The deep and extraordinary philosophy of Hinduism is often ignored and among the great philosophical texts are the Upanishads. This article was written in response to a question on why I wrote the book.
The main concept in the Upanishads is that of Brahman, which is both the ultimate goal of all existence, and the common aspect of all life forms. Brahman can be defined as the substratum of the world. The Upanishads agree that everything originates from Brahman, which is uncreated and always existed. It is eternal, infinite, and has no form or shape. It is beyond time and space. Its nature is sat-chit-ananda, that is ‘truth or true being, consciousness and bliss’. Even though  Brahman is responsible for the creation of the world, and is identical with or part of the soul in every living being, Brahman retains its original, unchangeable, eternal, nature. Brahman is beyond thought and words, which is why no description can ever reveal it. Only through a knowledge of it, would one know its reality. In the Brihadaranyaka Upanishad the rishi Yajnavalkya explains Brahman in many ways. He says that just as different types of smoke come from fire, in the same way everything including the Vedas comes from a limitless reality  which can be equated with Brahman. And everything merges with it, just as all sorts of water merge in the sea, as all sounds merge in the ear, all thoughts in the mind,  and as salt in water pervades all of it. He also explained  that when everything is Brahman, there can be no duality. Brahman is best explained in the Upanishads, though this idea is also known in other religions where different terms are used. A true understanding of this concept would remove all divisions and inequalities in society and would lead to respect and compassion, for if every person is of the same essence, there could be no awareness of differences based on religion, caste or even on economic status. Further, this same essence exists in every living being, which would lead to the protection of trees, plants, insects and animals.
India is a vast storehouse of sacred texts, belonging to many different religions, and ranging in date from the ancient to the modern and contemporary, and the Upanishads can be considered among the most interesting and valuable of these. This group of Sanskrit texts form part of Vedic literature, the most sacred texts of Hinduism. Veda comes from the Sanskrit root ‘vid’ to know, and the word Veda implies ‘divine knowledge’. The main texts of Vedic literature are the four Vedic Samhitas, that is, the Rig Veda, Yajur Veda, Sama Veda, and Atharva Veda, along with the Brahmanas, Aranyakas and Upanishads. All these texts are said to be ‘shruti’ or ‘ heard’, and are believed to be directly revealed from a divine source. The Upanishads, attached to one or the other of the Vedic Samhitas are the definitive texts expounding the wisdom of the Vedas.
Though a number of people are aware of the Upanishads, there are many more who are not. Even those who know about them are familiar with one or two, while there are actually almost 300 Upanishads, some very ancient, and others more recent. Out of these, a group of 108 Upanishads, listed in the 17th century Muktika Upanishad, are considered the most important. These  Upanishads are of different types, including early and late Upanishads, Upanishads focusing on a deity, or on the paths of sannyasa or yoga. These Upanishads include  numerous topics, such as  the source of all creation, the atman or soul, the jiva, or individual soul, the nature of consciousness, the different worlds, reincarnation, the body, the chakras and inner power centres, as well as meditations on deities, and a lot more, but the concept of Brahman can be said to be the most important aspect of these texts, and the main theme, that of ultimate realization and transcendence. Long ago, this common and main aspect was recognised and compressed into a single text, the Brahma Sutra, composed before the first century CE.
There are very few books dealing with all 108 Upanishads. Signe Cohen’s recent book looks at several of them, but is meant for academicians. T.M.P. Mahadevan’s book on the 108 Upanishads, does list them all, and provides a brief introduction and a translation of one or two verses of each, but my aim in this book is to go beyond this and  present a comprehensive overview of all 108 and of the Brahma Sutra, while at the same time situating these texts in the context of Indian philosophy. As all 108 are described, each person can focus on the one that suits them. It is not necessary to alter one’s way of worship or of devotion to a particular deity, but only to recognise, that at the highest level, every deity is Brahman.
The 108 Upanishads, thus provides an introduction to the texts, a starting point to delve deeper into the profound philosophy contained in them. It is an attempt to make the Upanishads along with the concept of Brahman, better known. The book is also a sequel to my book on the Vedic Samhitas (The Vedas, An Introduction to Hinduism’s Sacred Texts), which places the Vedas in a historical context, and examines questions regarding their date and origin..