Mandukya Upanishad
Sanskrit · Upanishad · ~500-200 BCE
The Mandukya Upanishad, belonging to the Atharvaveda, is the shortest of the principal Upanishads with just 12 mantras. Yet Gaudapada and Shankara considered it sufficient for liberation. It analyzes the syllable Om and its correspondence with the four states of consciousness: waking, dreaming, deep sleep, and Turiya (the fourth).
1 chapter