What is quintessential in Indian philosophy is its spiritual idealism; and the quintessence of its spiritual idealism is in the Upanishads. They are not a systematic, coherent treatise of philosophy or mysticism, but rather the records of the intuitions, speculations, spiritual experiences of the ancient seers and saints who lived between the 8th and the 3rd century BCE. They are heterogeneous, disorderly arranged and containing even some apparent contradictions. Many expressions and symbols are obscure to our present day mentality. Yet their messages are luminous and perennial. They are the infallible authority of revelation, they belong to sruti or revealed literature, and are the utterances of sages who spoke out of the fullness of their illumined experience. They are vehicles more of spiritual illumination than of systematic reflection.
The main concern of the Upanishadic sages is to find out the One ultimate principle of unity, underlying the wide ranging change and multiplicity experienced both int he world around us and within us. Their investigation was not just to satisfy their intellectual curiosity, but to seek liberation from the limitations of earthly existence, from the unreality that characterises it, from the darkness that envelops it, from the death that lurks within it. They were convinced that what they were seeking was to be found only in the ultimate unitary principle, the Supreme Reality.
In this search two trends are observed: one extrovert and objective, the Ultimate discovered is Brahman, the Absolute; and the other introspective and subjective, the Ultimate discovered is Atman, the Self.
The culmination of the search and the typical message of the Upanishads consists in the equation of Brahman and Atman: the Ultimate Reality which is at the heart of the Universe is the very Self of Man.