Duality; the condition of being double.
The belief that the world is ruled by a pair of antagonistic forces, such as good and evil; the belief that man has two basic natures, the physical and the spiritual.
The view that the world consists of, or is explicable in terms of, two fundamental principles, such as mind and matter or good and evil.
The mathematical equivalence of two seemingly different theoretical descriptions of a physical system.
A classification into two subclasses or opposed parts.
The interchangeability of points and planes.