In the early 1960s, while the world was entranced by the spectacle of human beings hurled into space in rocket ships, a series of philosophical earthquakes shook the sedate field of astronomy. Just when the skies were beginning to seem known and familiar, all at once things changed. Stellar astronomers suddenly faced unsettling data from new classes of objects called "quasars" and "radio galaxies."And now an abundance of exoplanets. Still, the greatest intellectual challenge to the worldview of modern astronomers came in the early sixties, not because of new space probes, telescopes, and computers but because of an idea.