Buddhism—Vaiṣṇava Vedāntist ācāryas such as Rāmānuja, Madhva and Baladeva have analyzed four types of Buddhist doctrine. These four are held, respectively, by schools known as the Sautrāntikas, Vaibhāṣikas, Yogācāras and Mādhyamikas. The first doctrine views mind and matter as having real but momentary existencei.e. with each moment, the reality of mind and matter changes. The second views matter as being knowable only through the mind; mind and the matter known through it are momentarily real. The third views matter as unreal, mind as absolute, and the perception of matter as momentary imagination. The fourth, known as Śūnyavāda, views the previous three doctrines as useless attempts at explaining what cannot be put into words. See Buddha, Māyāvāda philosophy, Nirvāna, Scepticism, Six systems, Voidism.