To know or tell the origin of the other divinities is beyond us, and we must accept the traditions of the men of old time who affirm themselves to be the offspring of the gods - that is what they say - and they must surely have known their own ancestors. How can we doubt the word of the children of the gods? Although they give no probable or certain proofs, still, as they declare that they are speaking of what took place in their own family, we must conform to custom and believe them. In this manner, then, according to them, the genealogy of these gods is to be received and set forth.
Oceanus and Tethys were the children of Earth and Heaven, and from these sprang...

Plato,
Timaeus 40d-40e

Benjamin Jowett translation