1. The genesis of the Upanishadic understanding, that the self and cosmic reality were one, is clear. First, the Shatapatha Brahmana stated that the most perfect ritual was, in fact, to be equated to the universe itself, visible and invisible. Second, the Aranyakas made clear that the individual initiated practitioner was the ritual itself. So, if the ritual equals all reality and the individual adept equals the ritual, then the notion that the individual equals all reality is easily arrived at. The Upanishads were arrived at, then, not by philosophical speculation, but by ritual practice. Later Upanishads of the orthodox variety (that is, early texts associated with a Vedic collection) omitted
most reference to the ritual aspect and merely stated the concepts as they had been derived.
Most importantly, the concepts of rebirth (reincarnation) and the notion that actions in this life would have consequence in a new birth (karma) were first elaborated in the Upanishads. This evidence shows that the concept of karma, or ethically conditioned rebirth, had its roots in earlier Vedic thought. But the full expression of the concept was not found until the later texts, the Upanishads, which are called the Vedanta, or the end or culmination of the Vedas. Therefore, the notion of reaching unity with the ultimate reality was seen as not merely a spiritual apotheosis, but also a way out of the trap of rebirth (or redeath).
(Adapted from Encyclopedia of Hinduism by Jones and Ryan 2007)
2. The Upanishads are a collection of texts of religious and philosophical nature, written in India probably between c. 800 BCE and c. 500 BCE, during a time when Indian society started to question the traditional Vedic religious order. Some people during this time decided to engage in the pursuit of spiritual progress, living as ascetic hermits, rejecting ordinary material concerns and giving up family life. Some of their speculations and philosophy were compiled into the Upanishads. There is an attempt in these texts to shift the focus of religious life from external rites and sacrifices to internal spiritual quests in the search for answers. Etymologically, the name Upanishad is composed of the terms upa (near) and shad (to sit), meaning something like “sitting down near”. The name is inspired by the action of sitting at the feet of an illuminated teacher to engage in a session of spiritual instructions, as aspirants still do in India today.
The books, then, contain the thoughts and insights of important spiritual Indian figures. Although we speak of them together as a body of texts, the Upanishads are not parts of a whole, like chapters in a book. Each of them is complete in itself. Therefore, they represent not a consistent philosophy or worldview, but rather the experiences, opinions and lessons of many different men and women.
Although there are over 200 surviving Upanishads, only 14 are considered to be the most important. The names of these Upanishads are: Isa, Kena, Katha, Prasna, Mundaka, Mandukya, Taittiriya, Aitareya, Chandogya, Brhadaranyaka, Svetasvatara, Kausitaki, Mahanarayana and the Maitri. These texts provide the basic source for many important topics of Indian philosophy and all major philosophical themes are covered in their pages. The purpose is not so much instruction as inspiration: they are meant to be expounded by an illuminated teacher from the basis of personal experience. The Upanishads do not claim that our brain is entirely useless; it certainly has its use. However, when it is used to unlock the great mysteries of life, the eternal, the infinite, then it simply is not enough. The highest understanding, according to this view, comes from direct perception and intuition.
(extract adapted from Ancient History Encyclopedia by C. Violatti, 2014)
3. Because the Upanishads constitute the concluding portions of the Vedas, they are called vedanta (“the conclusion of the Vedas”), and they serve as the foundational texts in the theological discourses of many Hindu traditions. The Upanishads became the subject of many commentaries and sub-commentaries, and texts modelled after them and bearing the name “Upanishad” were composed through the centuries up to about 1400 CE to support a variety of theological positions. Western scholars have called them the first “philosophical treatises” of India, though they neither contain any systematic philosophical reflections nor present a unified doctrine. Indeed, the material they contain would not be considered philosophical in the modern, academic sense. Contrary to the assertion of early Western scholars, the Sanskrit term Upaniṣhad did not originally mean “sitting around” or a “session” of students assembled around a teacher. Rather, it meant “connection” or “equivalence” and was used in reference to the homology between aspects of the human individual and celestial entities or forces that increasingly became primary features of Indian cosmology. The Upanishads present a vision of an interconnected universe with a single, unifying principle behind the apparent diversity in the cosmos, any articulation of which is called brahman. Within this context, the Upanishads teach that brahman resides in the atman, the unchanging core of the human individual. Many later Indian theologies viewed the equation of brahman with atman as the Upanishads’ core teaching.
Thirteen known Upanishads were composed from the middle of the 5th century through the 2nd century BCE. The first five of these—Brihadaranyaka, Chandogya, Taittiriya, Aitareya, and Kaushitaki—were composed in prose interspersed with verse. The middle five—Kena, Katha, Isa, Svetasvatara, and Mundaka—were composed primarily in verse. The last three—Prasna, Mandukya, and Maitri—were composed in prose.
(adapted from The Upanishads by P.Olivelle in Encyclopaedia Britannica)