The heart of philosophy is metaphysics, and at the heart of the heart lie two questions about existence. What is it for any contingent thing to exist? Why does any contingent thing exist? Call these the nature question and the ground question, respectively. The first concerns the nature of the existence of the contingent existent; the second concerns the ground of the contingent existent. Both questions are ancient, and yet perennial in their appeal; both have presided over the burial of so many of their would-be undertakers that it is a good induction that they will continue to do so. For...