Physical Continuity and Perdurantism: Understanding Identity


By David K. Johnson, Ph.D., King’s College

At many levels, it appears impossible to use continuity of memory or of psychology to establish or define what constitutes personal identity. Apart from other not very justifiable ideas such as the soul or love, we are then left to wonder if it is the body which can provide some answer to this, or some combination of the body and the mind. Let’s look at some of the possible theories.

A human body with polygonal shaped brain and heart.
Philosophers and science fiction have tried to give answers about identity based on such varied ideas as soul and mind. In some theories, some combination of the body and the mind have also been suggested as the root of identity. (Image: TZIDO SUN/Shutterstock)

Physical Continuity in The Prestige

For most people, and even in science fiction, the body is the site of identity. Even if a person’s body is wiped clean of the psychology of a person, that body intuitively seems to us to be the person still. This idea is explored in the Christopher Nolan’s movie The Prestige. The movie’s protagonist, a magician named Robert Angier, uses a “duplication machine” invented by a fictional version of Nikola Tesla, to perform the “real transported man” trick.

He wheels the machine on stage, steps in, and falls down a trap door. The machine creates his duplicate on the balcony, and that duplicate takes all the applause. The catch is this: that trap door leads to an inescapable water tank where the falling person dies. As the closing shot of the film shows, there is series of water tanks, under the stage, filled with many dead Angiers.

The machine creates a duplicate with a different body, who then proceeds to fall into the tank at the next performance. Because personal identity seems to follow one’s body, it’s crystal clear that the magician who steps into the machine gives up his life when the trick is performed.

Learn more about alternate worlds.

Embodiment Cognition

Image showing brain activity inside a human head.
The embodiment cognition hypothesis suggests that the entire body and the brain are necessary to create identity. (Image: adike/Shutterstock)

The idea that the preservation of your body is necessary for the preservation of yourself seems to align nicely with another philosophical theory called “embodiment cognition.” Proponents of embodiment cognition suggest that parts of the body beyond the brain play a significant role in the production of cognition.

Since the existence of our mentality seems necessary for our continued survival—when we die and all mentality stops, we cease to exist—if the embodiment cognition thesis is right, the preservation of our entire body would seem necessary for the preservation of our personal identity.

But it doesn’t necessarily follow that mental activity arises from different body parts. After all, people lose limbs all the time, but they don’t lose mental capacities. As long as your brain survives, you’d still be you.

This is a transcript from the video series Sci-Phi: Science Fiction as Philosophy. Watch it now, on Wondrium.

Continuity by Degrees

Peter Unger, a philosopher, points out that the brain doesn’t necessarily have to always be made of the same material. The material out of which our bodies are made, including that of the brain, is slowly replaced over time. But according to Unger, as long as the replacement is gradual enough, physical continuity, and thus identity, is preserved. But from here comes our first objection to this view.

Physical continuity comes in degrees, depending on how fast, or how much matter is replaced. Identity does not; either you survive, or you don’t. So it doesn’t seem that you can equate one with the other.

The Split-Brain Problem

A related problem is this. What if your brain splits in two? Which one would be you? They’re both physically continuous with your previous self, right? So, on this theory, they’d both be you. But that’s as logically impossible as all of your clones being you.

Interestingly, we don’t even need to go to science fiction to consider this possibility. Split-brain patients—where the tissue that joins a person’s two hemispheres is damaged or cut—are real. And experiments have shown that one half can not only see and hear things the other doesn’t, but can even have its own conflicting wants and desires.

In their daily lives, such patients can suffer from what’s called “alien hand syndrome,” where one hand of their body will act on its own without the other hemisphere being able to stop it. Without ways to answer these questions, the physical continuity theory seems to fall short—which leads us to believe that perhaps we have been conceiving of persons all wrong to begin with.

Learn more about personal identity.

Four-dimensional Thinking

A man with double face expression.
Perdurantism can help answer how to define the identity of split-brain patients. (Image: FGC/Shutterstock)

And that is exactly what the philosopher David Lewis suggested when he said we should not think of persons as entities that exist at particular places at particular times.

Instead, we should think of persons as four-dimensional objects, causally related, stretched across time. In the same way that a TV series isn’t a single episode, but instead a collection of episodes all bundled together, you are not just a body, sitting there, at this time, reading this. You are a collection—the set of all the moments, or episodes, of your existence, bundled together.

The view is called perdurantism, and it answers all kinds of questions about personal identity. What happens with split-brain patients? There are two persons because there are two sets: set 1 is all the episodes before the split and then the episodes of the right half, and then set two is all the episodes before the split and the episodes of the left half. And how do we make sense of clones? We just organize them into sets: a set of episodes for the original, and then Clone 1, and Clone 2, etc.

The Advantages of Perdurantism

Perdurantism does have some advantages. For example, it can make perfect sense of time travelers. One and the same person supposedly can’t be at two different locations at the same time, but that happens in time travel stories all the time.

But it can be seen as two different parts, or episodes, of the same person who can—at least in science fiction—coexist even in the same time, at different places. So, perdurantism helps to provide a clear answer about what constitutes a person—it is more than just physical or psychological continuity, or an essential soul.

Personal identity can be seen as a four-dimesional continuum of a person through all times, spaces, and psychological states.

Common Questions about Physical Continuity and Perdurantism

Q: What is the idea of physical continuity of identity?

The idea of physical continuity is that as long as the physical body is the same, the identity is the same.

Q: What is perdurantism?

Perdurantism is the idea that we should think of people as four-dimensional objects, causally related, stretched across time. A person is the set of all the moments, or episodes, of his or her existence, bundled together.

Q: What causes a split-brain in humans?

In split-brain patients, the tissue that joins a person’s two hemispheres is damaged or cut.

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