Metaphysics is a primary subject area in philosophy which investigates the nature, constitution, and structure of reality. It also deals with notions such as causation, time, modality (necessity and possibility), particulars and universals, and the relation of mind to body.
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Modailty deals with what is necessary and what is possible; e.g., a necessary truth is a proposition that could not have been false, a necessary existent is an entity that could not have failed to exist, etc.
Ontology is defined as the branch of metaphysics concerned with the nature relations of being or existence. It is the field of study concerned with the classification of entities.