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). Continue reading