She said, "It’s not the fact that I wouldn’t remember theexperience. If you’ve lived through something, you’ve lived through it.What worries me is all the other things I’d have to live through. Allthe other people I’d have to become."

Cass dated the advent of civilization to the invention ofthe quantum singleton processor. The Qusp. She accepted the fact thatshe couldn’t entirely avoid splitting into multiple versions;interacting with any ordinary object around her gave rise to anentangled system — Cass plus cloud, Cass plus flower — and she couldneverhope to prevent the parts that lay outside her from enteringsuperpositions of different classical outcomes, generating versions ofher who witnessed different external events.

Unlike her hapless ancestors, though, she did notcontribute to the process herself. While the Qusp inside her skullperformed its computations, it was isolated from the wider world — acondition lasting just microseconds at a time, but rigidly enforced forthe duration — only breaking quarantine when its state vector described oneoutcome, with certainty. With each operating cycle, the Qusprotated a vector describing a single alternative into another with thesame property, and though the path between the two necessarily includedsuperpositions of many alternatives, only the final, definite statedetermined her actions.

Being a singleton meant that her decisions counted. Shewas not forced to give birth to a multitude of selves, each respondingin a different way, every time she found her conscience or her judgmentbalanced on a knife edge. She was not at all what Homo sapienshad actually been, but she was close to what they’d believed themselvesto be, for most of their history: a creature of choice, capable ofdoing one thing and not another.