To-day, thanks to Einstein, we have definite reasons for believing that ultra-precise observation of nature has revealed our natural geometry arrived at with solids and light rays to be slightly non-Euclidean and to vary from place to place. So although the non-Euclidean geometers never suspected it (with the exception of Gauss , Riemann and Clifford ), our real world happens to be one of the dream-worlds whose possible existence their mathematical genius forsaw