For example, the geocentric paradigm contained problems that seem obvious to a modern mind, but because astronomers before Copernicus lived and worked inside that paradigm they had difficulty seeing the problems. Overthrowing an outdated paradigm is not easy because you must learn to see nature in an entirely new way. Galileo, Kepler, and Newton saw nature from a new paradigm that would have been almost incomprehensible to astronomers of earlier centuries.