Within the Upanishads, the universe is traced back to a single, ultimate reality called Brahman, described as eternal, infinite, and the unchanging ground of all existence. This Brahman is characterized as existence-consciousness-bliss and is portrayed as both the source of the cosmos and its inner support. It is said to be beyond all limiting categories, yet present in every aspect of the world as its deepest truth. Thus, the many forms and forces of the universe are understood as expressions of one underlying reality rather than independent, self-standing entities.

Corresponding to this cosmic principle is the inner principle of the individual, the Ātman, the true self. The Upanishads distinguish this Ātman from body, senses, mind, and ego, presenting it as pure, deathless consciousness. This self is not born and does not die, remaining untouched by the changing conditions that affect the personality and the physical form. The ordinary sense of identity, tied to transient experiences, is seen as a product of ignorance that veils this deeper essence.

A central philosophical insight is the fundamental unity of Brahman and Ātman. The great sayings of the tradition, such as “Tat tvam asi” (“Thou art That”) and “Aham Brahmāsmi” (“I am Brahman”), articulate the claim that the innermost self and the ultimate reality are not two separate principles but one. The experience of being a separate, isolated individual arises from ignorance of this identity and from the power that makes the manifold world appear as ultimately divided. When this ignorance is dispelled through direct realization, the apparent gulf between self and cosmos is overcome.

The phenomenal world, with its diversity and change, is linked to the power often called māyā, which makes the one reality appear as many. Through this power and through ignorance, beings become entangled in karma, the chain of actions and their consequences, which sustains the cycle of birth, death, and rebirth known as saṃsāra. As long as the true nature of the self is not recognized, this cycle continues, shaped by desire and past deeds. Liberation, or mokṣa, is described as freedom from this cycle, a state of bliss and fearlessness grounded in the realized unity of Ātman and Brahman.

The Upanishadic vision thus reorients spiritual life from outward ritual to inward knowledge. Ethical discipline, mental refinement, and contemplative insight are treated as essential conditions for recognizing the self as identical with the ultimate reality. The universe is seen as ordered and pervaded by consciousness, and human existence finds its highest meaning in awakening to that order within. Through such realization, the seeker is said to move from a life governed by transient appearances to one rooted in enduring peace and freedom.