The quality or state of being double, twofold, or consisting of two parts or aspects.
From 'double' plus the suffix '-ness,' which converts adjectives to abstract nouns. This word has been used in English philosophy and literature since the 14th century to discuss duality and division.
Philosophers love this word because it captures something deeper than just 'being two'—it describes the tension between paired opposites, like how light has the 'doubleness' of being both wave and particle.
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