Democritus of Abdera was a Greek philosopher who was an early proponent of atomic theory. He proposed that matter was made of discrete indivisible particles, after his teacher pointed out that a beach looks smooth from afar but is really made of discrete grains of sand. He called his particles atomos, meaning "cannot be cut." He believed these particles to be eternal, indestructible, and unchanging. Democritus helped resolved the question of how a world evidently in a state of flux could nevertheless have an underlying nature that was eternal and unchanging. His ideas were largely ignored until the scientific revolution of the 16th, 17th, and 18th centuries. He also pursued studies in physics, astronomy, zoology, botany, and medicine. In another impressive observation, Democritus maintained that the Milky Way galaxy was a conglomeration of stars.