Chandogya Upanishad

Along with the Brhadaranyaka
Upanishad, one of the two earliest
upanishads, the religious texts that form
the most recent stratum of the Vedas.
Internal textual factors indicate that
the Brhadaranyaka Upanishad is the
older of the two, and since large sections
of both texts are the same, the
Chandogya Upanishad is thought to
be dependent on the Brhadaranyaka.
Both texts are also much larger than the
other upanishads and much less
organized, rambling from topic to topic
without an apparent focus. Both
are written in prose rather than poetry,
with the text itself often presented as
a dialogue between various speakers.
Their profoundly speculative discussions about the nature of the
universe became important sources for
the later upanishads.