Whitehead characterizes Christianity as a religion seeking a metaphysic in contrast to Buddhism which is a mataphysic as seeking a religion (RM 40, 1927). What is the source of the system of the Buddhist metaphysic which Whitehead mentions?
As the primitive Buddhism which we know from Pali texts rejects metaphysics as a fruitless and empty speculation, we must specify the source of Whitehea's conception of Buddhism as "the most colossal example in history of applied metaphysics"(RM39).
One of the probable source is Th. Stcherbatsky's The Central Conception of Buddhism (1922) which introduced a Buddhist ontology of Sarvastivadins (the school which discusses all things, temporal or eternal, as beings). This book contains English translations of Vasbandhu's Abidharma-kosa as an appendix, which treats the problematic of time concerning the reality of past, present and future. Sarvastivadins accepted the reality of time including past and future as well as preset, and tried to lay the foundation of the dependent-arising (pratityasamtpada) in terms of temporal atomism.
The metapysical system of Sarvastivadins was criticized by Nagarjuna, who did not consider the ground of beings as an eternal being, but "emptiness(sunyata)" equated with "depending-arising". He identified nirvana(which was thougt by Salvastivadins as an absolute eternity) with samsara (relative temporality).
Nagarjuna was traditionally thought as one of the founders of Pure-land Buddhism in Japan. Pure-land Buddhism has been characterised by many scholars as a Buddhism most akin to protestant Christianity because of its conception of "Shin(faith as a gift of Amidha-Buddha)" and of sinners and evil ones as the very object of Amida's Vows of universal salvation.
So I think Whitehead's conception of "Buddhism as a metaphysic generationg a religion" may find a justification if we reflect the historical development of Budhism from Nagarjuna to Pure-Land Buddhism.
