The Brihadaranyaka Upanishad is considered the oldest and most important Upanishad, and it contains some of the most profound teachings on consciousness.

The Upanishads teach that the world is an illusion, that the soul is immortal, and that consciousness is the ground of all reality.

Brahman is the term used in the Upanishads to refer to the ultimate reality, which is described as being infinite, eternal, and unchanging.

The Upanishads teach that Brahman and Atman are the same, and that the individual soul is a manifestation of the universal soul.

The Upanishads teach that the goal of life is to achieve liberation from the cycle of birth and death, to gain knowledge of the ultimate reality, and to experience the bliss of Brahman.

The Chandogya Upanishad contains the famous teaching, 'Tat Tvam Asi', which means 'That Thou Art'.

The teaching, 'Tat Tvam Asi', teaches that the individual soul is a manifestation of the universal soul, that the world is an illusion, and that the goal of life is to achieve liberation from the cycle of birth and death.

The Mandukya Upanishad contains the teaching of the four states of consciousness: waking, dreaming, deep sleep, and turiya.

The four states of consciousness are waking, dreaming, deep sleep, and turiya.

Turiya is the state of pure consciousness, which is beyond the three states of waking, dreaming, and deep sleep.

The Taittiriya Upanishad contains the teaching of the five sheaths of the self: the physical sheath, the vital sheath, the mental sheath, the intellectual sheath, and the bliss sheath.

The five sheaths of the self are the physical sheath, the vital sheath, the mental sheath, the intellectual sheath, and the bliss sheath.

The Svetasvatara Upanishad contains the teaching of the three gunas: sattva, rajas, and tamas.

The three gunas are sattva, rajas, and tamas.

The Purusha Sukta Upanishad contains the teaching of the four purusharthas: dharma, artha, kama, and moksha.

The four purusharthas are dharma, artha, kama, and moksha.