How have the world's great thinkers, politicians, mathematicians, and religious figures reached their transformative moments of insight? Are there lessons to be learned from their experiences? William B. Irvine takes up these questions and others that relate to what he calls "aha moments," guiding us through the most striking examples of instantaneous intellectual breakthroughs that have shaped human civilization in six different domains, one at a time: religion,science, art, math, morality, and innovation.