I read Permutation City by Egan a few years ago, and it made a lasting impression. The ideas in that novel were big, at moments difficult to grasp, and most interestingly to me, philosophically and morally challenging and fascinating. I knew I'd be back for more Egan at some point.

Quarantine scratched the same itches that Permutation City did, and I'd argue that it's a more accessible book for most as well, thanks to how traditionally the narrative starts: there's a PI with a dark history, and a missing person he was hired to find. The (great) world building gets done while the main character is going through procedures familiar to anyone experienced with the mystery genre. But after about a quarter or so of the novel, the narrative takes a turn, and the science and philosophy quickly becomes the star of the show.

The core of the book explores some of the implications of the observer problem in quantum mechanics, which I am not even moderately qualified to talk about or attempt to explain. And I'm sure anyone who is qualified to talk about it would wave off the science in Quarantine as impossible, far-fetched, or misguided. (And if they wouldn't, this book's premise immediately gets more frightening.)

I enjoyed all of this stuff, even though I felt like I was barely keeping up with it, but the big idea that Quarantine explores that I'll probably continue to think about for a long time to come is choice. The book uses the observer problem to explore choice - every time you make a choice and act on it, you are effectively killing off all the versions of you that made different choices, and where you exist today is a result of the elimination of all of those possibilities.

What makes this idea even more interesting, is how outside influences impact those choices. In the novel, the character had literal modifications made to his brain that made him more "efficient." Thanks to these mods, he was able to survive challenging work situations, and not only avoid the feelings of despair from losing his wife, but to go on living happily with the ability to communicate with a simulated version of her that lives in his brain. (Think something along the lines of Ryan Gosling's program girlfriend in Blade Runner 2049.)

Without these mods, our hero would likely be dead, maimed, or deeply depressed. But is his life authentic? Is he real? What do concepts of authenticity and realness even mean in a world where brain modifications are possible and common?

From there, it's easy to apply that same kind of logic to the real world. We obviously don't have literal brain mods, but we do have medication that changes our brain chemistry and some kinds of brain surgery. We also have other kinds of influences like our jobs (in the book, our hero is given a brain mod that causes him to become life loyal to a company), our government, our religion, and our interpersonal relationships.

How much of a difference is there between a choice made because we are literally wired to make that choice, and a choice made because of social, emotional, or financial pressures that push us in directions we wouldn't have gone in otherwise? How many alternate personal histories have we given up because of these external forces, which inevitably become internal ones?

"What do you think: the average person sits down one day and constructs some kind of meticulously rational moral philosophy – which they modify appropriately, if and when they discover its flaws? That’s pure fantasy. Most people are just pushed around by the things they live through, shaped by influences they can’t control."