Śatapatha Brāhmaṇa ("of a hundred paths") 4.1.4

Mitra and Varuna, forsooth, are his intelligence and will; and as such belonging to his self: whenever he desires anything in his mind, as 'Would that this were mine! I might do this!' that is intelligence; and whenever that is accomplished, that is will. Now intelligence indeed is Mitra, and will is Varuna; and Mitra is the priesthood, and Varuna the nobility; and the priesthood is the conceiver, and the noble is the doer.

Now in the beginning these two, the priesthood and the nobility, were separate: then Mitra, the priesthood, could stand without Varuna, the nobility.

Not Varuna, the nobility, without Mitra, the priesthood: whatever deed Varuna did unsped by Mitra, the priesthood, therein, forsooth, he succeeded not.

Varuna, the nobility, then called upon Mitra, the priesthood, saying, 'Turn thou unto me that we may unite: I will place thee foremost, sped by thee, I will do deeds!'--'So be it!' So the two united; and therefrom resulted that graha to Mitra and Varuna.

Such, then, is the office of Purohita (placed foremost, domestic priest). Wherefore let not a Brâhman desire to become the Purohita of any one Kshatriya (he may meet with), as thereby righteousness and unrighteousness unite; nor should a Kshatriya make any Brâhman (he may meet with) his Purohita, as thereby righteousness and unrighteousness unite.--Whatever deed, sped by Mitra, the priesthood, Varuna thenceforward did, in that he succeeded.

Hence it is quite proper that a Brâhman should be without a king, but were he to obtain a king, it would be conducive to the success (of both). It is, however, quite improper that a king should be without a Brâhman, for whatever deed he does, unsped by Mitra, the priesthood, therein he succeeds not. Wherefore a Kshatriya who intends to do a deed ought by all means to resort to a Brâhman, for he verily succeeds only in the deed sped by the Brâhman.

Now he draws (the Maitrâ-varuna graha) from that (stream of Soma), with (Vâg. S. VII, 9; Rig-veda II, 41, 4), 'This Soma, O Mitra and Varuna, hath been pressed for you; ye holy, now hear my cry!--Thou art taken with a support!--Thee for Mitra and Varuna!'

He mixes it with milk. The reason why he mixes it with milk is this. Soma, forsooth, was Vritra. Now when the gods slew him, they said to Mitra, 'Thou also slayest!' But he liked it not and said, 'Surely, I am every one's friend (mitra): being no friend, I shall become an enemy (or, other than Mitra, Amitra).'--'Then we shall exclude thee from the sacrifice!'--Then said he, 'I, too, slay!' Thereupon the cattle went from him, saying, 'Being a friend, he has become an enemy!' Thus he was deprived of the cattle. By mixing (the Soma) with milk, the gods then supplied him with cattle; and in like manner does this (priest) now supply him (the sacrificer or Mitra) with cattle by mixing (the Soma) with milk.

As to this they say, 'Surely he liked it not to slay!' Thus, what milk there is in this (mixture) that belongs to Mitra, but the Soma belongs to Varuna: therefore one mixes it with milk.

He mixes it with (Vâg. S. VII, 10; Rig-veda IV, 42, 10), 'May we delight in the wealth we have gained, the gods in the offering, the kine in pasture! that unfailing milch cow, O Mitra and Varuna, grant ye unto us day by day!'--with 'This is thy womb: thee for truth and life!' he deposits it. Now as to why he says, 'Thee for truth and life,'--the truth is Mitra, since Mitra is the Brahman, and the truth is the Brahman (sacerdotium or sacred writ);--and life is Varuna, since Varuna is the year, and life is the year: therefore he says, 'This is thy womb: thee for truth and life!'