At Sāvatthī.
Then a mendicant went up to the Buddha, bowed, sat down to one side, and asked him, “Sir, how long is an eon?”
“Mendicant, an eon is long. It’s not easy to calculate how many years, how many hundreds or thousands or hundreds of thousands of years it lasts.”
“But sir, is it possible to give a simile?”
“It’s possible,” said the Buddha.
“Suppose there was a huge stone mountain, a league long, a league wide, and a league high, with no cracks or holes, one solid mass. And as each century passed someone would stroke it once with a fine cloth from Kāsi. By this means the huge stone mountain would be worn away before the eon comes to an end. That’s how long an eon is. And we’ve transmigrated through many such eons, many hundreds, many thousands, many hundreds of thousands.
Why is that? Transmigration has no known beginning. … This is quite enough for you to become disillusioned, dispassionate, and freed regarding all conditions.”
Sāvatthiyaṁ viharati …pe… ārāme …pe….
Atha kho aññataro bhikkhu yena bhagavā tenupasaṅkami; upasaṅkamitvā bhagavantaṁ abhivādetvā ekamantaṁ nisīdi. Ekamantaṁ nisinno kho so bhikkhu bhagavantaṁ etadavoca: “kīvadīgho nu kho, bhante, kappo”ti?
“Dīgho kho, bhikkhu, kappo. So na sukaro saṅkhātuṁ ettakāni vassāni iti vā, ettakāni vassasatāni iti vā, ettakāni vassasahassāni iti vā, ettakāni vassasatasahassāni iti vā”ti.
“Sakkā pana, bhante, upamaṁ kātun”ti?
“Sakkā, bhikkhū”ti bhagavā avoca.
“Seyyathāpi, bhikkhu, mahāselo pabbato yojanaṁ āyāmena yojanaṁ vitthārena yojanaṁ ubbedhena acchinno asusiro ekagghano. Tamenaṁ puriso vassasatassa vassasatassa accayena kāsikena vatthena sakiṁ sakiṁ parimajjeyya. Khippataraṁ kho so, bhikkhu, mahāselo pabbato iminā upakkamena parikkhayaṁ pariyādānaṁ gaccheyya, na tveva kappo. Evaṁ dīgho, bhikkhu, kappo. Evaṁ dīghānaṁ kho, bhikkhu, kappānaṁ neko kappo saṁsito, nekaṁ kappasataṁ saṁsitaṁ, nekaṁ kappasahassaṁ saṁsitaṁ, nekaṁ kappasatasahassaṁ saṁsitaṁ.
Taṁ kissa hetu? Anamataggoyaṁ, bhikkhu, saṁsāro. Pubbā koṭi …pe… yāvañcidaṁ, bhikkhu, alameva sabbasaṅkhāresu nibbindituṁ, alaṁ virajjituṁ, alaṁ vimuccitun”ti.
Pañcamaṁ.
I asked ChatGPT to calculate the time it would take to erode a mountain away with a fine cloth every 100 years.
Let’s assume:
- One stroke erodes about 1 micron of rock (1 millionth of a meter, which is generous).
- The mountain is a cube, 10 miles ≈ 16,000 meters per side.
That gives a volume:
V = 16,000³ = 4.1 trillion m³Let’s now say each 100 years removes a thin layer of 1 micron on one surface.
That would take trillions of years—possibly over 10⁹⁰ years—to wear it all away.
In this context, “stroke” means:
To gently move or rub your hand (or an object like cloth) across the surface of something.
So when we say:
“Once every 100 years, someone strokes the mountain with a fine cloth…”
It means:
Every 100 years, someone gently rubs or brushes the mountain one time with a soft cloth.
It’s not a hit or a strong rub—it’s like a gentle caress or soft touch across the surface.
Conclusion: Even with an unrealistic erosion rate, the mountain would take many countless trillions of years to vanish.
But according to the suttas, an aeon is even longer.