Assumption A1:
Every aspect of the state of a neuron which is important for how it will function in future as a part of the brain can be measured.
Assumption A2:
Every aspect of the neuron mentioned in A1 can in principle be affected in such way that a neuron can be in principle returned to some state which was measured at time t1.
Assumption A3:
We can measure the input and output signals of the neurons, and reproduce signals of same nature, in same timely manner.
So, we take a brain of John, and do this:
- We record internal signals between the neurons in John’s brain, and the signals that come from his senses to each of the brain neurons starting from time t0 for some time-span dt. John is conscious over the time dt. dt can be for example 30 seconds.
- As we now know the outputs of each neuron we construct replay neurons, which when started fire the exact same outputs as the original neurons did starting from time t0 till the end of the dt period, in same timely manner as the original neurons did. Inputs can be connected to those neurons, but they ignore those.
- We reset (A2), all of the neurons to their states at t0, and replay the inputs which come from the senses to any of the neurons for the dt time period. Also at the start we replay the starting inputs to each of the neurons that don’t come from the senses, and which were measured at t0. Will John be conscious for this replay-dt time period?
- We change one of the original neurons with it’s corresponding replay neuron, and repeat everything as in 3. Let me point that if we take the neurons as black-boxes, i.e. abstract from what is happening inside them, everything about the brain looks same. The inputs and outputs are the same, and John behaves same.
- We replace more and more of the original neurons with the corresponding replay neurons, and repeat everything as in 3.
Questions:
Q1: Would you agree that after changing all the neurons with their corresponding replay neurons, it can’t be true that John has conscious experience through the replaying?
Q2: If you agree with Q1, would you agree that it can’t be a matter of sudden turning off of conscious experience, but that it will be a gradual process?
Q3: If you agree with Q1, what is that which is lost in the John’s brain and which corresponds to having conscious experience?
A brood comb 
Well,
the brain is not the sole player in it’s own changes, Tanasje.
External factors like radiation, food and even the thinking that we do changes the brain’s structure.
But if the brain gets in a previously recorded state, it will be exactly the same as the previous situation. Of course, it’s neural connections must be the same, too; and this is impossible, since neural connections constantly change.
Goran.
Hi Goran,
The thought experiment here was supposed to concentrate on the features of the brain which on some accounts are essential for having conscious experience, and which would be part of explanation of having a conscious experience. I take it, for example, that it is safe to say that while food is required for working of the brain it is not part of explanation of why there is a conscious experience connected to the functioning of the brain. Even more so for some more incidental phenomena like radiation or being hit by an axe.
It well might be that this kind of idealized/abstract take on brain is not just practically, but also in principle impossible, and that this impossibility has some connection to the mind-body problem. In fact, I would agree with such view, and this thought-experiment is part of bigger argument by reductio going in that direction. If you are interested in my attempt to do more detailed analysis of this, you can check this essay that I wrote year ago or so.
“Assumption A2:
Every aspect of the neuron mentioned in A1 can in principle be affected in such way that a neuron can be in principle returned to some state which was measured at time t1.”
If the whole brain is “rewinded” into a previous form, i think that it will be the same as sending the person back in time. Things will be the same for him as it was in “t1”.
Goran.