how can you be certain of anyone else's existence but your own?
That system would then be able to identify cases where the latter exists in the absence of the former, and hence, to learn to distinguish between cases of veridical perception and cases of hallucination. Such internal monitoring is viewed here as constitutive of conscious experience: A mental state is a conscious mental state when the system that possesses this mental state is (at least non-conceptually) sensitive to its existence. Thus, and unlike what is assumed to be case in HOT Theory, meta-representations can be both subpersonal and non-conceptual.
No mechanism whereby a self could ascertain the extrinsic-thereto could exist extrinsic to it; therefore, the self cannot be (conclusively) said to perceive anything beyond itself. However, the self is an element of the phenomenology of consciousness and exists within the real only insofar as the "meta-representations" (Cleeremans 1, 4, 6-7, 10-1) that precipitate it so exist.