As early as 1755 the German philosopher Immanuel Kant proposed the hypothesis that the Milky Way is only one of many galaxies, in his book Universal Natural History and Theory of the Heavens. Arguing that a structure like the Milky Way would look like a circular nebula viewed from above and like an elliptical if viewed from an angle, he concluded that the observed elliptical nebulae like Andromeda, which could not be explained otherwise at the time, were indeed galaxies similar to the Milky Way.