Identity Across the Multiverse

guest post by Amy Kind

Philosophical thinking about personal identity often focuses on identity across time. What makes the baby named Dwayne, born to Ata and Rocky Johnson in May 1972, the same person as the WWE wrestler known as The Rock? And what makes them both the same person as the actor who voiced the part of the shapeshifting demigod Maui in the 2016 animated film Moana? Confronted with an ordinary case of aging like this one, we might naturally think that the answer lies in a combination of facts about biological and psychological continuity. Unfortunately, however, the situation is complicated by the fact that the biology and the psychology facts can come apart.

Examples arise in various real-life situations, such as when an individual in a persistent vegetative state has biological continuity with an earlier individual without having any psychological continuity with them. Or to take an even more mundane example: Consider a college student who wakes up after a night of such heavy partying that they had a complete blackout. Though they remember nothing of the events of the previous evening, they are nonetheless biologically continuous with the person who imbibed all that alcohol. An even vaster variety of such cases have been depicted in science fiction. When a Star Trek character is beamed from the USS Enterprise to the planet the starship is orbiting, they are dematerialized and sent to the planet as an energy beam. The planetary individual has all the thoughts and memories as the individual who stepped onto the transporter pad, but none of the same atoms. So, does that make transporting a quick means of travel, or should it instead be viewed as a quick means of death?

Unsurprisingly, philosophers have split primarily into two different camps on the issue of personal identity over time – with one camp following in a tradition associated with John Locke that focuses on psychological facts and the other camp instead focusing on the biological facts. (A third camp offers a theory based on the soul.) Though various considerations can be advanced in favor of each position, that’s not my interest here.[1] Rather, I want to explore how our thinking about personal identity becomes further problematized when we turn from the question of identity across time to the question of identity across worlds.

Cases of identity across worlds don’t seem to arise in real life (or at least not yet!). But just as identity across time has been insightfully treated in a variety of SF works – in Star Trek episodes like TNG: “Second Chances”, Robert Sawyer’s Mindscan, Philip K. Dick’s “We Can Remember it for You Wholesale,” and Arkady Martine’s A Memory Called Empire, to give just a few examples – science fiction also provides us with some interesting explorations of identity across worlds. In some works of science fiction, multiverse travel proceeds via something like a consciousness transfer. To travel to another world, one’s Earthly consciousness inhabits (or, one might say, takes over) the consciousness of one’s cross-world counterpart. This is how multiverse travel was imagined in Matt Haig’s 2020 novel The Midnight Library. But in other works, travel to other worlds operates more like travel to other places, though the mode of transportation is not a typical airplane or even spaceship. In Black Crouch’s Dark Matter, for example, the relevant vehicle is some kind of mysterious metal cube.

On either of these models, however, an interesting philosophical question arises: How should we view the relationship between a person in one world of the multiverse and their counterpart in another world of the multiverse?

On the one hand, there are good reasons to deny that the counterparts are identical to one another. Counterparts exist in different places at the same time. Their bodies may be qualitatively similar to one another, and perhaps even indistinguishable from one another, but they are not numerically identical to one another. Moreover, though counterparts may have very similar psychological make-ups to one another, there is no shared consciousness between them. Someone from one world cannot introspectively access the thoughts and feelings of their counterpart on another world. All in all, the relationship between an individual and their counterpart is more like a relationship between twins or clones than it is like the relationship between an individual and their past or future selves.

But here it’s worth noting that science fiction depictions of clones frequently sometimes the relationship between clones as one akin to something like identity and, moreover, identity in a non-metaphorical sense. SF clones sometimes seem to think of their own personal identity as unified across clones into a single self. In Ursula LeGuin’s short story “Nine Lives,” for example, when a group of genetically identical clones reports for a mission on the planet Libra, they introduce themselves as a single entity: “We’re a tenclone. John Chow’s the name.” Moreover, after nine of the ten die in an earthquake, the one remaining clone wants to die as well: “I am nine-tenths dead. There is not enough of me left alive.”

So that brings us to the other hand: There may well be good reasons to accept that cross-world counterparts also share personal identity. And this is how science fiction often presents the matter. One particularly compelling example comes in Micaiah Johnson’s The Space Between Worlds. (For another, see Greg Egan’s short story “The Infinite Assassin” from the collection Axiomatic.) In Johnson’s 2020 novel, the Eldridge Institute employs cutting-edge technology to explore the multiverse and bring information and resources back to their own world, what they call Earth Zero. But there’s one important catch. The technology prevents anyone from travelling to a world in which their counterpart is still alive.

Caramenta, the novel’s protagonist, is an especially useful “traverser” for the company. Since her counterparts have died in 372 of the 380 worlds to which travel is technologically possible, she has a greater capacity for world travel than other traversers. It’s hard to tease out exactly how we’re to understand Caramenta’s relationship with her remaining counterparts. She sometimes refers to a counterpart as “her,” sometimes as “me,” sometimes as “another me.” After one of her counterparts dies, her emotions are complicated. When she tells her sister that she and the counterpart weren’t close, her sister disagrees: “You are as close to her as anyone can ever be. You are her.”

When Caramenta does reflect explicitly on the matter, she notes that while she has always believed that her selves are separate, and that “they – we – exist independently,” in quiet moments she can feel her other selves and their experiences. In describing this feeling, she says something especially revealing: “I can feel it all happening. Not just my selves collapsing, but time collapsing, because past and future are other selves just as surely as those on different worlds.”

How should we take this suggestive remark? Is it meant to carve out more distance between our different temporal selves – and more disconnect in our personal identity over time – than we typically recognize? Or is instead meant to encourage us to think of our counterpart selves throughout the multiverse no differently from the way we think of our past and future selves throughout time? Ultimately, I’m not sure. But whichever way Caramenta’s analogy should be taken, it seems clear that our philosophical reflection about personal identity across time could benefit from more philosophical reflection about personal identity across worlds. To understand what makes us who we are, we need to understand not just who we were and who we will be, but also who we could have been.

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[1] I discuss this debate in some detail in my book Persons and Personal Identity.

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