There is no whatness of thatness, no 'beingness' or essence of being, as long as being is strictly understood as thatness. It's not that existence precedes essence, but most of what you think of as being is just essence, that's why you can think you can think of it. It is not a distinct kind of experience, as though experience had two modalities, but is the very nature of experience. The cogito points to it, but in such a way as to make confusion nearly inescapable. It is not known by knowing but only by itself, if you could call that knowing. It appears to be void, to be the perfectly dark background of experience but if it could be approached, if it approaches you, it reveals itself as light. If it could be said to resemble anything it resembles the space between the inbreath and the outbreath, an infinitesimal nothing that as you attend to ever more closely reveals itself as something vast and timeless. It is not subject to the distinction between truth and error, but if it were it would be wholly truth.