These source materials are thought to have appeared before the current era.
Uniting Yin and Yang (2nd Century BCE)
Tirumantiram by Thirumular (200 BCE)
The Symposium by Plato (360 BCE)
Taharat Hamishpacha (“kosher sex”) (~400 BCE)
Iphegenia at Aulus by Euripides (405 BCE)
The Way of Harmonizing Yin and Yang by Emperor Chuan Hsu (~1000 BCE)