Early in the 6th century B.C.E. Greek iatros (physicians) began to associate disease with base elements. This line of reasoning culminated in the system of four humors attributed to Hippocrates, the Father of Medicine. (Pythagoras later proposed a system of five humors, but it didn’t become popular until recently, when it was re-invigorated by another noted mathematician, Dr. Richard Garfield.) Understanding of the four humors became the basis of medicine in Europe for over two millennia. In the middle ages, the classical world was seen as a golden age, and the writings of Hippocrates and those who followed him became sacrosanct. Only in the 18th century, when empirical science began to take hold, did diagnosis by humors fall out of favor.