In propositional logic with the usual two truth values, negating a proposition twice preserves its truth value. Thus the complement of a set taken twice, in one fixed universal set, is the original set.
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Double negation is a logical principle stating that a proposition that is negated twice is equivalent to the proposition itself. In simpler terms, if you say "not not P," you are effectively affirming P. In formal logic, if "P" is a statement, then the double negation can be expressed as: ¬(¬P) ≡ P This principle is used in various fields, including mathematics, philosophy, and computer science.