The Most Important Century is a short, direct argument for taking the next hundred years seriously.

Written by Holden Karnofsky, co-founder of Open Philanthropy, the book lays out the case that advances in AI, biotechnology, and global coordination could shape the long-term future of humanity more than any period before it.

Rather than speculative sci-fi, this is a pragmatic piece of future-facing reasoning: probability, risk, leverage, and responsibility. Karnofsky isn’t trying to predict outcomes so much as shift how readers think about time, scale, and moral weight in technological decision-making.

Whether you agree with its assumptions or not, the book functions as a clean snapshot of early-2020s longtermist thought — useful both as an argument and as an artefact of a particular moment in tech ethics.