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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In some sense the idea of a separate self is an invention. Names are an invention - the idea that I can be represented as “Kevin” and I am different from other humans. The invention is so obvious nowadays that we take it for granted.

It isn’t unique to humans, though… at least parrots and dolphins also have sequences of sounds that they use to identify specific individuals. Maybe those species are much more “human-like” than we currently expect.

I wonder a lot where to draw the line for animal welfare. It’s hard to worry about planaria. But animals that have names, animals whose family calls to them by name… maybe that has something to do with where to draw the line.