If every cell in a creature's body were replaced (as in Theseus's Ship), would the creature still have its consciousness? E.g. a white blood cell would be destroyed and replaced by a nanobot that flows around the bloodstream, killing foreign substances; an interneuron would be replaced by a nanobot that sends an electrical impulse when it detects acetylcholine, etc.

Reality-checking Phyresis from Magic the Gathering here.

  • 23
    $\begingroup$ This question may be more appropriate on Philosophy - I think it's far too speculative even for this site. $\endgroup$ – ArtOfCode Aug 21 '16 at 22:26
  • 12
    $\begingroup$ @A.I.Breveleri so do you believe you are concious? $\endgroup$ – JDługosz Aug 22 '16 at 0:22
  • 3
    $\begingroup$ Far beyond our current understanding of consciousness. The only real answer we have today is "Don't know." $\endgroup$ – ApproachingDarknessFish Aug 22 '16 at 3:10
  • 2
    $\begingroup$ The real question is, why do you have to replace my cells to make a copy of me? Since that's all this is. $\endgroup$ – candied_orange Aug 22 '16 at 4:26
  • 2
    $\begingroup$ I downvoted this question because I think worldbuilding is becoming a drop-off site for random questions everyone thinks up while shitting. It can quickly degrade on Quora level. $\endgroup$ – Tomáš Zato - Reinstate Monica Aug 22 '16 at 13:24

11 Answers 11


Truth is - No one knows.

We are so far from perfectly replicating single neuron, and so far from understanding what consciousness is, that no one can tell.

Good thing is - both "yes" and "no" are believable answers in the (lack of) light of current science, so use whichever you need.

  • 1
    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Monica Cellio Aug 22 '16 at 20:15

Yes, by definition

If you're replace something with something that is functionally identical then by definition it must be functionally indistinguishable from the thing it replaces. If replacing the cells with nanobots doesn't reproduce consciousness then they're not functionally identical.

The more interesting question is whether it is possible to replace the cells with nanobots and preserve function.

  • 2
    $\begingroup$ This is the only really correct answer here. The OP has defined the answer in the question itself via their wording. $\endgroup$ – Vality Aug 22 '16 at 16:23
  • $\begingroup$ You would have to be very knowledgeable to understand all that "functionally identical" entails of course. I'm not a doctor, but I believe that an interruption of the electrical patterns in the brain would be comparable to a seizure. So the replacement must not only reproduce function, but also immediate state. There are very short lived aspects of the cell's state that have to be maintained to avoid a temporary interruption of consciousness. If replacement occurred one cell at a time over a long enough period of time I assume this wouldn't be an issue. $\endgroup$ – Stephen Lujan Aug 22 '16 at 17:29
  • $\begingroup$ But it would be a different individual UNLESS it could also copy the stored information (eg memories) from the original cell. $\endgroup$ – JDSweetBeat Aug 23 '16 at 14:20
  • $\begingroup$ @DJMethaneMan Oh right. A memory cannot be stored in an individual cell but the networks of synaptic connections between neurons collectively hold memories. So the difference between functionally identical to a brain cell and this exact brain cell, is the difference between retaining all memories and probably ending up dead from the medulla "forgetting" how to keep your heart pumping and lungs breathing. $\endgroup$ – Stephen Lujan Aug 23 '16 at 16:43
  • $\begingroup$ You should preface your answer with, "From a purely materialistic viewpoint", While this is the viewpoint I agree with, I can see no way to establish it as a certainty, $\endgroup$ – WhatRoughBeast Mar 2 '17 at 18:07

That depends. Do you believe that consciousness is a consequence of having a soul? If not, then if the nanobots faithfully simulate all cell activity, there is every reason to believe that consciousness would be preserved. In this view, consciousness is an emergent property of neural function given the organizational constraints which derive from our brain structure.

If you want something more mystical and less prosaic, then the existence of a soul would presumably not be supported by mere machinery. As to the point of replacement/failure of consciousness, that is not answerable until someone can quantify/qualify consciousness. And that does not seem to be likely any time soon.

  • $\begingroup$ We need to quantify/define consciousness to answer this question, I agree. A "good" definition would then automatically answer the question. $\endgroup$ – Artur Gower Aug 22 '16 at 11:20

Most of the body would not matter one bit. Is a man with a wodden leg still concious? You could change the limbs, bone, blood, intestines, etc. without having any effect on his mind.

As for the brain, replacing a brain cell with a nanomachine that works in exactly the same way would do the trick. It would have to connect to other cells in the same way and have the same behavior. A living cell is a complex nanomachine. So consider replacing the cell with another exactly like it.

