections have led many philosophers to reconsider the bodily view.

5. Revisiting the Bodily Criterion (Animalism)

Faced with the intractable problems of the psychological view, particularly fission and the thinking animal, some philosophers argue that we should bite the bullet on the brain-swap case and accept the bodily criterion.

Animalists argue that while our intuition in the brain-swap case is strong, it is outweighed by the deep philosophical problems raised by the psychological view. They contend that the simplest, most coherent picture is one where we accept what biology seems to tell us: we are living organisms, and our persistence conditions are those of an organism. Our rich psychological life is a phase we pass through, but it does not define our fundamental identity over our entire existence, from fetus to potential vegetative state.

Conclusion

We have journeyed through the core of the debate on personal identity, a place where metaphysics becomes intensely personal. The question "What am I?" forces a choice between two powerful, but flawed, pictures of our existence.

• Key Takeaways:
  • The problem of personal identity is about numerical identity over time, not qualitative similarity.
  • The Bodily Criterion (Animalism) holds that you are fundamentally a biological organism. Its strengths are in handling cases without psychology (fetus, PVS), but it struggles with the brain-swap thought experiment.
  • The Psychological Criterion holds that you are a continuous stream of psychological states linked by causal chains. It aligns well with our sense of what matters for survival but faces devastating challenges from the Fission and Too-Many-Thinkers problems.
  • Thought experiments are philosophical tools used to test the logical consequences of a theory, forcing us to clarify our concepts and confront counter-intuitive results.
  • There is no consensus. The debate reveals a deep tension between our identity as rational, conscious beings and our identity as biological organisms.

Preview of the Next Lesson:
Having explored what it means to be a person persisting through time, we will next confront another fundamental aspect of our nature: freedom. If our actions are part of the causal fabric of the universe, determined by prior events and the laws of nature, in what sense can we be said to have free will? We will examine the main positions in this classic debate: compatibilism, libertarianism, and hard determinism.