Introduction by Adi Shankaracharya:
The Mandukya Upanishad begins with the statement: “Aum, the word, is all this ” This treatise, consisting of four chapters, is the epitome of the substance [the oneness of jiva and Brahman] of the import of Vedanta.* ... [The Mandukya Upanishads contains the essence of the teachings of all Vedanta: "you are One with Brahman (God)".] What, then, is the end in view? It is thus explained: As a man stricken with disease regains his normal health when the disease is removed, so Atman, identifying Itself with misery, recovers Its normal state when the duality manifesting itself as the phenomenal universe is destroyed.** This realization of Non-duality is the end to be achieved. The manifoldness of duality is produced by avidya; it is destroyed by Vidya (the Knowledge of Brahman). Therefore this treatise is begun for the purpose of revealing the Knowledge of Brahman. [ * The goal to be attained through the study of the Mandukya Upanishad is Moksha, or Liberation. The means for its attainment is the practice of the Knowledge of the identity of Brahman and Atman. This Knowledge cannot be acquired directly through the study of scripture; yet scripture helps indirectly in this respect by demonstrating the unreality of the phenomenal universe and indicating the reality of Brahman. Thus scripture, too, indirectly helps in the realization of Brahman.
** Self-Knowledge is eternally existent. But it appears to be non-existent on account of man’s identification with the body, the mind, and other factors of the dualistic universe. This is all the result of avidya. Under the influence of this false identification a man regards himself as miserable and seeks happiness. Then, instructed by a compassionate preceptor, he practises the Knowledge of Non-duality, which destroys the illusion of duality. When the obstructions created by duality are removed, he recovers Supreme Bliss, which is the very nature of Atman.