Ontology is the philosophical study of existence, of being. It may seem a little outside the domain of sociology, but is relevant to social research: are there social facts out there to be discovered? Or instead of an objective reality, are there only social constructs? Do the apparent "facts" in social research only exist in their subjective social context? Questions of ontology are closely linked with those of epistemology: it's not just whether things objectively exist, but how we know they exist.