The Chandogya Upanishad is one of the oldest Upanishads of Hinduism. It opens with the recommendation that "let a man meditate on Om". It calls the syllable "Om" as "udgitha" (उद्गीथ, song, chant), and asserts that the significance of the syllable is thus: the essence of all beings is earth, the essence of earth is water, the essence of water are the plants, the essence of plants is man, the essence of man is speech, the essence of speech is the Rig Veda, the essence of the Rig Veda is the Sama Veda, and the essence of Sama Veda is the "udgitha" (song, "Om").