Keep looking behind the looker, the mystics say. Past the body. Past the thoughts. Past the one who is having the thoughts. There is a point at which the looker disappears and the looking continues.
What is left is the question every tradition deep enough has had to make a vocabulary for. The Upanishads call it Atman, the Self with a capital S, the witness that is aware of awareness, and then, scandalously, identify it with Brahman, the ground of being. Eckhart calls it the Seelengrund and is tried for heresy. The Sufis call its uncovering fana, annihilation, and al-Hallaj is crucified for naming it.
The Buddhist door is interesting because it refuses the noun. There is no Self with a capital S, the Buddha insists. But there is also, read closely, no self with a small s. What is left when you keep looking? Not-self. Not nothing. The open ground that the Hindu tradition reaches for from the other side, and that the Buddhist tradition refuses to let you reify.
Once you can see the move, you start to see it everywhere. Jesus saying I and the Father are one. Lao Tzu’s sage who has emptied himself. Plotinus’s intellect that is one with what it knows. Different doors, the same room, and the room is not somewhere you have ever not been.