Anacker (2005: p. 34), in introducing his English rendering of the "Method for Argumentation (Vāda-vidhi)" of Vasubandhu (fl. 4th c.)—a text composed in Sanskrit which is now only extant in a reconstructed composite extracted from Tibetan works, collated by Frauwallner (1957)—holds that: Dignaga formulated the 'three modes’ (trairūpya) which are three conditions required for a logical ‘sign’ or ‘mark’ (linga) to fulfill in order to establish the 'valid cognition' (pramana) of an 'inference' (anumana):