If Gǔ has appeared in your reading, the book is naming a moment when something has decayed — often something one did not create — and the task of restoration has fallen to the reader. The character itself depicts worms in a vessel: a corruption that has been there long enough to take shape, and that now must be cleaned out from within.

Classical commentary reads this hexagram as the test of generational responsibility. The decayed thing may be a household, a project, a tradition, an organisation. The original makers are not here to fix it. The work falls to whoever currently stands in the line of inheritance. The hexagram does not ask whether this is fair. It assumes that it is the situation.

What the book counsels is the long arc of careful preparation. 先甲三日, 後甲三日 — three days before the great day, three days after. The phrase reads as an instruction not to mistake the visible repair for the whole work. What sets the change up matters; what consolidates it afterward matters; the moment itself is brief and almost ceremonial.

Gǔ's promise — 利涉大川, fitting to cross the great river — is the book's confirmation that this work is worth doing. Restoration of a decayed inheritance is one of the few undertakings the book licences in such large language. The failure mode is impatience: trying to push the repair through without the long preparation, and finding that the worms come back.