Reading guide
A near-exact structural echo of SN 15.5
The same mendicant, the same opening question, the same request for a simile, and the same closing conclusion — this discourse is best understood as SN 15.5's direct pair, not a separate teaching.
Erosion versus depletion, two different processes
SN 15.5's mountain wears away continuously under a cloth's touch; this discourse's citadel empties one discrete mustard seed at a time — a meaningfully different physical process reaching the same conclusion about an eon's length.
An enclosed, countable image rather than an open one
A mountain's erosion is continuous and hard to quantify at any given moment; an iron citadel packed with mustard seeds is, at least in principle, countable — making this image's depletion feel more precisely measurable, even as the total count remains impossibly large.
The same identical wording carried through
Beyond the swapped image, this discourse's surrounding language — the century-long interval, the "why is that" transition, the closing refrain — matches SN 15.5 almost word for word.
Two images, one settled point
Having now measured an eon's length twice, by two genuinely different physical means, this saṃyutta turns in its next discourse to a different question entirely: not how long one eon lasts, but how many eons have already passed.
Key terms
The text
§1At Sāvatthī. Then a mendicant went up to the Buddha … 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.”
§2“But sir, is it possible to give a simile?” “It’s possible,” said the Buddha. “Suppose there was an iron citadel, a league long, a league wide, and a league high, full of mustard seeds pressed into balls. And as each century passed someone would remove a single mustard seed. By this means the huge heap of mustard seeds would be used up 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? This transmigration has no known beginning. … This is quite enough for you to become disillusioned, dispassionate, and freed regarding all conditions.”
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