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I'm sure everyone is familiar with the Ship of Theseus paradox, basically "If you replace parts of a ship over time until none of the original ship is left, is the end product still the same ship?"

I just read pop philosophy books, so spare me if this is an idiotic question, but could this be applied to the Brain and consciousness? If you were the take individual atoms/neurons, and somehow replace them with its exact copy one at a time, how would the person experience that?

I don't think there would be an interruption of consciousness, but at the end it's a completely separate (but identical) brain from the one you started with. Exactly as if you had just cloned the whole thing, right? But without interruption.

What interests me is the gradual transition of consciousness here, is it possible to know how that would work?

I think I remember hearing people argue similarly for both the existence of a transcendent soul and also AI consciousness. And the only other discussion I found of this was on a flat earth forum, so I don't know what to make of that. https://www.theflatearthsociety.org/forum/index.php?topic=61053.0

I hope I articulated my question clearly. Thank you!

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    This is what actually happens to every organ in your body, except the brain! Well, it was recently discovered neurons can regenerate, they just do it was less often than other cells. But do you notice your skin replacing itself over time? It always feels like “your skin”, right? The same skin as always? The brain isn’t philosophically different in this regard, though biologically it must be or it would regenerate just like skin. I think the biological issue is neurons are not fungible, because, unlike a skin cell, each neuron has a specific and important relationship to its neighbors. – Dan Bron Sep 10 at 12:01
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    The Ship of Theseus argument is about individual identity, and this it applies to every "individual", human body (and brain) included. The comment above about skin (and nails) applies to all the cells of our body: thus, the problem is the same. Our body is still "the same" also if (after a certian amount of time) it is composed of totally different atoms. – Mauro ALLEGRANZA Sep 10 at 12:10
  • The issue about consciousness is different : where it is located ? In the brain (maybe) but is it the brain itself or something on top of it ? – Mauro ALLEGRANZA Sep 10 at 12:11
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    Consciousness is associated with dynamic patterns of neurons firing, it is not composed of neurons the way the ship is of its parts. The pattern is not replaced piece by piece, so replacing the neurons does not even create the paradox. – Conifold Sep 10 at 17:37
  • I actually don't think you did articulate your question clearly enough for this site. What do you mean exactly by wanting to know how the "gradual transition of consciousness" would "work"? If you can be more explicit and define your terms, we can try to answer it, but otherwise I would vote to close. – Chelonian Sep 10 at 20:24
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David Chalmers discusses this type of thought-experiment in his paper Absent Qualia, Fading Qualia, Dancing Qualia, using it to argue that if there are "psychophysical laws" relating physical patterns to subjective experiences or qualia, then one might expect them to respect a principle of "organizational invariance" where computationally equivalent systems would experience the same qualia even if they are made of different kinds of physical structures (real neurons being gradually replaced by artificial substitutes that have the same input/output relationship, for example). You can read more about his views on consciousness and psychophysical laws in this paper and even more in his book The Conscious Mind.

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As consciousness is still somewhat a matter of philosophical speculation, I think two important philosophical concepts are at play here.

The first is functionalism or the idea that consciousness is state of the brain:

For (an avowedly simplistic) example, a functionalist theory might characterize pain as a state that tends to be caused by bodily injury, to produce the belief that something is wrong with the body and the desire to be out of that state, to produce anxiety, and, in the absence of any stronger, conflicting desires, to cause wincing or moaning. According to this theory, all and only creatures with internal states that meet these conditions, or play these roles, are capable of being in pain.

In this case, the structure of the neurons is not even strictly important as long as the neurons function in the same way. So, let's say you take away a neuron, and put it back. One could argue that even if it were put back elsewhere, or in a different manner, as long as the end result was the same (let's say the flow of neurotransmitters and neuromodulators was essentially identical), then yes, one could argue you would have the same mind. This is supported by the idea that the brain has neuroplasticity and has redundancy built into its operation. In fact, one can have an entire hemisphere removed from the brain, and thrive. One neuron? Seems unlikely to bring a collapse of the system any more than cutting a branch from a great oak and then grafting back on would affect the functionality of a tree.

On the other hand...

There is the butterfly effect (sensitive dependence on initial conditions) which posits that small perturbations in certain types of systems can cause radically different outcomes in the behavior of the system.

In chaos theory, the butterfly effect is the sensitive dependence on initial conditions in which a small change in one state of a deterministic nonlinear system can result in large differences in a later state.1

How do we know that consciousness isn't affected in such a way? Imagine pulling a fuse from a car, replacing, seemingly exactly only to have a small difference in seating result in a tiny arc of electricity that shorts the entire systems bringing electrical death to the car almost instantaneously?

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The principle of identity is that each thing is identical to itself. The ship that you have replaced all the parts of, bit by bit, over time, is no longer the same ship but not because you have replaced all its parts but because no thing is identical to what it was at any point in the past.

So, whatever you do to a thing, change it or leave it alone, it will remain identical to itself and it won't be identical to what it was at any point in the past.

The notion of an identity that would be carried over across time come what may is a fiction. Humans don't have such an identity. However, they do have the sense of their identity as a person. This sense of identity is not so different from the notion of identity whereby a thing is identical to itself. Our sense of identity tells us we are what we feel, what we remember, what we are able to do. In other words, we feel we are what we are.

Memory is normally the crucial element of our identity. We remember our name, who our family is, our friends, what we do in life, etc. Yet, suppose it was possible to replace all that we remember in one go. Suddenly, we would remember being... ourselves, because our brand new memory would just be the new ourselves. This would make no difference to our sense of identity. We would feel we know who we are. Other people would notice, of course, but only if their own memory wasn't also replaced.

The notion of identity the question is referring to is instead the notion of personal identity, whereby we are the same person as the person we were at any point in the past. Yet, we change. Our body evolves, and our mind is affected by experience of life. But we have the sense of our own identity, whereby we sense we are the person we remember being.

Mostly, it works fine. We behave as other people expect us to behave and we are not ourselves surprised at what we do, the way we do it, mostly. However, there can be mishaps. Mental illness can change us beyond recognition.

Suddenly, we no longer can do what we nonetheless clearly remember that we did routinely in the past. Or we develop an interest in something we distinctly remember we were never interested in. Other people notice, and we notice that they notice.

Yet, strictly speaking, we are still identical to ourselves because each thing is identical to itself. The problem is, our sense of identity now plays tricks on us. There is now a discrepancy between what we are now and what we remember, and what we remember still makes us feel we know who we are. But in such cases, we are no longer so sure.

So, you can replace all the atoms, or replace all the memories in one go. That won't matter. We would no care because we wouldn't even notice. Where we will notice is the less than philosophical conundrum of old age, senility and mental illness, where suddenly the fiction of our identity as a person just evaporates. Leaving us just identical to ourselves.

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