Brahma Knowledge, by L. D. Barnett, , at sacred-texts.com
§ 4. Foundations of Upanishadic Ideas.—The Ṛig-veda contains many strata of religious and philosophic thought. Its oldest element is a worship of nature-deities, such as the Sky-Father, Earth-Mother, Dawn, etc., who were inherited from the time before the division of the Indo-European stocks. These figures, however, are not as a rule living forces in religion; for the most part they are kept alive merely by conservatism and poetic convention. The most real gods of the Vedic pantheon are peculiarly Indian. Often indeed a distant connection can be traced between them and nature-deities in the other Indo-European races; but their myths and
legends have undergone a long process of development on Indian soil, by which they have acquired the characteristic stamp of the Hindu genius. But even in the naturalistic polytheism of this mythology we can trace in the Ṛig-veda, especially in its later parts, a tendency towards a pantheism merging all being into a Supreme Spirit of vaguely defined character, a primal Infinite.
Thus "there is one Existence, sages call it by many names" (Ṛig-veda, I. clxiv. 46; cf. x. cxiv. 5). The primitive spirit and source of being is called Hiraṇya-garbha ("Germ of Gold"), or Prajā-pati ("Lord of Creatures"), x. cxxi.; as his own firstborn he enters the universe created by him (x. lxxii., lxxxii., etc.). In x. cxxix. the first Being is neither existent nor non-existent, a watery void, from which arose a primal Unity, whence sprang Desire as first bond between being and non-being. Another poet (x. xc.) tells how the universe arose from Purusha, "Man," that is, an ideal human sacrifice offered by the gods. For as, to the Vedic mind, a sacrifice is a power controlling Nature, and the human sacrifice is the most powerful of all, then the greatest of all forces, the cosmogonic energies, must have arisen from an ideal offering of this kind made by the highest agents, the gods.
To the demand of philosophy for a final and absolute Reality beyond the temporary and merely relative reality of phenomenal experience
the Vedic poets thus gave almost the same answer as the early Greek thinkers. They asserted, in very diverse and often very mystic terms, the existence of a single cosmic matter or World-Spirit, whom they styled variously Prajā-pati, Brahmā (masculine), Purusha, Hiraṇaya-garbha, etc.