The nature of consciousness includes the demand for a ground, not in consciousness but of consciousness. The demand for a ground does not entail the existence of a ground, except that in this context existence cannot mean anything understood as existence in consciousness. So a ground may be impossible to doubt, but again this does not entail that such a ground is the exclusive ground of 'all' existence. It may be one among others. This correlates to a sort of polytheism of unlimited scope. If you admit the transcendental leap, then also what cannot be ruled out must be.