There is a Zeno-like paradox to Phenomenon, to appearance. Appearance cannot be the reality because in itself it is ungrounded, insubstantial, s not the kind of thing that would be needed to account for itself. You could say that it has being, but no substance. So there is something behind appearance. You could think of this as noumenon in some lofty sense, or as matter, or as events in a brain. Whatever it is it doesn't appear as it is, it has no power of self-exposure. But if it doesn't appear, has no dimension of appearance, then how can it give rise to appearance? How can what doesn't appear give rise to appearance? If what is real does not appear then it is complete in its non-appearance, there is nowhere for appearance to take hold of it. This is why some thinkers go for the panpsychic chimera, a sort of philosophic unicorn, or else deny that appearance appears at all, that the word has a stable meaning.  It is not enough to assert that something underlies appearance - you can assert this metaphysically, but metaphysics doesn't answer to what is required. It's not the being of appearance that needs explanation, which is what a theory of being tries to serve, but the appearance of appearance. If being, or the in-itself, were to construct a sort of screen on which it could appear, it could set this up as elaborately as possible, but whatever it did it could not create the appearance of the screen.