Ontology
From the Greek ontos and logos meaning the understanding or theory of being.
One of the major subdivisions of metaphysics, 'Ontology' investigates the nature
of those things which actually exist. There are two main ontological theories:
1) Monism - which maintains there is only one type of substance (usually thought
to be matter) in the universe, and 2) Dualism - maintains there are two
fundamental substances (matter and mind) in the universe.