Early in Dalton's carrer he discovered that he had a specific color blindness. Dalton started studying it, and he discovered he had a rare red-green color blindness that is known as "Daltonism" today. Dalton researched more and he saw that some of his tissue revealed that a photoreceptor for the color green was missing. Dalton concluded that it was heredity. It ran in his family. The most important discovery of Dalton's carrer was that he revived the Atomic Theory in 1803. It stated that matter is composed of atoms, and that atoms can be divided in the same variety. He brought the theory back from Democritus who created somewhere around 470 B.C.