Planarian worms are perfect regenerators. That is, they’re immortal. If you cut off a chunk of one, it will grow back, and the worm will continue as before.

This also means that a large enough chunk will grow into a separate worm. And in fact, some species of planaria reproduce this way, instead of sexually. They split in half, and each half becomes a full worm again.

So, a question: if Wormy the planarian splits in half and becomes two new worms, which one is Wormy?

You can’t just pick one, since they share equal amounts of flesh with Wormy. You could argue that Wormy is dead and that these are his children. But what if only one of the worms survived the process? Wouldn’t it seem odd to say he wasn’t Wormy? Or, a similar case: does a salamander who loses and regrows her tail become her own child? No.

If Wormy isn’t simply dead, and neither of the new worms is the true Wormy, we’re left with only one possibility: both worms are Wormy.


This seems reasonable, at first glance. Wormy is the name of a continuous process, which starts as a single worm and becomes two.

But what about Wormy’s predecessor? Let’s call him Wormy Prime; the little guy who split in half to produce Wormy and some other forgotten twin. Is he also part of that process? The answer can only be yes.

Now we’ve created a monster. Because the same applies to Wormy Prime’s predecessor, and the eventual successors of the newly-split Wormy. By our prior logic, Wormy is the name of a continuous process, which once was a single worm and now is many millions.

Okay, fine, let’s bite this bullet: Wormy is the name for this entire species of planaria. He’s a single entity who happens to have many spatially distinct parts. Similar to the way our original one-worm Wormy was composed of many cells.


But what about other species of planaria? At some point, they had a common ancestor with Wormy. For the asexual varieties, whatever distinctions we see as dividing them happened as a result of random mutations during this splitting/cloning process. Are they not Wormy?

This would seem somewhat arbitrary. If one of Wormy’s halves were to be struck by radiation and slightly mutated while regrowing, we wouldn’t say that he suddenly becomes severed from his Wormy-ness.

Similarly, we can return to our salamander friend: if she were mutated by radiation while regrowing her tail, we wouldn’t say she becomes a new salamander.

So, it seems that Wormy includes his neighboring species, as well.


Now we’re in another pickle: some of those species do reproduce sexually. Does this mean that their respective patriarchs, split asexually from Wormy-ancestors/aspects-of-Wormy, are themselves Wormy, but their children are not?

Imagine that Wormy is undergoing a split very near a nuclear testing site. One of his halves is hit by a blast of radiation so large that it immediately scrambles a significant portion of his genome. Fortunately, we’ve already decided that this doesn’t stop him from being Wormy.

(Similarly, our salamander friend would continue to be herself, even if hit with massively-mutating radiation during her tail regrowth.)

This clarifies the situation of Wormy’s sexually-reproducing subworms. A sexually-reproduced child is simply a clone with a set of significant and somewhat deterministic mutations.

We could object to the magnitude of the mutations. But in the nuclear scenario, it seems odd to cap Wormy’s continued Wormy-ness based on the force of the radiation he’s struck with.

Where would we draw that line? 50% seems arbitrary; can we really end Wormy’s Wormy-ness by increasing the blast intensity by a single notch? And besides, since planarian parents share much of their genetic code, sexual reproduction generally creates offspring who have far more than 50% of their genome in common with each parent.

So it seems that even sexually-reproduced children of Wormy are part of the Wormy process.


Another problem arises: when Wormy sexually reproduces, he must do so with another, apparently-distinct worm. Let’s call her Alexandra.

We decided, above, that Wormy’s child is also Wormy. But wouldn’t it equally be Alexandra’s child, and thus a continuation of Alexandra? After all, their reproduction is symmetrical.

Fortunately, Alexandra and Wormy, as fellow planarians, actually share a common ancestor. Given all that we determined earlier, this means that, in fact, Alexandra is Wormy and Wormy is Alexandra. They’re a single process.

So we’re simply talking about two aspects/subworms of Wormy, cloning themselves and continuing Wormy together.

