In our case, we are limited by our brains. If we can replace everything—cells, tissues—as we live, then there would be presumably no limits to how long we could live. But if we replace our neurons, we’re also rewriting our own experience in the process, and cease to be ourselves. I think that’s the real penalty for extending life beyond the natural maximal lifespan of a neuron, which is 120 years or thereabouts. That’s where I see the real limit. How do we prevent our brains from effectively losing mass over time, losing neuronal connections, losing synapses, which is where we’re storing memories and experiences?