Adi Shankara was a renowned Indian sage who consolidated Advaita Vedanta or non-dualism. He lived from roughly 788 to 820, travelling throughout India and re-establishing non-dualism and the value of monasticism. He wrote extensive commentary on the Brahma Sutras and Bhagavad Gita and won epic debates with other philosophers. His 4 primary students founded the 4 Shankaracharya seats (maths) in the cardinal directions of India. They operate to this day.
The core tenet of non-duality is that Atman and Brahman are one and the same. Ayam Atma Brahma.
The world is unreal
Only Brahman is real
The world is Brahman
The paradox of non-duality, often lost in modern neo-advaitism, is the third line. While many confuse non-duality with Self Realization, realizing Self or Atman is not realizing Brahman. True non-duality arises with the flowering of Brahman late in Unity. Only when Atman is recognized as of Brahman and even the subtle duality of existence is resolved does full non-duality arise.
Today, some aspects of Shankara’s teachings have become over-emphasized. Like that Maya means Illusion or that you have to be celibate to make spiritual progress.
But there’s another aspect that Shankara himself over-emphasized early in his life that is also still over-emphasized today. That’s the dry emptiness of Shiva. By itself, pure existence is a boundless empty void. This can certainly be one way Self Realization is experienced. But this is not complete.
There is a story told of Shankara walking by the water. One foot gets stuck and he is mysteriously unable to pull it free. He calls to a passing woman who chides him, saying it’s just an illusion. He realizes this is the divine mother herself. He goes on to write the poem Ananda Lahari.
Only when Shiva is united with Shakti does he have the power to create
Without her, Even an inch he cannot move…
Thereafter, Shankara speaks to the fullness of what is, of both aspects of the divine. That’s a lesson some have not yet recognized today.