ऋग्वेद

None knoweth whence creation has arisen;
And whether he has or has not produced it:
He who surveys it in the highest heaven,
He only knows, or haply he may not know.

-Last part of naasadiiya sukta (Hymn of Creation) from Rigved's X.129
Translation quoted from Macdonnell's Vedic Mythology, 1897, as given in "A source book of Indian Philosophy"
Eds. Radhakrishnan and Moore. Full hymn with its translation can be found here

Rigved is arguably the world's oldest compilation of wisdom. Quoted above is the last part of the naasadiiya sukta from it. This is the best theory of Cosmology of any ancient civilisation that I have come across. It has many correct conceptions not unlike the current theories of simultaneous creation of space and time, the indifferentiability of truth and untruth (under certain conditions) etc. But what is more important is that it asks the right questions too. Gods themselves came after the creation. How could they then know about it? Rather than hastily attributing the origin to some super being, it has the right amount of doubt that perhaps even that high being may not know how the universe came into existence.

Such wonderful pieces are really a tribute to our ancestors catching their love for knowledge and learning. The Vedas and especially the Upanishads are full of such gems exhibiting the wonderful grasp of philosophy they had even then (and reflecting on the social circumstances as well).