There is a gap between two thoughts. It may be very small — a fraction of a second in ordinary waking consciousness, wider in meditation. Something is there in the gap. The same something that was there before the last thought and is there after the next one.
What is it?
The question cannot be answered by thinking about it — because thinking about it fills the gap with a thought, which is precisely what the question is asking about the space before.
The Mandukya Upanishad describes Turiya as Prapanchopashamam Shantam Shivam Advaitam: the cessation of manifestation, peaceful, auspicious, non-dual. Not a state added to the other three. The ground in which the other three appear. In ordinary experience — the awareness that was present before the last thought and will be present after the next one — consistently overlooked because it is never a particular thing that calls attention to itself.
Nisargadatta Maharaj gave the same pointer his entire life in different forms: before the sense of I am arose, what were you? Not as a question about pre-birth cosmology. As a direct investigation of the present. Right now, before the mind asserts its claims — before all of that — what is there? The work the question does is not to produce an answer. It is to shift the attention from the contents of awareness to the awareness itself. And in that shift, something is recognised that no answer could contain.
That awareness. Right now. Before you name it. Before you decide what to do with it. Before the next thought claims it as its background.
That.