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The study of what ultimately exists or, alternatively, what a given theory or belief system must be committed to as existing. In the first sense, ontology is synonymous with metaphysics. In the second sense, ontology investigates what a set of beliefs or statements must assume exists for them to be true, but without committing to the truth of those beliefs or statements. Thus, in this second sense, one might investigate the ontology of the caloric theory, asking, for instance, whether the theory is committed to the existence of negative masses.