It must not of course be supposed that the Upanishads, either as a whole or individually, offer a complete and consistent conception of the world logically developed. They are rather a mixture of half-poetical, half-philosophical fancies, of dialogues and disputations dealing tentatively with metaphysical questions. Their speculations were only later reduced to a system in the Vedānta philosophy. The earliest of them can hardly be dated later than about 600 B.C., since some important doctrines first met with in them are presupposed by Buddhism. They may be divided chronologically, on internal evidence, into four classes. The oldest group, consisting, in chronological order, of the Bṛihadāraṇyaka, Chhāndogya, Taittirīya, Aitareya, Kaushītaki, is written in prose which still suffers from the awkwardness of the Brāhmaṇa style. A transition is formed by the Kena, which is partly in verse and partly in prose, to a decidedly later class, the Kāṭhaka, Īçā, Çvetāçvatara, Muṇḍaka, Mahānārāyaṇa, which are metrical, and in which the Upanishad doctrine is no longer developing, but has become fixed. These are more attractive from the literary point of view. Even those of the older class acquire a peculiar charm from their liveliness, enthusiasm, and freedom from pedantry, while their language often rises to the level of eloquence. The third class, comprising the Praçna, Maitrāyaṇīya, and Māṇḍūkya, reverts to the use of prose, which is, however, of a much less archaic type than that of the first class, and approaches that of classical Sanskrit writers. The fourth class consists of the later Atharvan Upanishads, some of which are composed in prose, others in verse.
The Aitareya, one of the shortest of the Upanishads (extending to only about four octavo pages), consists of
