When gods swear by the waters of the underworld river Styx, as the goddess Hērā does here, I.15.037, their oath must be irrevocable and therefore absolute. The basis for the absoluteness of the oath is the absolute imperishability of the material substance that has been chosen as the basis for the oath. That substance is the water of this river. And the imperishability is signaled by the adjective aphthito- ‘imperishable’, which is actually used as the epithet of the Stygian waters at Hesiod Theogony 805. Further, aphthito- ‘imperishable’ at Hesiod Theogony 397 is the epithet of Styx as a personified female divinity. On the epithet aphthito- ‘imperishable, unwilting’, see the comments on I.02.325, where further attestations are also cited.