A luminous guide to how the radical new science of counterfactuals can reveal the full scope of our universe. There is a vast class of properties, which science has so far neglected, that relate not only to what is true—the actual—but to what could be true: the counterfactual. This is the science of can and can't. A pioneer in the field, Chiara Marletto explores the extraordinary promise that this revolutionary approach holds for confronting existing technological challenges, from delivering next-generation processors to designing AI. But by contemplating the possible as well as the actual, Marletto goes deeper still, showing how counterfactuals can break down barriers to knowledge and form a more complete, abundant and rewarding picture of the universe itself.