The tradition of a defined group of "classics" in Chinese culture dates at least to the Warring States period, when the Zhuangzi has Confucius telling Laozi "I have studied the six classics—the Odes, the Documents, the Rites, the Music, the Changes, and the Spring and Autumn Annals". These six works were thus already considered classics by at least the 3rd century BC, although the Classic of Music did not survive the chaos of the Qin unification of China and was deemed lost during the Han dynasty. The remaining Five Classics were traditionally considered to have been edited by Confucius. Records from the late Han and Three Kingdoms period reference "seven classics", though they do not name them individually. By the Tang dynasty references to "nine classics" were common, though the nine works themselves vary depending on the source. The Kaicheng Stone Classics (833–837) comprise twelve works (all the above except the Mencius). By the time of the Southern Song dynasty, the number and specific books in the "thirteen classics" were universally established. The Thirteen Classics formed the texts used in the Imperial examinations, and their 600,000+ characters, in effect words, were generally required to be memorized in order to pass.