The negation of a proposition is its denial: classically, that proposition which is true when it is false, and false when it is true. The sign ¬ (for alternatives see Logical symbols ) expresses a truth-function, defined in the table: A proposition may be controverted in other ways than by asserting its strict negation. For instance ‘that's red’, is controverted by ‘that's green’, since both cannot be true together. But both can be false and one is not the direct denial of the other. See contrary, square of opposition.
Philosophy dictionary. Academic. 2011.