Mandukya Upanishad discusses the syllable Aum, presents the theory of four states of consciousness, and asserts that Aum is Brahman, which is the Whole, and that Brahman is this self. The Mandukya Upanishad is notable for having been recommended in the Muktikā Upanishad, through two central characters of the Ramayana, as the one Upanishad that alone is sufficient for knowledge to gain moksha, and as first in its list of eleven principal Upanishads. The text is also notable for inspiring Gaudapada's Karika, a classic for the Vedanta school of Hinduism. Mandukya Upanishad is among the often cited texts on chronology and philosophical relationship between Hinduism and Buddhism.