The Upanishads are second to none among the world's most important sacred scriptures. Most were written during the first millennium B.C.E., but some are as recent as the Modern period. They are the culmination of the Vedic tradition and serve as ritual text-books. The oldest Upanishads are associated with three of the four main classifications of the Vedas: the Rig Veda, the Yajur Veda, and the Sama Veda. More recent Upanishads are associated with the Arthavada Veda.
It is out of the ritualistic and allegorical earlier writings of the Vedic tradition that the more philosophical writings of the Upanishads emerged. The oldest Upanishads are the Brihadaranyaka, Chandogya, Aitareya, Kaushitaka, and Kena Upanishads with the Kena standing on the border with later Upanishads. These later ones are the Kathaka, Is'a, S'vetas'vatara, Mundaka, and Mahanarayana Upanishads. Later still are the Pras'na, Maitrayaniya, and Mandukya Upanishads. Two or three more are sometimes included in this list of the "principle" Upanishads. As with any large collection of sacred writings that were composed over centuries, the Upanishads include numerous philosophical views, many of which are contradictory; but in general, they present a broad and roughly coherent current of thinking regarding the fundamental nature of reality, the soul, and right action leading to salvation or emancipation from delusion and suffering.
To the neophyte, the Upanishads present a thicket of confusing, but thoroughly engaging prose and verse. One needs a guide to make good sense of them and Paul Deussen's landmark work The Philosophy of the Upanishads offers excellent guidance. First published in 1906, Deussen's work remains valuable today. By reading it in conjunction with the principle Upanishads, one can better grasp both the substance of the Upanishads and the basis upon which Deussen makes his arguments regarding the philosophical thrust of the Upanishads. A convenient anthology of the principle Upanishads is The Thirteen Principle Upanishads, translated from Sanskrit by Robert Ernest Hume.
Deussen's The Philosophy of the Upanishads is divided into five main sections, beginning with a general introduction followed by expositions of the theology, cosmology, psychology, and eschatology of the Upanishads. The central doctrine that lies behind all of these topics can be reduced to three propositions. (1) The atman is the knowing subject within us, (2) The atman, as the knowing subject, is unknowable, and (3) The atman is the sole reality. From these three proposition, the Upanishads spin out sublime doctrines regarding, life, death, illusion, knowledge, consciousness, right action, and numerous other metaphysical and epistemological questions.
Many of these doctrines express ideas similar to those of Western philosophy, particularly -- according to Deussen -- Plato and Kant, but on the whole the philosophic approach is at odds with the empirical realism that dominates Western scientific thinking. Furthermore, the Upanishad's general approach to religion is starkly different from the main currents of Islam, Judaism, and especially Christianity, where right action, or right intention, is central to salvation. Instead, the Upanishads call us to see beyond the delusions that are natural to human experience and intuit the empirically unknowable reality that is embedded in our consciousness. The salvation that this offers is akin to the relief that one might feel after waking from a nightmare and repeating the assurance, "it was only a dream, only a dream."
Deussen's book is dense and expansive, but it is worth all the attention one might give it, including repeated readings.