Well, one of the problems, anyway.

Assume for the sake of argument that practical immorality has been attained. Whether by magic, science or science indistinguishable from magic, doesn’t matter. But the end result is that  perfectly normal modern Homo Sapiens can get the treatment and become functionally immortal. They’ll be a physically healthy 30-year-old forever, or at least until killed by violence or impressive disease or whatnot. There are no Twilight Zone-esque twists or downsides; you don’t spend eternity in a box or in pain or insane or any such. It’s just honest to goodness proper eternal youth and health. Yay.

Now, there are a lot of well-established problems with that, such as population and cultural stagnation. But here’s the one I’m currently pondering: at what point does the brain run out of space?

There have been people well past 100 who still have pretty good minds and memories, so it seems the brain can handle a century worth of data storage – no doubt with a lot of compression and editing. A century makes sense… humans have not greatly extended the maximum of the human lifespan. Sure, the *average* lifespan has increased vastly; throughout most of history, making it past 40 would be a notable achievement. But there have always been old folks… Plato, for example, was about 80 when he died. But since 100 is approximately as old as humans get, there’d be no good reason for the brain to have more capacity than that; it’d be simply wasted capacity, and no reason why it might have evolved. If there is excess capacity that gets used for other purposes earlier in life, then as that brain-space becomes converted into storage space, capabilities used earlier will be lost.

So. Once humans are made immortal, at some point their ability to keep recording new memories is going to become a bit problematic. It seems unlikely that recording will simply stop. Instead, I imagine that at least initially, older memories will become fuzzy as they get compressed and overwritten, a process that’ll be slow and perhaps will go by unnoticed for a good long while. And then one day someone will ask the immortal about their childhood, and the immortal will realize that it’s all gone. Or perhaps after enough time the memories just start piling up into an uncorrelated mishmash, eventually turning the brain into a non-functional mess.

What to do here? I’ve seen some statisticians claim that if humans could avoid death by the effect of aging and the more common diseases, human lifespans might average anywhere from a few thousand to a few tens of thousands of years before the effects of accidents and such wipe them out. Even at the lower end, that means perhaps dozens of lifetimes of memories that will be simply lost. One approach might be to simply say “screw it” and let it ride. I’m *guessing* that some forms of memory will remain… you won’t forget how to speak your native language or do basic math, or forget how to walk or tie your space-shoes, because these will be memories that are constantly being refreshed. But friends and family, entire careers, might simply fade away to nothing. Joe Schmoe at age 400 might remember absolutely nothing at all about his life until around the age 320. The fact that he was President Of Earth at age 52, and spent his years from 124 to 197 in prison for a string of cannibalistic homicides and was a leading colonizer of Alpha Centauri from 244 to 297, might be completely unknown to him. Or perhaps those facts might be known to him in the same way that someone today might know the facts about the life of Abe Lincoln… he read about them in a book or saw a documentary, but the facts don’t have any particular meaning to the immortal. It was just some other guy, all those years ago. And this possibility raises further ponderables. Let’s say that Joe Schmoe was a major war criminal at age 263. Oversaw the slaughter of thousands, destroyed whole civilizations, talked loudly in theaters. But it’s not until he’s 700 years old that he’s finally caught. But his very last memory of his time as Head Honcho of the People’s Democratic Socialist Republic of Boulder faded into oblivion nearly 300 years ago. He’s been two completely different people since then. Is he still That Guy? Is he responsible, in a legal or ethical sense?

Another approach might be to record memories on some form of backup drive. A hundred years from now, chances are good that something you could stick in your head would be capable of recording a brains worth of data. But once you’ve accumulated a millennium of experiences… how the hell will a person sort it all out? And of course, if memories can be recorded artificially, they can be manipulated. Erased, tweaked, twisted. Transferred from one person to another. Heck, color-reversed, fade corrected, the hue and saturation jacked up and the volume turned down. And what will *that* do to people? We all have memories we’d love to ditch. Imagine having centuries worth of mistakes you’d just as soon not have, and the ability to delete them. Santayana might have something to say about that.