In terms of matter, all modern cosmologies are founded on the cosmological principle, which states that whichever direction we look from earth, the universe is basically the same: homogeneous and isotropic (uniform in all dimensions). This principle grew out of Copernicus's assertion that there were no special observers in the universe and nothing special about the earth's location in the universe (i.e., Earth was not the center of the universe, as previously thought). Since the publication of general relativity in 1915, this homogeneity and isotropy have greatly simplified the process of devising cosmological models.