This is even more obvious if we zoom in to the cellular level. Wormy and Alexandra are both composed of cells descended from the same batch; the two sub-batches are simply mixing again. They could just as easily have arrived at these exact cells in this exact configuration through the repeated splitting and regrowing of the original batch; in fact, this is literally what happened.


If this line of argument is making you squirm, that’s because you are, in fact, Wormy.

You may believe yourself to be meaningfully distinct from a planarian worm. For instance, you are not immortal, and you have several limbs. You think more abstractly. You do not wish to be a worm.

But we already agreed that mutations (including the special case of sexual reproduction) do not sever Wormy-ness. And all Earthly life forms are connected in this way.

We might object that our consciousness sets us apart. We have experiences that other aspects of Wormy do not, and we feel ourselves to be distinct individuals.

However, many people have nights when they are sufficiently intoxicated that they do not remember the course of events in the morning. Since the drunken person has experiences they do not share with the sober person, are they different individuals? No.

As for the feeling of distinctness: imagine a close friend suddenly waking up with a sort of strange amnesia, insisting that they are not the person you claim to know. They may feel this intensely, but it doesn’t make it true. If they were to “snap out of it” and regain their sense of identity, you wouldn’t say they had temporarily become another person.

Conversely, I may fervently believe myself to be the same individual as Lindsay Lohan’s character in “Mean Girls,” but this doesn’t make it so.

It seems that neither unshared experience nor a feeling of distinctness are determinants of identity. So, alas, we are Wormy.


In case it isn’t clear (which is understandable, since we are merely two aspects of a worm communicating through electrical signals), this thought experiment is meant to temporarily dissolve the typical narrow conception of identity and draw out a feeling of unity among all forms of life.

The idea that categories are arbitrarily drawn is not a new one. Neither is the idea that the self is less substantial than it seems.

But I return to this special case often. There once was a single cell that learned the planarian-esque trick of splitting and regrowing itself. It abruptly exploded into all of life on Earth. Each individual creature is composed of basically similar descendant cells, tweaked and arranged in novel and useful configurations. All part of a single process, like mold growing under a leaky sink.

This provides a similar perspective to the Pale Blue Dot reminders, except that it fills me with warmth and fellow feeling. And unlike many other objectively valid lines of argument (e.g. pointing out that all supposed objects are merely clusters of atoms), I understand it viscerally. It can feel just as intuitively correct as assuming these lives are distinct.

I think this is because it zooms in on the nebulous edges of my concept of identity, and forces me to collapse the contradictions in an unusual way. It’s not “true” that we are all a single entity, any more than it’s “true” that we’re individuals. In reality, the whole thing is quantum soup.

The categories are conventions, created by our animal brains to help us survive and reproduce — but as such, they have great emotional force. Typically, I behave as if the individualized version is correct; however, by exploiting its internal inconsistency, I can push into another stable configuration and redirect those primal energies along a different pathway.

It takes some effort to get into this mindset (hence the chain of arguments above), but once I’m there, it’s like an optical illusion: I can’t unsee it. At least for a little while.

My personal foibles become small: the struggles of a single mitochondria in a titanic blue whale. Not because my existence is meaningless, but because I’m part of something so vast, beautiful, and vibrant; more suffused with meaning than ever before. And in this light, I don’t feel altruism as a reciprocal obligation, but as a natural extension of this other, broader identity of mine.

Meditation is an excellent way to capture this feeling, as is philosophy. But it’s slippery. Worth approaching from as many angles as possible, each hopefully deepening our visceral understanding and easing the path from our default mode. I hope Wormy can help do that for “you,” as much as he has for “me.”

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23 comments, sorted by Click to highlight new comments since: Today at 6:59 AM

Identity is in the map. There is no objective answer there, just various models where the idea of a unique identity is more useful or less useful. It's not useful for describing a splitting worm. It's useful to describe the feeling of being the same person as yesterday. It is less useful in a hypothetical world with human clones. It is only marginally useful for multiples or for people with DID. It may not make sense in some cultures where people identify with a group more than with a body.

Let us not try to stretch a model beyond its domain of applicability.

