The Brihad-Aranyaka Upanishad is among the oldest of all the Upanishads and at 105 pages in translation, it is the longest. Anyone not familiar with its philosophical and religious tradition will find it rather curious, though a number of passages will stand out for their profound expression of a clear theological or cosmological concept. Its deeper value is in its expression of a metaphysical monism that characterizes Hinduism to this day.
The monism is the Upanishad’s answer to a deep philosophical question sometimes known as "the problem of the one and the many" which asks what is the fundamental nature of being – a multiplicity of ephemeral individual things or a permanent and unchanging single thing. The Upanishad describes the world metaphorically as the body of a person and by this metaphor, alludes to the idea that each component of the world exists primarily as a dependent element of the whole. As the heart would have no existence were it not developed as a part of a body, similarly, each component of the world exists only in its interdependence with all other parts of the world.
One is tempted to think of this as an early expression of the Gaia thesis which asserts that the world is a single living being; however, the Gaia thesis tends to describe the world in materialist terms. In contrast, a second strand of thought in the Upanishad clearly comes down on the side of a non-material reality. This is presented by describing objects in connection with, and as dependent upon, our senses. Bringing these strands together, the Upanishad understands the world as a whole that is a manifestation of a perceiver.
Finally, the interconnectedness of being and its origin in the perceiver leads to the conclusion that what appears to us as our individuate soul is in fact an illusion and that the basis of reality is a single universal soul or Brahma.
With a basic understanding of these concepts, a reader can experience the Brihad-Aranyaka Upanishad is an exhilarating piece of poetry, laced with beautiful imagery, and drawing inspiring conclusions about such things as the fate of the soul after death and the working out of karmic relations.