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.
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.