The period of the Brahmanas is a very important one in the history of Indian society. For in it the system of the four castes assumed definite shape, furnishing the frame within which the highly complex network of the castes of to-day has been developed. In that system the priesthood, who even in the first Vedic period had occupied an influential position, secured for themselves the dominant power which they have maintained ever since. The life of no other people has been so saturated with sacerdotal influence as that of the Hindus, among whom sacred learning is still the monopoly of the hereditary priestly caste. While in other early societies the chief power remained in the hands of princes and warrior nobles, the domination of the priesthood became possible in India as soon as the energetic life of conquest during the early Vedic times in the north-west was followed by a period of physical inactivity or indolence in the plains. Such altered conditions enabled the cultured class, who alone held the secret of the all-powerful sacrifice, to gain the supremacy of intellect over physical force. The Brahmanas in course of time themselves acquired a sacred character, and came in the following period to be classed along with the hymns as çruti or "hearing," that which was directly heard by or, as we should say, revealed to, the holy sages of old. In the sphere of revelation are included the later portions of the Brahmanas, which form treatises of a specially theosophic character, and being meant to be imparted or studied in the solitude of the forest, are called Aranyakas or "Forest-books." The final part of these, again, are philosophical books named Upanishads, which belong to the latest stage of Brahmana literature. The pantheistic groundwork of their doctrine was later developed into the Vedanta system, which is still the favourite philosophy of the modern Hindus. Works of Vedic "revelation" were deemed of higher authority in cases of doubt than the later works on religious and civil usage, called smriti or "memory," as embodying only the tradition derived from ancient sages.