Ivan was sitting on the cot, elbows on his thighs, slouched forward and looking at his hands. At Dr. Kemp’s entry, Ivan looked up and Kemp looked into his eyes.

Silver eyes. In a chrome face.

“I looked in the mirror this morning,” Ivan said. “I wanted to cry, but I can’t. I don’t think my new head supports the function.”

“That’s an odd way to phrase it,” Kemp said, sitting down on a stool.

Ivan gave him a sad smile. “Small joke. Very small. Apparently the transformation hasn’t improved my humor.”

Dr. Kemp attempted a quick smile in return. “You sound like you’re still Ivan. That’s something, anyway.”

“I’ve been going through my memories, which seem to be all there. There are no obvious holes or gaps, anyway. I’ve been thinking about stuff, and how I acted, and whether it seems to make sense now. I’m embarrassed about the same stupid things that I’ve done, proud of the same things, and so on. I still love my wife. I’m still worried about my family. All my past behaviors and attitudes seem to be consistent with how I think and feel now. I still seem to be me. But I can’t be, can I? There’s nothing left of me. I’m a machine that thinks it’s Ivan Pritchard.”

“Ivan, there’s an old saying that you replace every atom in your body every seven years. Reality isn’t quite that simple, of course, but if you could actually tag atoms, most of me wasn’t me a decade ago. Cells die, get replaced. Food comes in, builds more cells, and so on. I’m literally not the same person I used to be. So it’s not the specific matter that makes you, you.”

Kemp sat back. “There’s a thought experiment about this. Let’s say you suffer a stroke, and it affects, oh, I dunno, your sight. They’ve invented an artificial neuron, and it’s a drop-in replacement, so they replace the damaged part of your brain and your vision is fixed. But are you still you?”

Kemp tapped the side of his head. “Part of your brain is now artificial. But you’re still you. Now, a few years later, the same thing happens to a different part of your brain, with the same fix. Do that a few dozen more times over the years. Now thirty percent of your brain is artificial. Are you still you? Keep it going until ninety percent of your brain is artificial. At what point do you stop being you? There was no discontinuity. You didn’t wake up a million miles away. You were you from one moment to the next, you have continuous memories of everything, except when you’re sleeping, of course.