I am very concerned with the general attitude towards cryonics and body preservation in general. People who reject these as worthwhile as far as I can tell fall into two primary camps: the probability of revival is too low to justify the monetary sacrifice or that personal identity is not transferred in the revival process. The first issue does not worry me much. Restoring brain function or some equivalent is an engineering problem, a practical problem. Monetary cost is an unfortunate problem, but it is also a practical problem. The other issue however is more of a philosophical one. Even if the technology to restore a preserved brain or upload it into a simulation becomes viable technologically and monetarily people may still reject it for philosophical reasons. Practical problems can be solved through sufficient research and design, but philosophical problems may never go away.
Regarding synthetic brains or brain simulations, I have heard time and time again people claiming that any brain created in such a way will not have the same identity as the original. If someone's brain is scanned while he or she is alive and a synthetic or simulated brain is created and run, then I agree that two separate identities will form. The problem, I think, is that people imagine this particular situation and generalize its conclusion to all possible scenarios regardless of context. Obviously if the scan is performed after the original brain ceases to function there will not be any parallel consciousnesses to diverge from each other.
Some people will then argue that a synthetic brain or simulation cannot even in principle carry over the original consciousness, that personal identity is not transferred. I will try to provide an informal sketch of a proof here of the contrary, that personal identity for all intents and purposes can be transferred over to a synthetic or simulated brain.
#1 There is a brain device that manifests consciousness using neurons or some functional equivalent. It may be a natural biological, synthetic, simulated brain, or a mixture of these.
#2 There is a procedure that is to be performed on the brain device that will replace some neurons with functional equivalents such that neurons in the unaltered regions of the brain device will not behave any differently throughout time in the presence of the replaced neurons than they would if no neurons were replaced as long as the external stimuli (sight, touch, smell, etc.) is the same in both cases. This procedure, even if every neuron is replaced in one go, is completed faster than the individual neurons can react so that it won't lag behind and cause syncing issues between the unreplaced and replaced neurons. For the case of uploading one can imagine that neurons are removed and sensors are placed there to record what would have been the inputs to the removed neurons. A computer calculates what the outputs of the removed neurons would have been and sends this output to a biological interface connected to the unremoved neurons.
#3 There is a placebo procedure that gives the subject the appearance of the actual procedure having been performed without any neurons actually being altered.
#4 There exists a number N such that if any N neurons of a brain device without any degraded consciousness are altered while not affecting any other neurons, then the brain device will not suffer any significant cognitive impairment. This basically means that a small portion of the brain device can be altered without a significant loss to consciousness or identity, even if those portions are completely removed.
#̶5̶ ̶S̶c̶i̶e̶n̶c̶e̶ ̶a̶n̶d̶ ̶o̶b̶s̶e̶r̶v̶a̶t̶i̶o̶n̶ ̶i̶s̶ ̶n̶e̶c̶e̶s̶s̶a̶r̶y̶ ̶a̶n̶d̶ ̶s̶u̶f̶f̶i̶c̶i̶e̶n̶t̶ ̶t̶o̶ ̶e̶v̶a̶l̶u̶a̶t̶e̶ ̶c̶l̶a̶i̶m̶s̶ ̶r̶e̶g̶a̶r̶d̶i̶n̶g̶ ̶t̶h̶e̶ ̶p̶h̶y̶s̶i̶c̶a̶l̶ ̶w̶o̶r̶l̶d̶ ̶a̶n̶d̶ ̶t̶h̶e̶ ̶m̶i̶n̶d̶.̶
#5 Consciousness can observe and evaluate all aspects of itself relevant to itself.
Suppose the procedure is performed on N neurons of the original brain device. By #4 the subject does not incur any significant impairment. The subject does not notice any degradation in consciousness or identity or any change at all compared with the placebo procedure, for if it did then it would cause a behavior change to reflect this which is impossible since the replaced neurons are functionally equivalent to the originals and the unaltered neurons will behave the same as if no neurons were replaced.
