My brother was just in town, and we had our usual argument about Old Man's War, which he loves and about which I'm less enthusiastic (it was a fun read, but...). Perhaps one issue that keeps me from enjoying it fully is that whenever I think about it I think about an early scene, in which a character's consciousness was transferred from an old body to a new body. This is presented in the book as just one more futuristic miracle, but I can't stop thinking about the deeper questions it raises.
What does it mean to transfer consciousness from one body to another? Our current scientific understanding is that there is no consciousness separate from the underlying physical machinery, so such a transfer could not happen. But you might be able to create the illusion of a consciousness transfer, which I explain below. So we can make sense of Old Man's War if we assume that the doctors are deliberately lying about what is going on, covering up the murder that lies at the heart of the procedure.
Here's what might be going on (yes, I realize this is fiction, but good science fiction almost always has a thought experiment at its heart): It should be possible, at least in principle, to create a new body that has identical machinery to an existing body. This is would be new person who is a twin not only physically but mentally, down to having the same memories (by definition, since they have the same brains down to the microcircuitry). From the new person's perspective, he has finds herself suddenly in a "new" body. (This is much like the old philosophical puzzle, what if the world was created yesterday, all of us with artificial memories?)
So now we've got a consciousness that believes itself to have transferred into a new body from an old body. What happened to the consciousness in the old body? The doctors in Old Man's War claim that it is now a vegetable, with no consciousness inside, because that consciousness has transferred. Since that can't happen, they are lying: either the process of creating the new copy of the old brain destroys the old brain, or the doctors deliberately destroy the old brain to preserve the illusion of the transfer (after all, if transfer is impossible, why go through this procedure? It's very nice for your twin to have a new body, but it's not going to do you any good at all!).
Here's the question: does this matter? If John undergoes this procedure happens on a Wednesday, then the world on Thursday is much the same as the world on Tuesday: on both days, there is a consciousness calling itself "John" with roughly the same memories. It only gets tricky when you think too much about Wednesday. You might be tempted to say you have John 1 on Tuesday and John 2 on Thursday, who are duplicates but nonetheless not the same because they have different bodies. But, of course, John had a different body when he was 5yo than when he was 75yo, down even to being made up of different atoms. So if we're willing to call 5yo John and 75yo John the same person, why aren't John 1 and John 2?
This confuses the heck out of me, which is why I have difficulty paying attention to the novel itself.