For the modern philosophical tradition stemming from Descartes, our immediate and basic experience of subjectivity occurs in an internal or reflective experience whereby I discover, by a sort of turning back of my own mental attention upon itself, that I am able to see, immediately, incontestably, that I exist at least as a stream of conscious states. Thought discovers that it always has this power to attend to itself; the mind has the ontological structure of existence-for-itself. For consciousness to exist is for it to be continually aware of itself, to be its own witness.
This reflexive experience is immediate and non-discursive; let us call it immanent reflection. There is an eclipse of the body functions and their interactions with the world; thought experiences itself directly, without the interposition of the body or the world, and even comes to wonder if it could then exist without the body and without the world. In turning back reflexively upon itself it seems to form a circuit with itself such that it not only becomes its own witness, but closes itself to outside observation. Its essential nature, which is to exist reflexively for-itself, is not mediated by anything extended or sensibly observable; the mind seems, then, not only to be its own witness, but to be its sole possible witness.