But the specific concept of identity must map to something in the territory, otherwise it's empty, so it can't be only in the map.

If it was people who could split, I'd still be interested in knowing if I black out forever, or if I have a 50% of subjective experience probability of finding myself in each continuation. If someone told me that there was no objective answer there, I couldn't imagine what would that look like. I can imagine blacking out forever, or rolling the dice, but not there being no objective answer as to whether or not either of those two happens.

We could say that by switching the model, we can switch which answer is correct, but that's always true for every question (unless it's a logical necessity/impossibility), so I'm not sure what makes identity special in that sense.

But the specific concept of identity must map to something in the territory, otherwise it's empty, so it can't be only in the map

Yes, it corresponds to the part of the territory where there are single-mind non-clonable humans.

If it was people who could split, I'd still be interested in knowing if I black out forever, or if I have a 50% of subjective experience probability of finding myself in each continuation.

I don't think you can profitably use the notion of probability here. If you are one of the clones, and you remember being the pre-cloned human, you can claim that you have the same identity if that is what feels like to you. There is no "objective identity" here. Same with all other clones. There is no contradiction between multiple clones having the same identity if you define identity as "remembering being the pre-cloned human". Of course, once the experiences diverge, each one has their own identity.

We could say that by switching the model, we can switch which answer is correct, but that's always true for every question (unless it's a logical necessity/impossibility), so I'm not sure what makes identity special in that sense.

The difference is usefulness/accuracy of a given model. If an outdated model leads you astray, come up with a better one. Just because a model worked in one setting, doesn't mean it would work in a different one.

So your position would be that if you split, there is no fact on the matter as to whether you experience blacking out forever (like after a car accident), or whether you roll a dice as to which descendant you wake up as?

There is no objective "fact of the matter", no. Identity is an emergent subjective concept. There is no "experience" of a blackout, as far as we know, since there is apparently no memory being recorded during that time. The experience comes from waking up and reflecting on what you recall, and this reflection can lead to different feelings for different people (or different parts of the same person). Privileging identity over brain functioning is not a great approach.

Thank you for explaining this. I don't think this is coherent, but now I understand what you mean.

Well, I find my approach perfectly coherent, unlike the alternatives that pretend to be objective :)

That's valid.

We don't need to talk about splits to consider this sort of question.

Suppose a person at 2:00 viscerally feels that they aren't the same person as a second earlier, even despite having all the same memories from before 2:00, personality, skills, values, beliefs, and so on, except that they are utterly convinced that they came into existence just now with all of those.

Should we believe that the previous person has died? Is there anything in the territory that can make it either a fact or a delusion?

Going further: does it make any sense to say that a person could die and be replaced by another who does not have that belief, so seamlessly that nobody could possibly ever tell the difference including the person who replaced the deceased?

My strong suspicion is that there isn't anything in the territory that could make either of these true or false. That it's a mental illusion, a belief that points to nothing fundamental but is just a byproduct of a model with an extraneous parameter.

Even if something like souls exist, then in the first scenario there is still no reason to expect this sudden belief to be strong evidence of truth that a new soul was inserted into the body. I don't think souls are likely to come with clocks saying how long they've been present in a body, whereas people do observably experience hallucinations and delusions about all sorts of things.

In the second scenario, I'm not sure that it makes a difference at all, even if souls exist. If some entity has all the memories, personality, skills, values, beliefs, feelings and so on of a person, then as far as I can tell they simply are that person, regardless of whether or not they have the same "soul". Instantly destroying a soul and replacing it with another having all of those properties seems to be an operation that makes absolutely no observable difference to anyone, both objectively and subjectively.

My strong suspicion is that there isn't anything in the territory that could make either of these true or false. That it's a mental illusion, a belief that points to nothing fundamental but is just a byproduct of a model with an extraneous parameter.

I'm not sure that's coherent. If we perceive it, there must be an entity or a process in the territory corresponding to it, which, in turn, makes it true (or false, in case the referent is missing - it seems like I could be mistaken about whether I'm a continuation of a particular person (for example, if I wake up tomorrow believing that I'm you)).

