The Aitareya Upanishad belongs to the Rig Veda and the Upanishad proper consists of three chapters. The preceding parts deal with sacrificial ceremonies like mahavrata and their interpretations. It is the purpose of this Upanishad to lead the mind of the sacrificer away from the outer ceremonial to its inner meaning.
All true sacrifice is inward.
Sankara points out that there are three classes of men who wish to acquire wisdom. The highest consists of those who have turned away from the world, whose minds are free and collected, who are eager for freedom. There are others who wish to become free gradually by attaining to the world of Hiranya-garbha . For them the knowledge and worship of the prana, life-breath is intended. There are still others who care only for worldly possessions.