Everything we know informs us that consciousness arises from the physics of the atoms making up the brain. (See this question to explore otherwise; and comments and references concerning this fact.) So if a brain cell were replaced with a different nanomachine that was in fact behaviorally identical, it would make to difference.


You are talking about replacing every single cell in a body, which means nothing original would be left. In that case, how is it different from creating a new clone?

If you create a new clone, you would probably have a new human there. The new human would probably be considered conscious, because it moves, but the old human doesn't get unconscious either, if it is a good clone that doesn't affect the original body. So the problem is, is it just the same consciousness, or a new one?

Meanwhile, you can't say that the person becomes unconscious. Consider a person whose neurones are all replaced except the last one. Each other cell is perfectly cloned. As a cell unit, the last cell is still conscious.

If the person is perfectly cloned, to other people it seems that he is still conscious, because he will act like what he was before. But what about the original consciousness? Well, that's the same question as what happens after a person dies ‐ nobody knows.

If you believe in souls, you can mention that neurones are just a physical carrier of a soul, and that nanobots can or cannot carry a soul, depending on your requirements. Does the soul get surgically removed when the brain cells are surgically removed? This is up to you as well. It might even get torn apart by the surgery. Someone has once hypothesized that a cloned human would have the physiological capabilities like an animal, but not the wisdom of a human. Nevertheless, considering the hypothesis that intelligence memory is probably stored in the brain, this might not be true as well.

  • 1
    $\begingroup$ If you suppose there is something beyond the physical functioning of the components, try addressing it as an answer to this question. $\endgroup$ – JDługosz Aug 22 '16 at 7:39

Consciousness is the result of the brain, a mixture of senses,memories and a complex communication between the neurons.

To make an example see neurons as people, having 10 people there doesn't do anything.

But if you give those 10 people something to talk about they will create a conversation, this conversation is the consciousness of the group.

If one of the ten people dies and gets replaced the conversation and so the consciousness will change and lose or gain new qualities.

Over time these 10 people will die and be replaced causing the conversation to deviate completely from the original group.

In short, we all die slowly over time until there's nothing left of what we used to be.

Replacing the neurons of a person with nanobots would be exactly like killing that person and having someone replace them that thinks to be them.

another Example:

1-Jimmy is alive and knows it.

2-Every single cell in jimmy's body gets replaced by nanobots.

3-Jimmy is killed by the nanobots.

4-The nanobots give life to Jimmy-bot.

5-Jimmy-bot is alive and knows it.

6-Jimmy-bot doesn't remember about the existence of the original Jimmy therefore now Jimmy-bot became the true original Jimmy leaving the old one in the oblivion.

  • $\begingroup$ Even after you reach adulthood, your cells are being routinely replaced all the time. This is true, to a lesser extant, even in the brain. Adult Neruogenesis $\endgroup$ – Stephen Lujan Aug 22 '16 at 17:08

This is essentially a new take on an ancient (and I do mean ancient) thought experiment.

The Ship of Theseus. Discussed by Plato and other ancient philosophers.

If you have a ship/boat, and every single component is replaced with an identical replacement (no upgrades). Is it still the same boat?

There are other variations, such as the grandfather axe, where both the axe handle and axe head are replaced.

Your question asks about whether the nanobot body would still be conscious. I think another very important thing to take into account is, is it even the same consciousness?!

  • $\begingroup$ When I first read your question. I didn't see you had Theseus ship in brackets. Did you edit the question?! I think I need to go back to bed... $\endgroup$ – EveryBitHelps Aug 21 '16 at 23:17
  • $\begingroup$ It is not like Theseus ship, quantum mechanically $\endgroup$ – MolbOrg Aug 22 '16 at 1:58
  • $\begingroup$ @MolbOrg, "if every cell is replaced"!!! How is that not Theseus ship. If every cell is replaced the quantum mechanics would be acting on different cells...the actions of the quantum mechanics might be the same regardless, and it could be different...but isn't that the entire point of the thought experiment? Is it the same, or is it different? $\endgroup$ – EveryBitHelps Aug 22 '16 at 6:54
  • $\begingroup$ long story, definitively not for the comments, and probably mater of perception, but if about ship example - if u replace parts not with the wood parts, but steel - I would say at the end it will be different ship, then at the begin. Active parts of cells, everything that makes them work - are not macro objects but numerous molecules. This molecules interaction mesh is that what makes us happening. Even(highly doubt in that) it is possible to find same solution for that system in other therms(other molecules), the whole reason for such replacement is just incompatible with such solutions. $\endgroup$ – MolbOrg Aug 22 '16 at 11:49

Technically, your body does this every ~1 year. And do you lose consciousness every year?

