Later Buddhist philosophical traditions developed complex phenomenological psychologies termed 'Abhidharma'. Mahayana philosophers such as Nagarjuna and Vasubandhu developed the theories of Shunyata (emptiness of all phenomena) and Vijnapti-matra (appearance only), a form of phenomenology or transcendental idealism. The Dignāga (c. 480–540) school of Pramāṇa promoted a complex form of epistemology and Buddhist logic. This tradition contributed to what has been called an "epistemological turn" in Indian philosophy. Through the work of Dharmakirti, this tradition of Buddhist logic has become the major epistemological system used in Tibetan Buddhist philosophy and debate.