Metaphysics
What is the true nature of reality? For instance, is spacetime a "substance" or does it merely represent the relational facts that obtain between genuine substances, such as fundamental particles? Do the non-modal facts ground the modal facts, e.g. about what is possible or whether things have essences or powers? And how do things persist through time, i.e., how do things retain their identities while undergoing intrinsic change over time?