Nice lecture by Maharishi: https://www.youtube.com/watch?v=v4Ao9M_wjfc https://www.youtube.com/watch?v=v4Ao9M_wjfc
Some Sanskrit concepts: From Māṇḍūkya Upaniṣad shivaM shaantam advaitam caturthaM manyante sa aatmaa sa vijñeyaH https://en.wikipedia.org/wiki/Mandukya_Upanishad https://en.wikipedia.org/wiki/Mandukya_Upanishad
