Ontology is the philosophical study of the nature of being, existence or reality in general, as well as of the basic categories of being and their relations. Recognized as a part the of philosophy known as metaphysics, Ontology deals with questions concerning what exist or can be said to exist.
Materialism holds that the only things that exist are matter and energy, that all things are composed of material, that all actions require energy, and that all phenomena (including Consciousness) are the result of material interactions.
The earliest known comprehensive treatments of the subject of Existence are from Plato and Aristotle. Aristotle developed a comprehensive theory of being, according to which only individual things, called substances, fully have being. Other things such as relations, quantity, time, and place have a derivative kind of being, dependent on individual things.
In Aristotle’s Metaphysics, there are four causes of existence or change in nature: the material cause, the formal cause, the efficient cause and the final cause.