what is ontology in philosophy

Ontology is a branch of philosophy that deals with the study of being, or what types of entities exist, how they are grouped into categories, and how they are related to one another on the most fundamental level. It is a part of metaphysics, which is a branch of philosophy that looks at the very nature of things, their being, cause, or identity. Ontology seeks the classification and explanation of entities, and it concerns claims about the nature of being and existence. Many classical philosophical problems are problems in ontology, such as the question of whether or not there is a god, or the problem of the existence of universals. Ontology is a deductive science, knowable a priori and based on two fundamental principles: the principle of non-contradiction ("it cannot happen that the same thing is and is not") and the principle of sufficient reason ("nothing exists without a sufficient reason for why it exists") .