The Upanisads being diverse in character and outlook recognize the intuition rather than reason as a path to ultimate truth.

Even though there are supposed to be 108 or more Upanishads, twelve of them are recognized as the principal units.

The Isa Upanisad emphasizes on the fact that this world is enveloped by the Supreme.

The Kena Upaniad depicts the Goddess Uma Haimavati in the form of Supreme Knowledge expounding the doctrine of the Brahman or Supreme Entity.

The Katha Upanisad embodies the aspiration of Nachiketa who declined his father's offer of property and went to meet Yama, the God of Death. Through his intelligent conversation with Yama asks for the secret of immortality.

The Mundaka Upanisad is the form of the Bhagavad-Gita. It reveals the rules of life for pursuing a futile path.

The Taitiriya Upanishad has the basic doctrines expounded. The Svetasvatara Upanisad has the summary of the main Upanisadic doctrines, and the idea of devotion to a personal God is also developed.

The Chandogya Upanisad reveals that the main doctrines of the Upanisads were first expounded by the Kshatriyas and not by the Brahmins.

The Kausitaki Upanisad emphasizes on the fact that the Brahmins took up the intensive study of philosophy.