The essential theme of the Upanishads is the nature of the world-soul. Their conception of it represents the final stage in the development from the world-man, Purusha, of the Rigveda to the world-soul, Atman; from the personal creator, Prajapati, to the impersonal source of all being, Brahma. Atman in the Rigveda means no more than "breath"; wind, for instance, being spoken of as the atman of Varuna. In the Brahmanas it came to mean "soul" or "self." In one of their speculations the pranas or "vital airs," which are supposed to be based on the atman, are identified with the gods, and so an atman comes to be attributed to the universe. In one of the later books of the Çatapatha Brahmana (X. vi. 3) this atman, which has already arrived at a high degree of abstraction, is said to "pervade this universe." Brahma (neuter) in the Rigveda signified nothing more than "prayer" or "devotion." But even in the oldest Brahmanas it has come to have the sense of "universal holiness," as manifested in prayer, priest, and sacrifice. In the Upanishads it is the holy principle which animates nature. Having a long subsequent history, this word is a very epitome of the evolution of religious thought in India. These two conceptions, Atman and Brahma, are commonly treated as synonymous in the Upanishads. But, strictly speaking, Brahma, the older term, represents the cosmical principle which pervades the universe, Atman the psychical principle manifested in man; and the latter, as the known, is used to explain the former as the unknown. The Atman under the name of the Eternal (aksharam) is thus described in the Brihadaranyaka Upanishad (III. viii. 8, 11):-- "It is not large, and not minute; not short, not long; without blood, without fat; without shadow, without darkness; without wind, without ether; not adhesive, not tangible; without smell, without taste; without eyes, ears, voice, or mind; without heat, breath, or mouth; without personal or family name; unaging, undying, without fear, immortal, dustless, not uncovered or covered; with nothing before, nothing behind, nothing within. It consumes no one and is consumed by no one. It is the unseen seer, the unheard hearer, the unthought thinker, the unknown knower. There is no other seer, no other hearer, no other thinker, no other knower. That is the Eternal in which space (akaça) is woven and which is interwoven with it." Here, for the first time in the history of human thought, we find the Absolute grasped and proclaimed. A poetical account of the nature of the Atman is given by the Kathaka Upanishad in the following stanzas:-- That whence the sun's orb rises up, And that in which it sinks again: In it the gods are all contained, Beyond it none can ever pass (iv. 9).