There is not, even in principle, a method for observing a degradation of consciousness or identity after N neurons are replaced by the procedure since the replaced neurons are functionally equivalent to the originals. If the subject noticed any change whatsoever then the subject could, for example, raise a finger to signify this. But the subject's behavior is the same whether the actual procedure or placebo were carried out. As long as the subject is given the same external sensory information, the subject cannot distinguish which procedure took place. From an internal point of view the consciousness cannot distinguish any degradation or change of any kind in itself. By #5, there must not have been any alteration relevant to consciousness. Assuming that consciousness and identity is an aspect of consciousness, then there is no degradation of either.
Assume that the procedure will not degrade the mind if performed on kN neurons, where k is some positive integer. Suppose the procedure is performed on kN neurons of the original brain device. The resulting brain device does not have degraded consciousness. Perform the procedure on an additional N neurons with a negligible lapse in time since the former replacement. By assumption #2, altering N neurons on a non-degraded brain device will not cause any significant effect to its mind so the mind is still capable of evaluating any potential changes to its consciousness. Furthermore, since the N neurons just replaced are functionally equivalent to the originals the behavior of the brain device cannot be different from the placebo procedure that gives the subject the appearance that the N neurons were replaced. Since the behavior is indistinguishable from the placebo, the subject cannot have noticed a change or degradation in consciousness for if it did a difference in its behavior would signify this. As explained previously, there is no method even in principle for the subject to observe any degradation since its behavior is unaltered in any case. By #5, the procedure of replacing (k + 1)N neurons will not cause any degradation or change of consciousness or identity.
By mathematical induction, the procedure performed on kN neurons will not cause any degradation to consciousness or identity for all positive integers k where kN is less than or equal to the total number of neurons in the brain device.
I do not know how high N can be for human brains, but based on brain damage survivors it is likely to be quite high. N is at least 1. Therefore any number of neurons can be replaced by the procedure in a single iteration without any observable degradation. This implies that the entire brain device can be replaced in one go without any degradation.
This informal proof can be made much more general and rigorous. For example by replacing closed volume regions instead of individual neurons since the brain uses more than just neurons to function. Regions could be replaced with devices that interact with the region boundaries in functionally the same way as the original material. One can go into arbitrary detail and specialize the argument for cryonically preserved people, but I think the general point of the argument is clear. The argument can be extended to neurons that have partially random behavior. The conclusion would be the same regardless.
Imagine that someone developed such a procedure. How would one evaluate the claim that the procedure does or does not degrade consciousness or identity? A philosophical or metaphysical system could be applied to generate an absolute conclusion. But how could one know that the philosophical or metaphysical system used corresponds to the actual universe and actual minds? Observation must decide this. If one accepts that different philosophies of mind with different conclusions each have a probability of being true, then observation must be what narrows down the probabilities. If one is less than certain of one's own philosophical conviction, then one must observe to decide. My proof was a thought experiment of what would occur if one were to experimentally test whether the procedure affects consciousness. Consciousness itself is used as the standard for evaluating claims regarding consciousness.
Do you all find this reasonable? Crucially, do you all think this might convince the people who deny synthetic and simulated brains for philosophical reasons to not choose death for the sake of philosophy. Dying for philosophy is, in my opinion, no better than dying for religious dogma. Science, observation, and grounded reason should be the evaluator of physical and mental claims, as I hope my arguments reflect.
After reading the comments and thinking over the matter I can see how people can justifiably disagree with this.
I used the term consciousness vaguely. Replacing a part of the brain with a functional equivalent does not alter the future behavior of the neurons that are unreplaced. However, the unaltered part of the brain not being able to tell a difference does not necessarily imply that consciousness was not altered. One can conceive that the removed part had consciousness inherent in it that may not be manifested in the same way in the new replacement part even though the rest of the brain does not react differently.
Corpus callosotomy severs the connection of one half of a brain from the other half. People seem to retain consciousness after the procedure and each side of the brain then acts independently, presumably with independent consciousness. This implies that consciousness is manifested throughout the brain.
If the right side of the brain is replaced with a synthetic one that interacts with the left side of the brain in the same way, then the left side doesn't notice a difference. However, the left side does not necessarily know if the consciousness in the right side is now manifested in the same way or manifested at all.