Aristotle's God sets the universe in motion not by means of mechanical causality but rather by, in its self-contemplating perfection, being worthy of yearning. We can only ever wager - rather than be certain - that there really is an ultimate object of this kind, and maybe the requisite wager is part and parcel of the desire itself. It would not be possible to yearn for God were God not potentially absent, or at least apparently so. The world exists because God is apparently absent from it.