"Centrism") philosophy, also called Śūnyavāda (the emptiness doctrine) is the dominant Buddhist philosophy in Tibetan Buddhism. In Madhyamaka, the true nature of reality is referred to as Śūnyatā, which is the fact that all phenomena are empty of inherent existence or essence (svabhava). Madhyamaka is generally seen as the highest philosophical view by most Tibetan philosophers, but it is interpreted in numerous different ways. The other main Mahayana philosophical school, Yogācāra has also been very influential in Tibetan Buddhism, but there is more disagreement among the various schools and philosophers regarding its status. While the Gelug school generally sees Yogācāra views as either false or provisional (i.e. only pertaining to conventional truth), philosophers in the other three main schools, such as Ju Mipham and Sakya Chokden, hold that Yogācāra ideas are as important as Madhyamaka views. 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. Two tenets belong to the path referred to as the Hinayana ("lesser vehicle") or Sravakayana ("the disciples' vehicle"), and are both related to the north Indian Sarvastivada tradition: Vaibhāṣika (). The primary source for the Vaibhāṣika in Tibetan Buddhism is the Abhidharma-kośa of Vasubandhu and its commentaries. This Abhidharma system affirms an atomistic view of reality which states ultimate reality is made up of a series of impermanent phenomena called dharmas. It also defends eternalism regarding the philosophy of time, as well the view that perception directly experiences external objects. Sautrāntika (). The main sources for this view is the Abhidharmakośa, as well as the work of Dignāga and Dharmakīrti. As opposed to Vaibhāṣika, this view holds that only the present moment exists (presentism), as well as the view that we do not directly perceive the external world only the mental images caused by objects and our sense faculties. The other two tenets are the two major Indian Mahayana philosophies: Yogācāra, also called Vijñānavāda (the doctrine of consciousness) and Cittamātra ("Mind-Only", ). Yogacārins base their views on texts from Maitreya, Asaṅga and Vasubandhu.