There are some objective means by which we can test properties associated with "X is a continuation of Y", such as finding out whether X can answer questions consistently with having been Y. Those aspects are in the territory, but I don't think they capture anybody's concept of personal identity usefully.

With strong enough models of the mind we may even be able to objectively predict whether X considers themself to be a continuation of Y. This still does not mean that personal identity is in the territory, any more than the blegg/rube distinction is in the territory in https://www.lesswrong.com/posts/yA4gF5KrboK2m2Xu7/how-an-algorithm-feels-from-inside. 

If you can imagine doing every possible test and still not knowing the answer, that seems likely to be a sign that the question is referring to a distinction that is only in a model and isn't in the territory at all.

That seems to be the case here. Most of humanity has a model of personal identity that works well for their ordinary experience: for each person X_now, and for each time t > now, there is exactly one person X_t such that X_t is a continuation of X_now and reversed for birth/conception < t < now. Many even extrapolate it past bodily destruction.

This relation has nice properties such as its symmetric extension (X continues Y or Y continues X) being an equivalence relation that partitions the world of people into distinct equivalence classes.

It is a less useful model in the presence of probably physically possible but currently implausible circumstances such as minds that can be copied, edited, overwritten, or merged. If a question is based on such a restricted model applied to the wider space of possibilities, it isn't likely to have any answer.

I see, for me, if an ontology says that there is no fact on the matter as to whether X is a continuation of Y, it means the ontology has to be discarded and replaced by another one, but I can see how some people would be unbothered by their next experience being undefined, as long as it doesn't happen too often.


You're leaning heavily on transitivity. In areas, such as maths where identity can be defined precisely transitivity holds: If a=b and b=c, then a=c. But it doesn't have to where identity is fuzzy. If I define identity99 as "a is 99% similar to b", then "a is identical99 to b" and "b is identical99 to c" don't imply "a is identical99 to c".

temporarily dissolve the typical narrow conception of identity 

I think a lot of discussion around here is already different from the typical narrow conception of identity.  You'll often find people talking about "past me" as an indicator that there are aspects of identity which are best modeled as a different person.  One unsolved problem in probability is "to what extent is this reference class useful for this prediction", and that gets to the underlying point I think you're trying to make: identity is a cluster of different types of similarity,  which only make sense when specifying what you intend to infer with the concept.

and draw out a feeling of unity among all forms of life.

Wow, that's a stretch.  In me, at least, it draws out a feeling of isolation from previous and future versions of me.  In dissolving the notion of identity, why would that make me closer to things I now even more clearly recognize as "other"?

Short form, I agree with shminux that identity is in the map, and I'd say the evidence we have is consistent with identity being illusory, but we think it is there because of our tendency to find essences.

Link here:


Do planeria not age? That’s surprising; even bacteria age! And it’s remarkably interesting for research if it’s true.

As for the splitting, that happens with humans as well, due to Everett branching. We can’t observe it directly like we can with the worm, but it seems to be the case.

There are a range of animals that do not seem to show increased likelihood of death as they age. Hydra, naked mole rats, sea urchins, Aldabra giant tortoises, aspen trees, Greenlank sharks, and planaria are examples. It may ultimately turn out that we're just not looking hard enough for evidence of aging in these organisms.

It may seem surprising that complex organisms can show less aging than simple organisms, given that it seems harder to maintain all the parts of a complex organism in working order.

However, the complexity of multicellular life also permits higher-fidelity regeneration and maintenance mechanisms that are unavailable to simpler life forms. Also, multicellular life may derive a survival advantage from investing in structures that promote longer or indefinite individual life expectancies, while single-celled life may benefit from maximizing the rate of reproduction and genetic adaptation to changing conditions.

For bacteria, individual cells die easily, but colonies can live indefinitely as damaged macromolecules are diluted between daughter cells during cell division.

I've heard the claim about naked mole rats, but not the details:  is it possible to have a naked mole rat that lives for an arbitrarily long time if you keep them in a sterile environment where it's hard to get significantly injured?  That would seem to be a straightforward implication of them not aging, but I've never heard that this has been done. 

