Metaphysics and its sub-domain ontology study reality, what parts comprise it, and how they fit together systematically. This includes what entities there are in the world (e.g. human beings, time, space), what categories they may fall under (e.g. substances or properties), what relations there may be between them (e.g. causal, explanatory, or type-token relations), and what different modes of existence (e.g. necessary or contingent) they may have. One important topic for metaphysics is human beings and our capabilities, including whether we have free will and what sort of natural laws would have to be in place (e.g. deterministic or probabilistic) to allow it. But any aspect of reality is fair game for metaphysics, including the metaphysics of social reality, asking for example whether there is anything real underlying such social categories as gender or race.

Most religions posit a transcendent or divine reality. The metaphysics of religion investigates what conditions would be needed for this reality to exist, what such a reality could or could not be like, and what (metaphysical) arguments there might be for or against its existence. Other examples of metaphysical explorations of religion include the relationship between the divine and humans (e.g. whether humans can have free will if God knows the future), whether there actually is a single thing called “religion”, and the metaphysics of death and the afterlife.