That department of the science of metaphysics which investigates and explains the nature and essential properties and relations of all beings, as such, or the principles and causes of being.
a subphilosophical academic discipline. It is the philosophical study of the nature of being, becoming, existence and/or reality, as well as the basic categories of being and their relations. [it] often deals with questions concerning what entities exist or may be said to exist and how such entities may be grouped, related within a hierarchy, and subdivided according to similarities and differences.
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Did Adam and Eve produce daughters? These were the great, complex questions of ontology that tormented him.
Joseph Heller. Catch-22, p.268 (1961)
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The shapes, while they resemble those of crystals and snowflakes, are ontologically meaningless; and the “reconjungation ” and “redissolusingnesses”, however complex, offer no nutriment to the intellect. They are fortuitous rather than dramatic actions;