On the ride into work today I was thinking about some of the Great MysteriesTM. Things like, "Why does anything exist?" or "What the hell is up with quantum mechanics?"
I've always been partial to the mysteries surrounding consciousness and personal identity. Yes, things like quantum entanglement are weird and interesting, but for me they don't pack the same punch I feel when I think about how there could be something like consciousness. All the mysteries about the physical universe seem to concern the same basic sort of "stuff". But the idea that there should be a non-extended, subjective, first-person-only viewpoint on things is really strange.
Even stranger than that—at least in my opinion—is that there should be something like personal identity. I say "like" because we're not even sure what we're dealing with here. The paradoxes of subjectivity creep me out a lot more than the paradoxes of quantum mechanics.
For instance, each of us has the immediate impression of being a continuous person through time, each at the center of their own experience. I not only have memories of being a 6 year old. I also have the intuition that the self who perceived, thought, and lived as a 6 year old is the same person who perceives, thinks, and lives now. It's not as though my "soul" "died" at some point along the way and was replaced by a new one. It's still "me". Part of what it means to be conscious or to be a consciousness is that a whole set of things are mine and are always only mine—in principle.
And yet what is this "me"? Whatever goes on in the mind—be it perception, consciousness, the sense of self, etc.—is at least caused by (if not identical with) what happens in the brain. And yet the contents of those neurons are being turned over all the time, and the cells themselves are also replaced over the course of months or years. None of the atoms composing me as a child are in my body now. If there's nothing permanent in me that is physical, then how could there be anything permanent in me that is subjective, either?
Sartre argued for the same outcome, though he did so from a phenomenological rather than a physical perspective. In his essay "The Transcendence of the Ego" he argues that intentionality is the sole structure of consciousness. It's the "aboutness" of consciousness that makes it what it is, not that it belongs to an ego. There could be an ego in consciousness, like when I reflect on the fact that I'm typing a blog entry. But there's no way to objectify the ego of that second-order consciousness without invoking a third-order consciousness. In other words, consciousness is a purely internal, subjective phenomenon, and as soon as you turn it into something objective—like when you locate the ego in consciousness—you have turned it into the object of a new consciousness, or a new intentionality. The idea that there is a transcendental ego that subsists outside of each conscious state and acts as the "hub" of the multitude of states is an illusion. Each consciousness is its own "thing", absolutely free from other consciousnesses, even within the same "person".
The early Buddhist position is similar to Sartre's. This is a useful book on the subject. Buddha disagreed with the idea that consciousness is pure intentionality, but he arrived at a similar conclusion with regard to the impermanence and illusory nature of self.
Perhaps we can salvage the idea of a self if we assume it is a form or pattern of matter rather than the matter itself. If my sense of self persists despite the constant turnover of matter in my brain, then perhaps it is the arrangement or form of that matter that makes up who I am rather than the stuff itself. But there are problems with this solution, too.
Imagine we have the ability to destructively scan your brain at a resolution good enough that we can recreate its exact patterns (arrangement of matter) in a new brain. Presumably something like this happens in Star Trek. Your old body is wiped out, and a new one replaces it. According to a patternist view of self, you should experience no break in continuity between your old self and new self. It's the same pattern, after all.
But is this true? How do we know the new you is really you? It will act exactly like you. It might even really be conscious. But from the interior, subjective perspective of the old you, maybe you went to sleep and never woke up, and this new person came into existence, having all your memories, thinking it's really you (but it's not). It would be like a replicant from Blade Runner, thinking it had a past when it really didn't.
But, you might say, don't we turn over our matter all the time? Each person goes to sleep every night and wakes up the next morning. Maybe the person who goes to sleep is not the same person who wakes up. But we have good (internal) evidence that this is not the case. Therefore, there's nothing to be concerned about in this scenario. It's nothing that doesn't happen all the time. It just happens all at once rather than taking days, weeks, months, or years.
Even if this is true, we can upset this self-assurance further by adding a detail to the thought experiment. Assume the scan is non-destructive, so that the original you is preserved when the new you is created. Imagine this happens when you are asleep, so you don't even know it's happened. You wake up and find you standing over you! The new you says, "I have excellent news. The consciousness transfer worked. I'm over here now. We won't be needing you anymore!" (I suppose all this depends on your personality. Some people might be more excited to have an army of clones of themselves.)
In that case we have two identical patterns (at least at the point of creation—they'll diverge after that) but also two identical consciousnesses. Clearly you (your consciousness) aren't in two places at once. There's only one "you", laying helplessly in bed as the new you decides the "transfer" was successful. There must be something more to personal identity than a mere pattern.
This goes back to the ontological gap between objective and subjective things. Subjective experiences are essentially internal and personal. They belong to a person, but a person is something that endures over time, throughout change. This concept doesn't show up anywhere in a purely physical understanding of the world. Of course there are scientific ways to study people. My body is also an object of scientific (and generally empirical) inquiry. But the personhood of my person is not. Insofar as patterns are empirical things, they're not capable of capturing personhood.
So this doesn't leave us with a whole lot to work with. In that regard it's not much different from other puzzles, like those mentioned above. But the fact that this puzzle reaches down to who we are on the deepest, most essential level is what's most disturbing.
Why are you you? That's strange.