Each layer exists because the one below broke

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The sentence the walk is proving

A villager could not settle a debt by making more promises. A bank could not settle a debt by creating more of its own deposits. One village could not settle a debt by creating more of its own currency. — Gundersen, The Hierarchy of Money

Each of those three sentences is a rigidity. Credit, bank money, reserves, a reserve currency, and fiat are the patches. None of them is needed on day one. Stones would have been enough if everyone always had some, and if shops always trusted a promise, and if harvest never drained the vaults, and if the next village took your stones at par.

They don't. So the stack exists.