  • 2
    $\begingroup$ +1, but it’s worth noting that some cells in the body aren’t replaced. Their constituents are, but continuously, not abruptly. And, as Mołot commented elsewhere, this is a potentially important caveat. $\endgroup$ – Konrad Rudolph Aug 22 '16 at 10:51
  • $\begingroup$ Could you expand a bit on it? Here on Worldbuilding, we usually like more consistent answers. And without it, yours looks more like a comment than an actual answer. $\endgroup$ – clem steredenn Aug 22 '16 at 11:55
  • $\begingroup$ Yes neurons being replaced are the very first reason for why many memories don't simply get lost but are completely erased. $\endgroup$ – Charon Aug 22 '16 at 12:17
  • $\begingroup$ Brain cells do not regenerate, when they die, they are gone. $\endgroup$ – Cem Kalyoncu Aug 22 '16 at 13:05
  • $\begingroup$ Someone argues that cells are not that important, only connections are, and brain connections continue to grow during whole life. (just hypotesis, one of the most active research area) $\endgroup$ – GameDeveloper Aug 22 '16 at 14:39

Depends on your definition of consciousness.

If you define consciousness as a state of matter (materialist way), then yes. However from that point of view, consciousness itself is meaningless word and everything can be said to have it, even rocks.

However if you believe there is some unique spiritual value bound to consciousness (eg. soul) then no, because method you propose allows (or would allow) for duplication of this entity and we said it's unique.

  • $\begingroup$ no one prevent a spirit from being bound to a particular state of matter, so we can have both spirit and consciousness in a body replaced by nanobots $\endgroup$ – GameDeveloper Aug 22 '16 at 15:18
  • 1
    $\begingroup$ The problem is that if you can replace body with nanobots, you can probably also create a copy of it. That's where the problem is, if there are two copies, which one is the original? $\endgroup$ – Tomáš Zato - Reinstate Monica Aug 22 '16 at 15:31
  • 1
    $\begingroup$ exactly, who will be the spirit in the body? the nanobot spirit or the flesh one? $\endgroup$ – Charon Aug 22 '16 at 16:08
  • $\begingroup$ If you think the second paragraph might be the case, please weigh in on this question. $\endgroup$ – JDługosz Aug 22 '16 at 21:03

Assume you retain no consciousness after the process, since you interact with the world using your human interface (you know hands, feet, eyes etc.) and you already stated that cells keeps their functionality, then replacing each cell by keeping the same functionality keeps the interface functioning with the rest of the world, then you appear to be conscious even if you are not.

So I could ask about your conciousness and you would still answer like if you were conscious making that not distinguishable.

Conclusion: Either you are not conscious already or you will remain conscious after the replacement.


I think there is a fundamental flaw in your question, and that is that each cell is replaced by one nanobot. While that might (might) work for a red blood cell, or a WBC, or a muscle cell, there are no cells in the body with as many interconnections as neurons.

Each neuron may be connected to up to 10,000 other neurons. One nanobot cannot possibly perform the same as a biological neuron.

So no, there would be no mind or consciousness above the level of primitive organisms.

  • 11
    $\begingroup$ why can't some fictional nano bot have 10,000 connections? It seems like a large jump $\endgroup$ – sdrawkcabdear Aug 22 '16 at 2:10
  • $\begingroup$ "up to 10,000 other neurons. One nanobot cannot possibly perform" - yes, it can - the OP said so. This is part of the givens of this question. A cell is a nanobot. $\endgroup$ – AnoE Aug 22 '16 at 12:32
  • $\begingroup$ a nanobot is nothing more than a cell made of different materials..... $\endgroup$ – Charon Aug 22 '16 at 12:33
  • $\begingroup$ Is the objection just that a nanobot the size of a neuron arguably is a "microbot", not a "nanobot"? If so then I think we have to surrender to the imprecise use the prefix "nano" to mean "so small as to be futuristic" ;-) $\endgroup$ – Steve Jessop Aug 22 '16 at 12:35
  • $\begingroup$ @SteveJessop Thanks, I was/am unaware of the terminology. I will do more searching before answering in the future. :) $\endgroup$ – anongoodnurse Aug 22 '16 at 12:45

Not the answer you're looking for? Browse other questions tagged or ask your own question.