Though the Sarvâstivâdins can hardly have accepted idealist metaphysics, yet the evidence of art and their own version of the Vinaya make it probable that they tolerated a moderate amount of mythology, and the Mahayanists, who like all philosophers were obliged to admit the provisional validity of the external world, may also have admitted their analysis of the same as provisionally valid. The strength of the Hinayanist schools lay in the Vinaya. The Mahayanists showed a tendency to replace it by legends and vague if noble aspirations. But a code of discipline was necessary for large monasteries and the code of the Sarvâstivâdins enjoyed general esteem in Central Asia and China. Three stages in the history of Indian Buddhism are marked by the names of Aśvaghosha, Nâgârjuna and the two brothers Asanga and Vasubandhu. It would be easier to give a precise description of its development if we were sure which of the works ascribed to these worthies are authentic, but it seems that Aśvaghosha represents an ornate and transitional phase of the older schools leading to Mahayanism, whereas Nâgârjuna is connected with the Prajñâ-pâramitâ and the nihilistic philosophy described in the preceding chapter. Asanga was the founder of the later and more scholastic system called Yogâcâra and is also associated with a series of revelations said to have been made by Maitreya. As mentioned above, tradition makes Aśvaghosha,[203] one of the most brilliant among Sanskrit writers, live at the court of Kanishka[204] and according to some accounts he was given to the Kushans as part of a war indemnity. The tradition[205] is confirmed by the style and contents of his poems and it has been noted by Foucher that his treatment of legends is in remarkable accord with their artistic presentment in the Gandharan sculptures. Also fragmentary manuscripts of his dramas discovered in Central Asia appear to date from the Kushan epoch. Aśvaghosha's rank as a poet depends chiefly on his Buddhacarita, or life of the Buddha up to the time of his enlightenment. It is the earliest example of a Kâvya, usually translated as artificial epic, but here literary skill is subservient to the theme and does not, as too often in later works, overwhelm it. The Buddha is its hero, as Râma of the Râmâyana, and it sings the events of his earlier life in a fine flow of elaborate but impassioned language. Another of his poems,[206] discovered only a few years ago, treats of the conversion of Nanda, the Buddha's half-brother.