Sacred texts are always subjected to reflection and re-thinking, because they become a basis for the spiritual creativity of numerous people. Any sacred text must contain a mystical part within it; otherwise, it will not have the necessary depth. The Upanishads became such a mystical part of the Veda, and therefore on the basis of the deciphering and development of ideas contained in them, quite a few philosophical schools appeared. Advaita was one of them, and the founder of it was the wise man Gaudapada, who advanced a number of quite categorical claims. Any seasoned debater knows that the categorical nature of claims seems to prove their truth, and we will not forget that debates of wise men in ancient times were commonplace. And why just in ancient times? Osho repeatedly made long trips around India in order to take part in such debates. But that, so to speak, is a small comment, possibly not having direct relationship to the essence of the matter.
Thus, proceeding from the phrase “tat tvam asi” (“you are this”), Gaudapada deduced the thesis of the identification between God and a person’s individual soul. On this, essentially, all of Advaita stands to this day, and in that sense, nothing has changed since the very beginning. We cannot know whether Gaudapada himself experienced what he talked about, and whether he sensed the commonality of the nature of the soul and God, their indivisibility, perceived at a certain highest level of awareness. All claims were directly or indirectly based on references to the Upanishads and the further reflections of Gaudapada himself. But his own judgements about God and the world enable us to suppose that the wise man did not have any experience, and was involved in pure philosophy, not confirmed by mystical experience. The claims about God and the world only underscore the purely philosophical nature of all of Gaudapada’s constructs. Proceeding from the texts of the Upanishads, where it is said that God does not have any characteristics or qualities, and that true reality also does not have them, which is what He is, the wise man makes the conclusion that the world visible to us and its abundance of qualities and characteristics is an illusion. To this is appended various amusing justifications such as that a sleeping person perceives his dream as actual reality, although there is no reality in a dream whatsoever. Not a single mystic will deny the reality of our world; only a philosopher is capable of this, for whom his own mental constructs are generally the only thing that is really real.
Gaudapada’s constructs seemed impressive, but not very convincing regarding the illusory nature of the world. This idea, in fact, attracts many people to this day, those who like mind games with strong ideas claiming certain extreme things. Another wise man named Shankara set about correcting the situation. He made the concept of the reality of things more complex, introducing the concept of true reality which belongs only to Brahman, the main God, and also distinguished the categories of conditional reality and transparent reality. Conditional reality is all of our world, along with its creator, Ishvara, who is the facilitating god, and also all kinds of knowledge and principles guiding human life. Thus, Shankara tried to overcome one of the claims of Gaudapada in which it was said that the eternal cannot give birth to the non-eternal. The eternal God, who has no cause cannot engender causes for the existence of numerous objects of the world; otherwise He himself disappears into them and loses His eternity. That is, God cannot be the cause for the emergence of the world, because then He himself must have a cause. We can find lots of senseless but impressive reflections by Gaudapada on this theme. By introducing an intermediary god, Shankara seems to eliminate the question of the eternal, true, reality of Brahman, untouched by anything. But of course this is just the usual philosophical trap, because the intermediary god must have his cause by which he arose. Otherwise, how is he different from the main, almighty God, existing without cause? Philosophers always have had problems with God which never arose with mystics.
According to Shankara, phantom reality is relegated to a purely philosophical category of obviously false claims, whereas true claims are relegated to relative reality. To these, by the way, are relegated dreams, which in Shankara have a relative reality, but one nonetheless.
The relative reality itself is the product of Maya – a special substance without consciousness, which comes from the god Ishvara. Maya creates projections and illusions, due to which the individual soul senses itself as alienated from the main God, and does not realize that it is one whole with Him. Maya itself is fundamentally unknowable and indescribable. Scholars believe such a description is closest to the sense of what is laid out in the Upanishads, but even so, this all remains pure philosophy, with which there is nothing to be done, although Shankara himself proposes a way out of the situation in which people find themselves. The way out is through liberation (moksha), which is achieved through realization that you and Brahman are one. Well, or that you are the same thing.
For those who want to come to liberation, Shankar recommends first studying the Vedas properly, and only then coming to performance of jnana yoga. Other forms of yoga are not particularly encouraged by him, and it could hardly be otherwise; after all, the yoga of knowledge (as jnana is translated) is, to a certain extent, the yoga of philosophers.