Upanishads are philosophical-religious texts for Hindus. As they come at the end of Vedas, they are also called as Vedanta (meaning end of Vedas). There are over 200 knows Upanishads, and of them a few are considered as the principal Upanishads. These are:
- Isavasya Upanishad
- Kena Upanishad
- Katha Upanishad
- Prasna Upanishad
- Mundaka Upanishad
- Mandukya Upanishad
- Taittiriya Upanishad
- Aitareya Upanishad
- Chandogya Upanishad
- Brihadaranyaka Upanishad
The shortest of all Upanishads is Mandukya Upanishad, but it is no less important. In fact, it is among the most important ones. It explains two ideas: one, the Aum (ॐ) symbol, and two, through this symbol, the four states of consciousness. The major theme of the Mandukya Upanishad is an exposition of Aum sound, with a view to training the mind in meditation, for the purpose of achieving freedom, gradually, so that the individual soul is attuned to the Ultimate Reality.
It begins with: “Aum, the word, is all this ” and then goes on to explain the four states of consciousness through the metaphor of each syllable of AUM.
A is Waking State.
U is Dream State.
M is Deep Sleep State.
The silence that follows AUM is Pure Consciousness (Turiya)
Turiya cannot be described by any instrument of empirical knowledge, because of Its unique nature. When Turiya is realized, there no longer remains any distinction between the knower, knowledge, and the known. You become one with the universe.
Kalampedia 