Divisions of life into stages were popular with Romans. According to tradition, they went back to Pythagoras, who allegedly divided human life into periods each of twenty years. The medical theories of Hippocrates and Galen elaborated upon this division, to which they gave a physical basis, combining the principle of the four seasons with the science of the four corporeal humours (blood, phlegm, yellow and black bile) and the corresponding states of mind. Hence numerous texts stick to the basic scheme outlined.