The most “radical version of Non-dual teaching” goes back to Revered Gaudapada (of 5th or 6th CE) who forms a watershed mark in the Advaita tradition. With him started the human form lineage of teacher-disciple. (Before him it was a lineage of Sages preceded by the lineage of Gods – see here).
Gaudapada says that we are prisoners of an unwavering belief in cause-effect relationships. He avers that cause-effect relationships do not exist. For example, he writes:
नास्त्यसद्धेतुकमसत्सदसद्धेतुकं तथा ।
सच्च सद्धेतुकं नास्ति सद्धेतुकमसत्कुतः ॥ — 4.40, Gaudapada kArikA on mANDUkya Upanishad.
Meaning: The unreal cannot have the unreal as its cause. Nor can the real be produced from the unreal. The real cannot be the cause of the real. And it is much more impossible for the real to be the cause of the unreal. (Translation: Swami Nikhilananda).
The Swami Ji adds the following Notes of explanation:
1. An unreal cannot be the cause of another unreal – This refutes the contention of the Buddhistic nihilists.
2. An object (say, a jar) which is perceived and which is the effect of an unreal object (like the horns of a hare) is never existent – This is the refutation of the nyAya school.
3. An object which is perceived and which is the effect of another perceived object is itself non-existent – This refutes the sAmkhya school of causality.
4. It is not possible for the existence of a real object as the cause of an unreal one – A class of Vedantin-s hold that the ever-existent brahman is the cause of these illusory phenomena. This is the refutation of that school of thought.
[All the four systems of thought refuted above believe in causality of one form or other.]
A belief in causality itself is “ignorance,” says Gaudapada. Because of such an ignorance people talk that Consciousness (aka Awareness aka Self aka brahman) is the cause for the objects of perception (including thoughts) and the world. They think that these percepts arise as a result of the movement in Consciousness. He firmly and unambiguously tells us that Consciousness NEVER MOVES NOR IS IT A CUASE for anything! In his kArikA at 4.56 on mANDUkya Upanishad, he writes, “As long as there is faith in causality, the endless chain of births and deaths will be there. When that faith in causality is destroyed through right Knowledge, birth and death become non-existent.” Shankara adds, while amplifying on that verse that “the world also ceases to exist for want of any other cause for its existence.” Thus, true understanding of the Knowledge of the Self and the appearance of a world don’t go together!
Gaudapada explains that the world is nothing but a flimsy and diaphanous appearance. It is like the forms that emerge when a firebrand is rotated. Those forms do not come from the firebrand nor merge into it. They have no substance (kArikA 4.50)! Similarly, the world and the percepts do not emerge from Consciousness nor do they go back into it.