What does the Taoist classic three-hole classification mean?

"Three-hole Classification" was founded in the Catalogue of Three-hole Classics by Helu. "Three caves" refers to the truth of caves, the mystery of caves and the gods of caves. Liang collated the second volume of "The Essentials of Daoism" and said: "Those who call points generally take the same as the meaning." In other words, this hole means the same thing. The Tao Te Ching specifically explains the specific meaning of the three caves: the so-called caves are interlinked, and "truth" means purity and purity, leading to the path of true immortals, hence the name "cave truth"; "Xuan" means neither condensation nor stagnation, leading to the mysterious way, hence the name "Dong Xuan"; "God" is mysterious and connected with the gods, hence the name "Cave God". The so-called point-to-point, that is, all three holes can "pass the mortal to win" and "pass the mortal to be holy", making people immortal. It can be seen that the names of the three caves only express vague religious teachings, which have nothing to do with the contents of the classics under their jurisdiction or the names of the sects they represent, but are only symbols of the theological realm. Lu used this mysterious "three-hole classification" to unify the three schools of Confucian classics popular in Jiangnan since the Jin Dynasty. "Dongzhen" collected classics, "Dong Xuan" collected Lingbao classics, and "Shen Dong" collected Huang San classics, which reconciled the contradictions among Qing, Lingbao and Huang San schools.