The School of Naturalists or the School of Yin-yang (陰陽家/阴阳家; Yīnyángjiā; Yin-yang-chia; "School of Yin-Yang") was a Warring States era philosophy that synthesized the concepts of yin-yang and the Five Elements; Zou Yan is considered the founder of this school. His theory attempted to explain the universe in terms of basic forces in nature: the complementary agents of yin (dark, cold, female, negative) and yang (light, hot, male, positive) and the Five Elements or Five Phases (water, fire, wood, metal, and earth). In its early days, this theory was most strongly associated with the states of Yan and Qi. In later periods, these epistemological theories came to hold significance in both philosophy and popular belief. This school was absorbed into the alchemic and magical dimensions of Taoism as well as into the Chinese medical framework. The earliest surviving recordings of this are in the Ma Wang Dui texts and Huang Di Nei Jing.