“Cartesian what?”

“That consciousness is not caused by physical processes but by something else — some sort of soul if you will.”

“I guess not.”

“Well, then I guess I shouldn’t-“

“You promised!”

“Ok, but don’t say I didn’t warn you. There’s a famous thought experiment that goes like this: In the future, a teleportation device has been invented. It works by creating a perfect copy of whatever is put into it at the receiving end and then destroying the original at the sending end. If you use the machine to travel to Hawaii or something, what comes out will have all your memories, behave exactly like you—it would be impossible for anyone to tell the difference. Now the question is whether this is you—in other words, would you be willing to use this machine?”

“Um…if the thing that comes out is a perfect replica of me I don’t see why not”

“Really? Doesn’t your body at the sending end get destroyed though?”

“Well, my body gets slowly replaced piece by piece every few years anyways. If what comes out is exactly like me for all practical purposes, then there is no reason to think it isn’t me. This is a cute example and all, but I really don’t see the point.”

“Well, Ok then, let’s try another scenario. You figure out one day that there is a perfect replica of earth in some alternate universe. Of course this other earth has a perfect replica of you, your family, your hometown—everything you care or know about. Unfortunately, someone tells you that it isn’t Ok for these two planets to be exactly the same and that to make them different, either you or your replica has to die. He then surprisingly offers you the choice. Which would you want to survive?”

“Oh, interesting, I guess my copy will probably be getting the same talk and question, but I would clearly want myself to live. Whether this actually effects the outcome is unclear. I understand the other example better now, but even so, there are never going to be two of me after I step into the teleporter.”

“I’m not sure you’re getting the point, doesn’t the example with the alternate universes show that even a perfect copy of you isn’t necessarily the same as you?”

“Hmm, I guess. There’s still no reason to think that what comes out of the receiving end isn’t me though.”

“Well, let’s go back to the point about there being two of you. Imagine if the teleporter sometimes developed problems and destroyed the sending end copy seconds to minutes after the receiving end copy was created. Now which one is you?”

“Ah, so in this situation the sending end copy is me and the machine would basically be killing me. But it seems so artificial for whether the receiving end copy is actually me to depend on the specific timing of things. Therefore I guess I would definitely never want to use such a machine.”

“Right.”

“Wait, the scariest part about this is that if for whatever reason I did teleport, the me that comes out the other end would have all my memories and therefore remember going through the teleportation machine safely. It would actually think that teleportation is a good idea!”

“Yeah, you can imagine some poor sucker who thinks he’s gone through thousands of safe teleportations, not realizing his experience only started at the end of the last one and that he just has a bunch of fake memories of all the previous copies of himself.”

“Wow, that’s sort of terrible.”

“Anyways, so now that we are on the same page about that—“

“Wait a second, suddenly my consciousness is tied up to the fixed physical elements of my current body. Why isn’t that silly for the reasons I was talking about before?”

“Oh, so here it helps to think of your consciousness as some instance of some complicated process run using the bits and pieces that make up your body. While the pieces may switch out over the years the specific instance of the process continues. When your body is completely destroyed by the teleporter, the instance of the process stops and you stop experiencing permanently—in other words, you die.”

“Ok, fine, I guess that makes sense. The only problem is that I know of thousands of times in my life that my instance of this process has shut off.”

“Ah, you seem to have hit the crux of the problem. Every time you sleep, right?”

“Yeah.“

“Are you sure this isn’t a problem with the argument but instead something that the argument is telling you about sleep?”

“Stop trying to scare me, I have personal experience of surviving sleep thank you very much.”

“Um…do you remember the sucker who thought he had been through hundreds of safe teleportations? Another instance of the process drawing on the same intact pool of memories does start when you wake up, after all.”

“…”

“Anyways, good night! I guess since there is a good chance that I won’t ever be seeing you again, I should be saying good bye too.”