| dualism | (noun) the doctrine that reality consists of two basic opposing elements, often taken to be mind and matter (or mind and body), or good and evil | - |
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| dualist | (noun) an adherent of dualism | - |
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| duality | (noun) (physics) the property of matter and electromagnetic radiation that is characterized by the fact that some properties can be explained best by wave theory and others by particle theory | Synonyms: wave-particle duality |
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| (noun) (geometry) the interchangeability of the roles of points and planes in the theorems of projective geometry | - |
| (noun) being twofold; a classification into two opposed parts or subclasses | Synonyms: dichotomy |