Metaphysics (Ontology) is basically the study of the possible assumptions one can make when faced with reality. The three most commonly seen metaphysics being different forms of Materialism, Dualism, and Idealism. Some other notable metaphysics are Vitalism, Naturalism, Historicism, Objectivism, Subjective Relativism, and Personalism.
For the purpose of convenience I will explain in brief detail the differences between these metaphysics. The basis of all metaphysics is to describe what is real and more importantly what is most real. before I get to it; for those of you who ask why this is important (Philosophy bakes no bread) these metaphysics are the basis of religions ways of dealing with life, which some people will kill for.
First the most simple which is the basis of modern science: Materialism. Materialism, in metaphysics, is characterized by defining matter (stuff) as most real. For hard materialists this means that the only things that are real are those things which you can touch. When one goes to look at consciousness from this perspective, it can only be a result of matter and chemicals. Hard materialists don't believe in things like heaven or God. The most notable materialist is Karl Marx who also resembles a historicist, more on that later. Also from the actual texts we have Gautama Siddhartha (The Buddha) appears to be materialist.
Second we cover the opposite end of the spectrum Idealism. Idealism, in metaphysics, is characterized by defining soul/consciousness/form (non-stuff) as most real. Most idealists are soft idealists believing that matter exists, but that it for the soul purpose of creating ideas, or that it is an illusion created by non-stuff. Their are several notable idealists Hegel, Hindus, Berkeley, and others. Berkeley goes beyond most idealists by basically denying the existence of stuff. As I recall Kant is in this same line of thinking. Hegel develops the concept of Historicism (which can also assume one of the other two basic metaphysics) declaring that Guist (or world spirit) is behind everything. The Hindus believe in Brahman, a consciousness that is everything and is behind everything in the material world.
Third is the last of the most basic metaphysics, and perhaps the most commonly believed: Dualism. Dualism is the belief that there is both matter (stuff) and spirit (non-stuff). They have one major problem in interaction (if there is both stuff and non-stuff how/why do they effect each other). This view is that of the Hebrew Religions (Judaism, Christianity, and Islam) but perhaps more accurately a Dualism based on Personalism. Notable dualists include St. Augustine, St. Thomas Aquinas, Descartes (although some might place him with the idealists), Soren Kierkegaard, Martin Hiedegger (also a Personalist.), Plato, Aristotle, and the list goes on and on. This view includes things like God, man, Heaven, Earth, mind/soul, body, form, and object (this list was ordered by the non-stuff and opposing stuff).
As for the others here is what they elevate above all else, and their primary influence(s):
- Vitalism: the elevation of vitality (sex, sort of). This a more accurate version of what the Hindus believe.
- Naturalism: A belief in natural laws, and an elevation of natural above all else (and the denial of the supernatural), a friendlier Materialism. The common view of reality in Greece before the appearance of Socrates.
- Historicism: The elevation of history above all else. Hegel uses the patterns of history and envisions World Spirit (a sort of god). Marx does something similar but denies the existence of God.
- Subjective Relativism: The elevation of the subject above all else. The objective is denied and all things believed true by the subject are then true (note: nothing can have any real truth or falsehood in this system).
- Hedonism: The elevation of self above all else. This concept (popularized by Hedon) is what modern Capitalism's morals most closely resemble. Their is only stuff, and so you should do whatever feels good.
- Objectivism: Ayn Rand's philosophy, just go to the node.
- Personalism: The node does not yet cover the metaphysic, but hopefully it will. Personalism is the elevation of people above everything else. This idea entered into religion with Jesus. The long and short of it is love one another.
That ends this short lesson in the metaphysics, I hope that you can take something from it.