This isn't quite what it means to not experience aging. Instead, "not aging" means that the likelihood of dying in a given year does not increase with age. So a naked mole rat kept in a cage for an indefinite period of time can still die, and yet not be aging.

The oldest known naked mole rat in captivity is 40 years old, while most naked mole rats live 2-5 years in the wild.

Thanks!  Do you know what tends to kill them then?  I was under the impression that organisms usually die of disease, injury, cancer, transposon propagation or programmed aging.  I notice I am confused, because keeping a naked mole rat in a safe environment should protect them from disease and injury, and transposons and programmed aging would do more damage over time.  Do they die of cancer then, or am I missing something big about biology here? 

Captive NMRs are kept in colonies, and can inflict injuries on each other.

They do get cancer at very low rates.

Animals that haven't been specially reared to have had zero contact with pathogens can also harbor dormant diseases, and the current longest-lived NMR has not spent his whole life in a sterile environment.

Complex organisms may be at a constant low-level risk of deadly internal injuries (i.e. a random stroke).


Very interesting; thank you!

I commend you sir, because what you've done here is found a critical failure in materialism (forgive me if you're not a materialist!). As a hard dualist, I love planarians because they pose such challenging questions about the formation and transfer of consciousness, and I've done many thought experiments of my own involving them, exactly like this. Obviously, though, my logical progression isn't going to lean into the paradox as this formulation does. Rather, the clear answer is to decide one way or the other at the point of the first split which way Wormy goes. In a width-wise split, the answer seems fairly obvious: Wormy stays with the head end and regenerates, and the tail end regenerates into a new worm. A perfect lengthwise split is much more conceptually puzzling, but it can be solved for all but the final step with the following principle: An individual simply needs a habitable vessel. In a perfect lengthwise split, either side ought to be immediately habitable, but the important point is that both sides are habitable enough that Wormy could go with one or the other. The other becomes a new worm. All we are left with not knowing is which side Wormy ends up in, but there are tons of other things we don't know about planarian psychology also (for example, all of them), so I can't say I'm terribly bothered by leaving myself guessing at that point.

For a more close-to-home analogue than OP gives: Consider a hemispherectomy, which is a very real surgery performed on infants and young children with extreme brain trauma in which an entire cerebral hemisphere is removed. Now, you can probably predict the results, to a point. If the left brain is removed, the child lives with the right brain which remains in the body, because the right brain remains a habitable vessel while the left is not. If the left brain is removed, the child lives with the left brain, which remains a habitable vessel while the right is not. Easy intuitive conclusions both, but they illustrate the habitability principle to a tee; clearly, neither hemisphere contains the determinant of identity, but rather, something that is using the biological system, and simply needs there to be enough functional material to superimpose onto, regardless of what it is. That something... is you. Now here's the bit that I bet you couldn't predict, unless you've specifically studied the neuroscience of this operation (I'm a BA in neuro): regardless of which hemisphere is removed, the child will likely develop fairly normal cognition! I am shitting thee not, the left brain of a right hemispherectomy survivor will develop typically right-brained functions, and vice versa. Take a second to think about what is going on here. There is a zero percent chance that a genetic adaptation evolved to serve as a fail-safe for losing half your brain in infancy, because that is not a thing that ever happened in the ancestral environment to be selected for. So we're left with the only logical conclusion being that this is a dualistic interaction system playing Tinkertoys with good old-fashioned childhood neuroplasticity - the mind has native functions that it needs a working brain to represent faithfully, and it has only half of one to work with, but a half with a lot of malleability, so it MacGyvers what's left into a reasonable approximation of the standard 1:1 interface it's meant to be using. Yeah, nature's fricking metal.

The mechanics of hemispherectomy form one of the absolute best indirect arguments for dualism (not to say the direct evidence is lacking), and it's hiding in plain sight right under neuroscientists' noses. And the exact same dynamics are most certainly at play in planarian fission. It's all spectacularly fun to analyze.