For centuries, geometry was based on Euclid's postulates, which seemed eternal and irrevocable. However, one of the postulates (the fifth) has always seemed "a little less natural" than the others, and hundreds of mathematicians have tried in vain to do without it by deducing it from the other postulates. In the mid-19th century, Bernhard Riemann came up with a novel idea: let's imagine it's false! This was the birth of "non-Euclidean geometries", which would later have major applications in physics.