An icebreaker
This philosophy holds that one undivided awareness underlies everything, and that the sense of being a separate self is a kind of mistake in seeing rather than a fact of the world. A teacher in the 8th century systematized it; teachers still transmit it today.
In commentaries on the Upanishads, the Bhagavad Gita, and the Brahma Sutras, he argued that the individual self and ultimate reality are one undivided consciousness, and that ordinary separateness is appearance rather than final truth.
Writing before Shankara, he argued from the experience of dreaming that what feels completely real while asleep dissolves entirely on waking, using that comparison to defend non-dual reality as the only truth.
Living at the South Indian mountain Arunachala for over fifty years, he taught one instruction above all others: "Who am I?" — tracing every thought back to the awareness noticing it, until only that awareness remained.