Brhadaranyaka Upanishad ("Great Forest Book").
By far the earliest of the Upanishads, the theoretical religious writings that make up the Vedas' most recent layer.
At least four pieces of evidence support the Brhadaranyaka Upanishad's chronological priority: its length, lack of structure, archaic language, and connection to previous Vedic works.
The first section of the book compares the world to the sacrifice horse in detail, with obvious similarities to previous Brahmana literature.
Furthermore, the Brhadaranyaka Upanishad's name ("great forest book") denotes a shift from the Aranyaka ("forest books") literature that preceded Brahmana literature.
This upanishad is a significant source for the evolution of the tradition since it answers many of the issues addressed in subsequent writings.
It is written in prose rather than poetry, and the instructions are frequently given in the form of conversation between different speakers, unlike most of the later Upanishads.
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