Mandukya Upanisada is the shortest of all Upnisadas, consisting only 12 slokas. It is a part of Atharva Veda and the three stages of consciousness that are vividly described in it, are also talked elsewhere in Chhandogya Upanisada. Mandukya Upanisada famously maintains that there are three stages of consciousness : Jagruti ie. wakefulness, Swapna ie. dreamy state and Sushupti ie. deep slumber. There is a fourth stage too, which is called Turiya. No attributes are given of Turiya except that it is the sum total of all above three stages. A sort of collective consciousness.
Despite of being shortest, Mandukya Upanisada is considered one of the most important Upnaisadic texts. Out of 108 Upnisadas, 10 are considered as the principal texts. Mandukya is one of them. Out of 11 Upnisadas that Sankara chose to write a commentary upon, Mandukya was one.
And therein lies a subtle link which connects Sankara with Buddhist philosophy.
Acharya Gaudapada was guru of Sankara. He has written a celebrated commentary of Mandukya Upanisada called Agama Shastra. This is well known that Sanakara’s Advaita was a philosophical counter-attack on chiefly two philosophical schools of then India : Mimansa and Buddhism, the latter more than the former, being a non-Hindu atheist philosophy.
But what does Sankara’s own teacher does in his own commentary on Mandukya Upanisada? In his 19th Karika he alludes Buddha. In 90th Karika he referes to Agrayana (Mahayana), the principal philosophical branch of Buddhism. In 98th and 99th Karika he again refers to Buddha and say : “All things are pure and revealed deep into their elements, only Buddhas know it.” But when Sankara writes his own commentary on Mandukya, he simply does not mention Buddha at all.
Why?
It wasn’t without reasons that Gaudapada kept offering his reverence to Buddha in his commentary on Mandukya Upanisada. Buddhist philosophy has a peculiar prejudice of negation to it, especially it’s theory of emptiness which was explored to fullest by Nagarjuna. The basic philosophical premise for Buddhism is “anatman” (no self), “a-nitya” (impermanence), “anishwara” (atheism), “a-vidya” (misconceptions) and “Pratitya-Samutpada” (dependent origination) etc. Mandukya Upanisada also use expressions like “a-drishta”, “a-vyavharya”, “a-lakshana”, “a-chintya” and so on. This way of defining things in their negation is so close to Buddhist tradition that it was almost impossible for Gaudapada to overlook Buddha’s influence in his Karika on Mandukya. Even Sankara also talks about terms like “a-dwaita”, “a-vidya”, “a-jnan”, “a-jaati” and all. The doctrine of Ajaatiwaad of “Majjhim Nikaya” and Sankara's Ajaatiwaad are said to be one and the same. So where is the difference?