In Tibetan Buddhist scholasticism, Buddhist philosophy is traditionally propounded according to a hierarchical classification of four classical Indian philosophical schools, known as the "four tenets" ("drubta shyi"). While the classical tenets-system is limited to four tenets (Vaibhāṣika, Sautrāntika, Yogācāra, and Madhyamaka), there are further sub-classifications within these different tenets (see below). This classification does not include Theravada, the only surviving of the 18 classical schools of Buddhism. It also does not include other Indian Buddhist schools, such as Mahasamghika and Pudgalavada.