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 Vedanta 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 Brihadaranyaka, Chhandogya, Taittiriya, Aitareya, Kaushitaki, is written in prose which still suffers from the awkwardness of the Brahmana style. A transition is formed by the Kena, which is partly in verse and partly in prose, to a decidedly later class, the Kathaka, Iça, Çvetaçvatara, Mundaka, Mahanarayana, 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, Maitrayaniya, and Mandukya, 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 three chapters. The first represents the world as a creation of the Atman (also called Brahma), and man as its highest manifestation. It is based on the Purusha hymn of the Rigveda, but the primeval man is in the Upanishad described as having been produced by the Atman from the waters which it created. The Atman is here said to occupy three abodes in man, the senses, mind, and heart, to which respectively correspond the three conditions of waking, dreaming, and deep sleep. The second chapter treats of the threefold birth of the Atman. The end of transmigration is salvation, which is represented as an immortal existence in heaven. The last chapter dealing with the nature of the Atman states that "consciousness (prajna) is Brahma."