Ontology, as etymology suggests, is the study of being, of what there is. The ontologist asks: What entities or kinds of entity exist? Are there abstract entities, such as sets or numbers, in addition to concrete entities, such as people and puddles and protons? Are there properties or universals in addition to (or instead of) the particular entities that, as we say, instantiate them? Questions such as these have divided philosophers down the ages, and divide them no less to this day.