The Upanishads state they are “the secret of the Veda”. This implies that there IS a secret hidden within the Veda, that it is not all on the surface to be seen, and that there is an esoteric meaning to be understood by those who have the ears to hear, the eyes to see, the mind to grasp, and the heart to understand. The Veda has always been something of a mystery to the modern mind, due to the symbolic language utilized and the imagery conveyed which spoke to a different age of humanity. With his substantial insight into the symbolism employed, Sri Aurobindo was able to unlock and reveal the “secret of the Veda”. It turns out that it is a scripture of spiritual growth, development, aspiration and experience of reality from a different standpoint than what we ordinarily experience in our focus on the outer life of work, family, and society.
Another implication of the Upanishadic statement is that they actually are able to reveal the secret of the Veda. If that is the case, we should be able to understand the meaning of the Upanishads and thereby grasp the meaning locked within the Veda. The Upanishads have been revered as scriptural authority for some thousands of years, and in some cases, with their connection to and use of symbolism from the Veda, they have become obscure to us as well. However, it is important to recognize that the obscure references are a small part of the much larger picture provided by the Upanishads.
Certain recurrent themes become obvious and these can guide the seeker to the vast spiritual truths that the Rishis saw and experienced. “One without a second” is one such truth. The One includes, encompasses, creates, and directs, while concurrently exceeding any limitations of the outer world. “All this is the Brahman” makes it clear that the world itself, and all the beings that inhabit it, and the entire universal creation, and the entire action of birth, life, decay and death is indeed the Brahman. “Neti, Neti”, “Not this, Not that” makes it clear that the Brahman cannot be limited by any specific form, definition or line of action, as the Brahman transcends all.
The Upanishads also provide a guidebook for spiritual development, as well as a description of the experiences of realised souls. The practices of raja yoga are delineated. Various states of consciousness, waking, dream, sleep and transcendent are described and shown to be both part of the external world, and descriptions of inner states of spiritual experience.
Sri Aurobindo focused on certain major Upanishads out of the much larger body of works collected under that name. He was not predominantly interested in philosophy but rather, in finding keys to the nature of, growth and development of consciousness. He integrated what he found in the Upanishads by incorporating numerous quotations in his own magnum opus, The Life Divine, in particular in chapter headings to tie in the ancient knowledge to the systematic approach and development he was setting forth therein.
In the end, the Upanishads are not about creating a line of thought or philosophical understanding, but about preparing the inner being for the “knowledge by identity” that transcends the limits of the mind and the speech, and opens the seeker to the vast, indeed infinite, and eternal consciousness of the Brahman. The student of the Upanishads need not be an adherent of a particular religious persuasion, as the Upanishads are not about religion. The Upanishads are concerned with the truth of our existence, and the ability of the individual to experience that truth directly, and thus, the Upanishads are open to anyone from any tradition, background or religious direction, who truly wants to know.
Sri Aurobindo, The Upanishads, Summary and Conclusions