The term dualism has several uses:
In theology, dualism can refer to the belief that there are two basic principles in the universe, usually personified as gods, that work in opposition to each other. One god is good, the other evil; some religions hold that one god works for order, the other for chaos. Both the Zoroastrian religion, three millennia old and still extant, and the essentially dead gnostic (and its variations such as Mandaeanism, Manichaeism, Bogomils, Cathars etc.) religions are dualistic. The third-century Christian heretic Marcion of Sinope held that the Old and New Testaments were the work of two opposing gods.
Alternatively, dualism can mean the tendency of humans to perceive and understand the world as being chunked into different categories. In this sense, it is dualistic when one perceives a tree as a thing separate from everything surrounding it, or when one perceives a "self" that is distinct from the rest of the world. In traditions such as Zen, a key to enlightenment is overcoming this sort of dualism. This is notoriously difficult, and may require a lifetime of practice.
In philosophy of mind, dualism refers to a narrow variety of views about the relationship between mind and matter, which are seen as totally different kinds of things. See Dualism (philosophy of mind). This is in contrast to monism, which views mind and matter as being ultimately the same kind of thing.
In philosophy of science, dualism often refers to the dichotomy between the "subject" (the observer) and the "object" (the observed). Some critics of Western science see this kind of dualism as a fatal flaw in science. In part, this has something to do with potentially complicated interactions between the subject and the object, of the sort discussed in the social construction literature.
