Rig Veda 10.129 is the least characteristic thing in the Rig Veda and probably the most quoted.
It opens by denying the categories: there was neither non-existence nor existence then, no air, no sky beyond, no death and no immortality, no distinguishing sign of night or day. It describes That One breathing without breath by its own impulse, darkness hidden by darkness, an undifferentiated flood. Then desire arose, the first seed of mind, and the poets searching in their hearts found the connection between existence and non-existence.
Then it stops and asks who could possibly know. The gods, it points out, came after the creation of this world, so they are in no position to say. And the final verse asks whether the one who oversees this in the highest heaven knows, or whether he does not.
That last line is startling in any religious literature and unique in the Vedic corpus. A hymn ends by raising the possibility that God does not know how the world was made.
The hymn belongs to Book 10 and is generally placed among the latest strata of the Rig Veda, on the same linguistic and stylistic grounds that place the Purusha hymn there. It is a speculative composition rather than a liturgical one, it invokes nobody, asks for nothing, and accompanies no rite, and it stands closer to the questions the Upanishads would take up than to the sacrificial hymns around it.
Crediting the discovery of the link between being and non-being to poets searching in their own hearts locates cosmological insight in human reflection rather than revelation, which is what makes the final scepticism intelligible: a hymn that grounds its cosmology in human enquiry is in a position to admit the enquiry may not reach far enough.