DUALITY
Duality generally refers to the concept of two complementary aspects or principles that exist together. It's commonly used in various contexts. In philosophy, duality often refers to the idea that reality is composed of two irreducible, contrasting principles or essences. For example, mind and body, good and evil, or spirit and matter.
The concept of duality is well reflected in the phrase "without shadow there is no light". This phrase suggests that opposing or contrasting elements are interdependent and define each other. It implies that we can only understand and appreciate one aspect of something by being aware of its opposite.