structural:summarization, information:extraction
Upanishads

The central concern of all Upanishads is to discover the relations between ritual, cosmic realities (including gods), and the human body/person. The texts do not present a single unified theory, rather they present a variety of themes with multiple possible interpretations, which flowered in post-Vedic era as premises for the diverse schools of Hinduism.

Paul Deussen states that the concept of Brahman in the Upanishads expands to metaphysical, ontological and soteriological themes, such as it being the "primordial reality that creates, maintains and withdraws within it the universe", the "principle of the world", the " absolute ", the "general, universal", the "cosmic principle", the "ultimate that is the cause of everything including all gods", the "divine being, Lord, distinct God, or God within oneself", the "knowledge", the "Self, sense of self of each human being that is fearless, luminuous, exalted and blissful", the "essence of liberation, of spiritual freedom", the "universe within each living being and the universe outside", the "essence and everything innate in all that exists inside, outside and everywhere".

Gavin Flood summarizes the concept of Brahman in the Upanishads to be the "essence, the smallest particle of the cosmos and the infinite universe", the "essence of all things which cannot be seen, though it can be experienced", the "Self within each person, each being", the "truth", the "reality", the "absolute", the "bliss" (ananda).

According to Radhakrishnan, the sages of the Upanishads teach Brahman as the ultimate essence of material phenomena that cannot be seen or heard, but whose nature can be known through the development of self-knowledge (atma jnana).

The Upanishads contain several mahā-vākyas or "Great Sayings" on the concept of Brahman:

The Upanishad discuss the metaphysical concept of Brahman in many ways, such as the Śāṇḍilya doctrine in Chapter 3 of the Chandogya Upanishad, among of the oldest Upanishadic texts. The Śāṇḍilya doctrine on Brahman is not unique to Chandogya Upanishad, but found in other ancient texts such as the Satapatha Brahmana in section 10.6.3. It asserts that Atman (the inner essence, Self inside man) exists, the Brahman is identical with Atman, that the Brahman is inside man—thematic quotations that are frequently cited by later schools of Hinduism and modern studies on Indian philosophies.

This whole universe is Brahman. In tranquility, let one worship It, as Tajjalan (that from which he came forth, as that into which he will be dissolved, as that in which he breathes).

Man is a creature of his Kratumaya (क्रतुमयः, will, purpose). Let him therefore have for himself this will, this purpose: The intelligent, whose body is imbued with life-principle, whose form is light, whose thoughts are driven by truth, whose self is like space (invisible but ever present), from whom all works, all desires, all sensory feelings encompassing this whole world, the silent, the unconcerned, this is me, my Self, my Soul within my heart.

This is my Soul in the innermost heart, greater than the earth, greater than the aerial space, greater than these worlds. This Soul, this Self of mine is that Brahman.

Paul Deussen notes that teachings similar to above on Brahman, re-appeared centuries later in the words of the 3rd century CE Neoplatonic Roman philosopher Plotinus in Enneades 5.1.2.