Symmetry and Numerality
Philosophers like to split systems (especially the universe and other grand concepts) into fundamental components. They do this by identifying the many components of the system, and then reducing them (that is, combining those that are related). When the components can be combined and reduced no more, the number that remain is the numerality of the system. The components of a system are often opposed to one another, or in some way opposites, and may display symmetry.
Simplest is monism, which says that the universe is only one thing. An example of monism is the unrealized Theory of Everything in physics, which would explain all the different stuff in the universe as different aspects of one thing.
Ever-popular dualism splits concepts in two. Dualism is apparent all around us: true/false, positive/negative, good/evil, past/future. Our bodies are also bilaterally symmetrical, so we have two arms, two eyes, and two lobes to our brains.
In (some versions of) Zoroastrianism there are two equal but opposite supreme gods, Ahura Mazda and Angra Mainyu, who represent good and evil respectively. Likewise Manichaeism says that the universe is comprised of only light and darkness - light is associated with goodness and the soul; darkness with evil and the body.
This mind/matter dualism is also seen in the western philosophy referred to simply as Dualism. Dualism says that our thoughts, memories and souls are distinct from the rest of the physical universe.
My favourite is triadism. See Triad for more information.
A system involving four-way division would be, I suppose, called quadrism. In physics forces are divided into four fundamentals: electromagneticism, weak, strong, and gravity (though the first four have in fact been unified). The EsNa is another quadrism.