Yangism was a philosophical school founded by Yang Zhu, existent during the Warring States period (475 BCE - 221 BCE), that believed that human actions are and should be based on self-interest. The school has been described by sinologists as an early form of psychological and ethical egoism. The main focus of the Yangists was on the concept of xing, or human nature, a term later incorporated by Mencius into Confucianism. No documents directly authored by the Yangists have been discovered yet, and all that is known of the school comes from the comments of rival philosophers, specifically in the Chinese texts Huainanzi, Lüshi Chunqiu, Mengzi, and possibly the Liezi and Zhuangzi. The philosopher Mencius claimed that Yangism once rivaled Confucianism and Mohism, although the veracity of this claim remains controversial among sinologists. Because Yangism had largely faded into obscurity by the time that Sima Qian compiled his Shiji, the school was not included as one of the Hundred Schools of Thought.