The negation in intuitionistic logic is stronger than the classical negation. It is intuitionistic and intelligible, while those in the other two logics are not intuitionistic. The reason is that the former can be defined by classical negation but the good property is absent from the latter two negations. Any non-classical negation is clear only if it can be explained by classical negation. It is not precise that the law of noncontradiction doesn't hold for paraconsistent logic. The meaning of the contradiction here is different from the usual.