“Nibbāna is the untranslatable expression of the Unspeakable, of that for which in the Buddha’s own saying there is no word, which cannot be grasped in terms of reasoning and cool logic, the Nameless Undefinable (cp. the simile of extinction of the flame which may be said to pass from a visible state into a state which cannot be defined. Thus the Arahant passes into that same state, for which there is “no measure” (i. e. no dimension): atthangatassa na pamāṇam atthi . . . yena naṃ vajju: taṃ tassa n’ atthi SN 1076. Yet, it is a reality, and its characteristic features may be described, may be grasped in terms of earthly language, in terms of space (as this is the only means at our disposal to describe abstract notions of time and mentality).
“It is the speculative, scholastic view and the dogmatising trend of later times, beginning with the Abhidhamma period which has more and more developed the simple, spontaneous idea into an exaggerated form either to the positive (i. e. seeing in nibbāna a definite state or sphere of existence) or the negative side (i. e. seeing in it a condition of utter annihilation). Yet its sentimental value to the exuberant optimism of the early Buddhists (Rh. Davids, Early Buddhism, p. 73) is one of peace and rest, perfect passionlessness, and thus supreme happiness. As Heiler in the words of R. Otto (Das Heilige etc. 1917; quoted l. c. p. 41) describes it, “only by its concept Nirvāna is something negative, by its sentiment however, a positive item in most pronounced form.”—We may also quote Rh. Davids’ words: “One might fill columns with the praises, many of them among the most beautiful passages in Pāli poetry and prose lavished on this condition of mind, the state of the man made perfect according to the B. faith. Many are the pet names, the poetic epithets, bestowed upon it, each of them—for they are not synonyms—emphasizing one or other phase of this many-sided conception—the harbor of refuge, the cool cave, the island amidst the floods, the place of bliss, emancipation, liberation, safety the supreme, the transcendental, the uncreated, the tranquil, the home of ease, the calm, the end of suffering, the medicine for all evil, the unshaken, the ambrosia the immaterial, the imperishable, the abiding, the further shore, the unending, the bliss of effort, the supreme joy, the ineffable, the detachment, the holy city, and many others. Perhaps the most frequent in the Buddhist texts is Arahantship, ʻthe state of him who is worthyʼ ; and the one exclusively used in Europe is Nirvana, the ʻdying out,ʼ that is, the dying out in the heart of the fell fire of the three cardinal sins—sensuality, ill—will, and stupidity” (Saṃyutta iv.251, 261), (Early Buddhism pp. 72, 73.) And Heiler says (p. 42 l. c.): “Nirvāna is, although it might sound a paradox in spite of all conceptional negativity nothing but eternal salvation, after which the heart of the religious yearns on the whole earth.”
—F. Heiler, Die Buddhistische Versenkung (München 1922), pp. 36–42