Debates Commentary - Controversy on “Assurance of assured one”
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There are two types of assurances (niyāmas) as:
the assurance of incorrectness (miccatta niyāma) which is the kamma of immeadiate retribution (anantariyakamma)
and the assurance of correctness (sammatta niyāma) which is the ariyan-path (ariyamagga)
There is no other assurance (niyāma) exepting these two.
All other phenomena of Three-planes are non-assured (aniyata).
(Undetermined/ Uncertain).The person contained with those, is also a non-assured one (aniyato).
The Bodhisatta who has been said “this being will attain Buddhahood in the future” by the Buddhas using own power of wisdom, is called “an assured person (niyato)” due to their abundant merits.
Quesion by sakavadi and the anwer by the other one are for the people like pubseliya and aparaseliya who hold the view "assured one falls into assurance "
thinking that “bodhisatta in his last birth is capable of falling into the assurance”.
by taking this mere term (niyato).
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Kathāvatthu atthakathā - Niyatassa niyāmakathā
663-664. … Tattha duvidho niyāmo – micchattaniyāmo ca ānantariyakammaṃ, sammattaniyāmo ca ariyamaggo. Ime dve niyāme ṭhapetvā añño niyāmo nāma natthi. Sabbepi hi sesā tebhūmakadhammā aniyatā nāma. Tehi samannāgatopi ¶ aniyatoyeva. Buddhehi pana attano ñāṇabalena “Ayaṃ satto anāgate bodhiṃ pāpuṇissatī”ti byākato bodhisatto puññussadattā niyatoti vuccati. Iti imaṃ vohāramattaṃ gahetvā “Pacchimabhaviko bodhisatto ¶ tāya jātiyā bhabbo dhammaṃ abhisametu”nti adhippāyena “Niyato niyāmaṃ okkamatī”ti yesaṃ laddhi, seyyathāpi pubbaseliyāparaseliyānaṃ; te sandhāya pucchā sakavādissa, paṭiññā itarassa. …
For a better translation see PTS-Debates-Commentary-BC-Law (175 page)