As has been explained, Agni and Indra represent heat or Buddhi in association with Purushic and Prakrtic Ether respectively; and this leads to an examination of the twofold character of Ether more closely, in connection with its magnetic energy and its twin-born senses of knowledge and action. The former is personified in Heaven and Earth or Dyava-Prthvi, and the latter in the two Asvins.
Figure of Dyu and Prthvi: Heaven and Earth are called Dyu and Prthvi in the Vedas; and they might be represented by the following diagram according to the letters composing their names:
The last i of Prthivi is merely a sign of the feminine gender, and so has no special value. The most interesting idea in this connection is that the Supreme Purusha, by his mere presence, produces an electro-magnetic current in the Golden Egg, which creates. The universe becomes divided into two halves, Purushic and Prakrtic or Dyu and Prthvi; and it is for this reason that they are conceived as separate and apart.
It will be noticed that the line of Dyu's name extends from the region of Buddhi (d, y) to that of Purushic Ether (u); while that of Prthvi from Prakrtic Ether (ri) to Water (v); and the former is Purushic and the latter Prakrtic, being on the Right and left side of the Golden Egg. As Prthvi is the name of our planet Earth, we see that it includes (Prakrtic) Ether and the elements.
Their Connection with Ether: It has been said that Dyu and Prthvi represent the two halves of the universe, Purushic and Prakrtic, in connection with Ether; and it has been shown that the latter, in relation to Go or Cow, are represented by milk and ghee. Accordingly Dyu and Prthvi, who represent this twofold character of Ether, are spoken of as rich in milk, and filled with fatness; they are enclosed in fatness, mingle with fatness, increase fatness, and pour out fatness.
Two Bowls: Heaven and Earth are called two bowls, of noble kind, the two world-halves, which by Varuna's decree, unwasting, rich in germ, stand parted from each other; and the idea may be illustrated by the following figure:
Parents of the World: The whole system of Hindu thought arises out of an examination of Ether; and as Heaven and Earth represent its twofold character, they may be regarded as the parents of all systems of thought; and so are spoken of as the Father and Mother, the two parents, born before all others. The double birth of Agni is connected with them and Indra is the son of Dyu, and his mother Aditi is identified with Prthvi. Again, as the idea of the other Vedic gods is derived from all that is associated with Ether, Mind, Buddhi, and the energy of the Heart, they are said to be the parents of all.
Their Origin: But even the Vedas do not claim to know the ultimate. All their systems of thought relate to the union of Purusha and Prakrti in the manifestation of life, and no one knows whether Purusha or Prakrti is the first and original Cause. Accordingly it is said of Heaven and Earth,
Whether of these is elder, whether later?
How were they born? Who knows it, ye sages?
These of themselves support all things existing;
As on a car the Day and Night roll onwards.
Dyu and Prthvi in Post-Vedic Literature: The idea of Dyu and Prthvi in post-Vedic literature agrees with their conception in the Vedas. They are spoken of as the parents of Agni and Indra, and Dyu is a Vasu and this is repeated in the story of the Mahabharata, where he is born as Bhishma, the last surviving son of Ganga and Santanu.