[1864] "Does the Arahant (arahā) attain final Nibbāna (parinibbāyati) while established in the imperturbable (āneñja)?"
"Yes (āmantā)."
"But did not the Blessed One attain final Nibbāna immediately after emerging from the fourth jhāna (catutthajjhāna)?"
"Yes."
"If the Blessed One attained final Nibbāna immediately after emerging from the fourth jhāna, then indeed, good sir, it should not be said: 'The Arahant attains final Nibbāna while established in the imperturbable.'"
The Discussion of the Imperturbable (Āneñjakathā).
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The Discussion of the Comprehension of the Dhamma (Dhammābhisamayakathā)
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.