David Deutsch, an Oxford physicist and one of the pioneers of quantum computing, makes the case that the rise of good explanations is the most important development in human history and the engine of essentially unlimited progress. The book builds on his earlier The Fabric of Reality, taking up Karl Popper's epistemology and pushing it in unfamiliar directions. Deutsch's central claim is that knowledge consists of explanations that are 'hard to vary'—accounts whose details are so tightly constrained by what they purport to explain that small changes destroy them—and that the discovery of how to generate such explanations, in seventeenth-century Europe, opened an unbounded frontier. Once humans began to demand explanations that survive criticism, every problem became, in principle, solvable, given enough knowledge. Deutsch then applies this framework to a sweep of topics: the foundations of quantum mechanics, the multiverse, the philosophy of mathematics, evolutionary biology, aesthetics, political institutions, and the prospects for artificial general intelligence. He attacks fashionable doctrines—cultural relativism, ecological pessimism, the idea that humans are insignificant in the cosmos—as failures of imagination. The book has become a touchstone among technologists, effective altruists, and members of the 'progress studies' movement, valued for its uncompromising defense of reason and its insistence that nothing in physics rules out an open-ended future for human civilization.