If functionalism is correct, then digital brain emulations will experience the same consciousness as biological brains. Yet functionalism does not address questions of personal identity and cannot help us determine if an upload maintains continuity of consciousness with the original brain (Chalmers 2010). If I go to sleep at my local upload center in the year 2080, will I wake up in a computer simulation or will a newly created conscious entity awake for the first time complete with false memories? What will happen if I choose nondestructive uploading and keep my original self intact? The three traditional views of personal identity cannot help us answer these questions. Many of the same problems encountered in split-brain syndrome recur and there are also new dilemmas unique to uploading.

Psychological identity asserts that a destructively uploaded brain retains continuity of consciousness since it maintains psychological continuity. If we nondestructively upload and keep the original brain intact, psychological identity cannot tell us which of the two retains continuity of consciousness (recall the psychological theory has the same problem when both Righty and Lefty survive). The closest continuer theory would also state that a destructively uploaded brain retains continuity but is similarly unable to determine whether the upload or biological brain retains continuity of consciousness in nondestructive uploading.Footnote 11 The psychological and closest continuer theories can also claim that neither the brain nor upload maintain continuity of consciousness after nondestructive uploading. However, it seems ad hoc to claim that where there are two equally identical psychological or closest continuers of a brain, neither retains continuity. This could be added on as an extra axiom but it does not logically follow from either theory. Claiming neither the brain nor upload maintain identity leads to the same problems the theories encountered when denying continuity to Right and Lefty: once again continuity of consciousness in one entity depends on what happens in space and time to a completely independent entity. The last option would be to claim that both the brain and the upload maintain continuity of consciousness. This is the only solution that can make specific predictions about identity that avoid dependence on what happens in space and time to an independent entity. Yet this possibility is often ruled out as absurd and many philosophers add a non-branching axiom stating identity can never branch (Swinburne 1984; Olson 2010).

We have just shown that psychological and closest continuer theories are unable to make clear predictions in nondestructive uploading as long as they adopt the non-branching axiom. It can also be shown that that the two theories lead to absurd conclusions if they accept the non-branching axiom. There is a strong affinity between these theories and the identity theory of mind we reviewed previously when we defended functionalism. Psychological and closest continuer identity both claim that continuity of consciousness is identical to a particular property.

This identity claim leaves both theories open to a type of fading/dancing qualia argument that we will call popping qualia. We will examine popping qualia arguments in each of the four possible outcomes of nondestructive uploading: only the brain maintains identity; only the upload maintains identity; neither brain nor upload maintains identity; and both brain and upload maintain identity.

To invoke the popping qualia argument, first gradually nondestructively replace all the neurons in the brain with nanomachines. Now assume only the biological brain maintains continuity of consciousness. Suppose we turn off the neurons as they are replaced by the nanomachines. At some point in this process when greater than half the brain has been replaced (from split-brain syndrome) the continuity of consciousness will suddenly terminate. If functionalism is correct, a new conscious entity will be created at this point in the brain/machine hybrid with false memories. This new entity will be functionally identical to the original and have the exact same conscious experiences. Once the brain is completely replaced by our nanomachines, we start turning the neurons back on one by one. The newly created consciousness could continue during this process or be replaced by another conscious entity. Neither the psychological or closest continuer theory makes clear predictions in this building up of a brain scenario. For the current argument it doesn’t matter if a third conscious entity is created or not. To invoke the fading/dancing qualia argument, we repeat this brain replacement and reinstatement at an accelerated pace. There is no reason in principle why we could not complete one step (replacement or reinstatement) every 30 ms (or less). This time period is less than the minimum time increment of conscious perception (Efron 1970; Ruhnau 1995; Breitmeyer and Ogden 2000; Pockett 2003; Coren et al. 2004; Dainton 2014). Psychophysical studies also show that events that occur within 40 ms or less of one another are perceived as simultaneous (the exact time varies by stimuli type) (Efron 1970; Ruhnau 1995; Breitmeyer and Ogden 2000; Pockett 2003; Dainton 2014).

Returning to our example, two things could be happening as we repeatedly replace and reinstate the brain. The first possibility is that a new conscious entity could be appearing within the minimum phenomenal time span of consciousness. This would create a bizarre form of popping qualia where the newly created entity bursts into existence in the middle of the minimal temporal phenomenal experience. In this case either the experience is shared between two conscious entities (the last moment of one and the first moment of the second) or the new conscious entity begins at the next moment of experience. Neither option seems desirable. Yet things get much worse when we consider the effect popping qualia have on the stream of consciousness. Because our stream of consciousness extends for several seconds in working memory, the new conscious entity would understand that the last several seconds of time was not experienced by them. By the design of our experiment the functional structure of the brain/machine hybrid is identical at all times and would not deviate in any way from the original brain. Therefore the subject could not act on these bizarre popping qualia experiences and we have the same absurd situation created in the fading/dancing qualia arguments. In addition, many complicated concepts take the full capacity of working memory to understand. If a new conscious entity was created in the middle of understanding a difficult sentence or mathematical equation, they would seem to have access to only half the needed awareness. Once again there would be no behaviorally detectable difference and these missing qualia would be another violation of Chalmers’ principle of structural coherence.

The second possibility of our replacement and reinstatement of the brain example is that we are already Nagel millisecond-people who only exist for one quantum of phenomenal experience and therefore nothing would change in our experiment.Footnote 12 In this case personal identity has always been an illusion. So we are left with either popping (and missing) qualia or an extreme skepticism about identity, neither of which seems plausible. The popping qualia argument would be identical if we were to claim only the upload maintains identity in nondestructive uploading. It can be easily seen that the argument also works if we were to claim neither brain nor upload maintains identity. The argument does not work if we claim both the brain and upload maintains continuity of consciousness. Therefore we have shown psychological and closest continuer theories lead to absurd conclusions if they include non-branching axioms.

We have been focusing on the psychological and closest continuer theories but we can apply many of the same arguments to the biological theory of identity. Biological identity is the only theory that seems to give a determinate answer in nondestructive uploading: only the original brain will retain continuity of consciousness. Yet biological identity shares the same links with the identity theory as the psychological and closest continuer theories and is therefore also vulnerable to popping qualia arguments.Footnote 13 Proponents of biological identity would claim that only the brain maintains identity in nondestructive uploading and we just saw that this leads to popping and missing qualia. Biological identity would also deny continuity of consciousness to a destructive upload and it can easily be seen that this would also lead to popping qualia.