I watched an episode of "The Blacklist" which is a popular show on the TV streaming service Netflix. In the episode, the main character refers to a very tiny marine creature commonly known as the "Immortal Jellyfish". In extreme life threatening situations, an immortal jellyfish is able to revert to a polyp state within seconds. A miracle of nature which is not fully understood. You can read more about it here: https://en.wikipedia.org/wiki/Turritopsis_dohrnii

My question is: if I had a pet named Jimmy and Jimmy is an immortal jellyfish and at some point returns to its polyp state, and eventually transforms back to a mature jellyfish, is it still Jimmy or a different jellyfish?

I'm also interested in how this would apply to humans. If we had that ability and could transform back to an earlier zygote state, would John Doe still be John Doe? There is physical continuity if nothing else.


Welcome to SE Philosophy!

This is what is known in philosophy as a question of identity and is related to the metaphysical discipline of ontology, or the study of what is. In essence, identity is the question of what 'is' is, and is a source of much debate. Are equality and equivalence the same thing?.

Questions of identity related to personhood and the self are addressed in these SE posts.

Questions related to things that change over time are frequently tied to the Ship of Theseus and is old indeed. To wit:

In the metaphysics of identity, the ship of Theseus is a thought experiment that raises the question of whether an object that has had all of its components replaced remains fundamentally the same object. The concept is one of the oldest in Western philosophy, having been discussed by the likes of Heraclitus and Plato by ca. 500-400 BCE.

Note that this isn't really a question of biology, because if a fish becomes a polyp and returns to being a fish, biologists agree it is the same organism. But, are things that change really the same if they become different? With the Ship of Theseus, there are four general responses: yes, no, depends, and it's a meaningless question. That's the universal tell that any answer you get is going to be a result of your metaphysical presuppositions.

This is an interesting question because it is the ship-of-theseus problem in the guise of debate over the nature of biological immortality in the animal kingdom. From an analytically philosophical standpoint, the answer of the question is yes, because it follows from the definition of organisms which is a category which fundamentally accepts the change in state from birth to death. (Biologists would be in trouble linguistically if they had to classify that same living things as different organisms depending on development since there are no clear categories.) In the ship-of-theseus problem, it is less clear because objects are not defined so narrowly in such a way.


Technically, this IS a question about biology. You see, there's a thing called genetics that is very useful in identifying not just species but individuals.

I don't know much about jellyfish reproduction. Maybe they're all "identical twins," for all I know. But is they have genetic variation, then each life stage would presumably have the same genetic code.

Of course, you have to decide whether individual identity is ultimately based on DNA or something else.


Eternal life and youth, rejuvenation to the youthful state - all these are hot topics of the current cell biology and medicine, see, e.g. https://www.nature.com/articles/s41556-018-0206-0 especially after discovering Yamanaka factors that lead cells back into pluripotent state (fully and partially - without preserving identity and functionality and with preserving them). This article https://jetpress.org/v28.1/edelman.htm argues that eternal life leads to the loss of identity due to the bounded amount of memory and memory is the necessary factor of identity. All those questions are dare to the transhumanist and technoporgressives movement, there is also political party of transhumanists.

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    Your statement "memory is the necessary factor of identity" doesn't seem true to me, unless you only mean identity subjectively, for instance: I, the subject, identify myself as John Smith; were I to lose all memory I could not identify myself. Therefore my identity has changed. Not sure if I quite buy that but I could see there being an argument. On the other hand, I identify my grandma, the object, as grammie Doe, even if she suffers from dementia and has lost all memory. In that case, I can and do properly identify regardless of the object's memory loss.
    – ferris
    Oct 17 '19 at 17:15

This is only a question of what we mean by "the same". All "objects" are continuously changing and are not exactly the same instant to instant. They not only change internally but also by interaction with the environment. For simplicity we refer to patterns of experience as being "the same" object if they don't change too much and fall out of whatever catagory that we have placed them in. Our catagories are necessarily ambigious so transitions between them will always be somewhat ill defined.

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