One of the premier texts in Advaitic literature is the Māṇḍukya Kārikā, attributed to the 6th century CE philosopher and scholar of the Advaita (not two) *Vedanta (end of the Vedas) school of Hindu philosophy—Gauḍapāda, also referred to as Gauḍapādācārya. It consists of four chapters, the first of which focuses on the Māṇḍūkya Upaniṣad. A small work, utilizing just 12 short stanzas, it actually consists of the absolute essence of the Upanishadic teachings. In itself, the work has never been surpassed and remains to this day the peak of awareness-expansion.
*Vedanta—It literally means the last portion of the Vedas which is identical with the Upanishads. The word also signifies the essence of the Vedas. The conclusion of Vedanta can be summed up in four words “All this is Brahman”: Only the non-dual Brahman exists. Vedantic truth is different from Kantian dualism which makes a distinction between noumena and phenomena. Vedantist does not negate the world which, being Brahman, can never be negated. It only asks the student to know the real nature of the world. (Swami Nikhilananda)
An important textual question concerns the status of the Māṇḍūkya Upaniṣad first twenty-nine Kārikās (short, metrical and aphoristic verse—inclusion mine), of Gauḍapāda’s text. The Māṇḍūkya Upaniṣad is composed of prose passages, or mantras, concerning the mystic syllable om that relate each letter of the syllable to one of the four states—waking, dream, deep sleep, and the fourth, or liberation—referred to here by technical terms of Upaniṣadic ontology—viśva, taijasa, prajña, and turiya. (Potter, Karl. H. (1981), Gaudapada, Encyclopedia of Indian Philosophies: Advaita Vedānta up to Śaṃkara and his pupils, pg. 104)
The Māṇḍūkya Upaniṣad, like other Upanishads, discusses the problem of Ultimate Reality. The knowledge of Brahman or Ātman, the goal of existence, is its theme. Unlike most of the Upanishads, it does not relate any anecdote or any imaginary conversations to elucidate the subject-matter. It is also silent about rituals and sacrifices in any form as they are irrelevant to the metaphysical or philosophical discussion of Reality. It goes Straight to the subject. (Sound familiar?—inclusion mine) (Nikhilananda, Swami; (1990). Māṇḍūkyopaniṣad: with Gauḍapāda’s Kārikā and Saṅkara’s commentary. From preface.
There is considerable Buddhist influence in the Māṇḍukya Kārikā, most notably in books two to four—especially in the latter. Scholars have argued back and forth whether or not Gauḍapāda was Buddhist, but I think that the Vedantic side wins out. This doesn’t downplay the matter, however, as there is undoubtedly considerable Buddhist influence and terminology—but in the final analysis this does not mean that Gauḍapāda is any less Vedantin, just as my taking on this subject matter doesn’t make me any less a Lankavatarian. His work is divided into four chapters: 1. Āgama, or (scripture); 2. Vaitathya (unreality); 3. Advaita (unity); 4. Alātasānti (the extinction of the burning coal). We shall see that these principles have much to bear on our own practice as they are universal in scope and effect.
Since there presently exists no independent work earlier than these Kārikās, the work is the most important source for clarifying the thought of the early Advaita school. In particular, we must carefully note how it at times presents an extremely systematic and organized exposition, and also on occasion is highly polemical. While absorbing the ideas and being influenced by the thoughts of various schools, it also sharply criticizes and attacks other philosophical standpoints. It profoundly influenced later Indian thought, and in subsequent periods was considered to be itself a Śruti, thus having an authority equal to that of the Upanisads. It is not only an extremely important work in the history of the development of Indian thought, but apart from that, the work itself has the sparkle of genius. Very directly and loftily it proclaims with bold assurance that it is the Truth. Certainly it is one of the brilliant lights in the history of human philosophical thought. (Nakamura, Hajime (2004), A History of Early Vedanta Philosophy, Part Two, pg. 212)