It's not always obvious, but every experiential moment, thought or perception or act of understanding, requires a basis, is what it is only in relation to a prior certainty. For all of our ordinary experience this basis is self-certainty, that is the inner intuitive certainty of self-existence, or the cogito in its most general sense, having nothing to do with thinking as such. This self-certainty is so common as to be almost entirely overlooked, and is prior to all the terms that arise in phenomenology, noesis, noema, horizon, Dasein etc. This basis is assumed equally in materialism as idealism, and in particular materialism in any form, even the strictest only makes sense in relation to this inner certainty, as if it is precisely this that it is displayed to. The thing is that if you look more closely it is clear that this inner certainty is no such thing and actually requires an appeal to an even deeper basis. This is not an infinite regress, it only goes one step beyond your individual self-certainty, but it is a leap.