A thought is occurring right now. Something is aware of that thought. The thought is the object. Something is the subject. What is the subject?
Not another thought — that would produce an infinite regress. Not the brain — the brain is itself an object of awareness. Not the body — the body is largely unconscious while whatever is aware of thoughts is not.
The Kena Upanishad poses this directly: It is not known by those who know it. It is known by those who do not know it. Not a paradox for its own sake. A precise description of why the usual approach fails. The usual approach treats awareness as an object to be examined — looks at awareness from the outside. But awareness cannot be examined from outside because there is no outside. Everything that could be examining it is inside it.
Ramana Maharshi spent fifty years pointing at this from every available direction. His instruction was always the same: turn the attention around. Not outward toward objects. Toward the awareness itself. Not to find it as a thing. To notice that the looking is itself the awareness — and the awareness does not need to be found because it is what is already doing the finding. The recognition does not produce new experience. It produces the recognition of what has been present in every experience.
The investigation does not require sitting on a mountain. It requires exactly what you are doing right now: noticing that something is reading these words. Noticing that something is noticing. Staying with that noticing long enough for the difference between the content of awareness and the awareness itself to become apparent.
That difference is the beginning of everything the tradition has been pointing at.