Logicians use ' negation' in different senses. Sometime it is used to negate a proposition whereas at other times it is used to negate its two parts [subject and predicate]. When it is used to negate a proposition, it is used in the form of negating either its quantity or quality. However, when it is used to negate its two parts, it acts in the form of either ' negation' or ' diversion'. After relating the different uses of ' negation' in classic and modern logics and presenting their solutions for differentiating among them, this article presents their theory concerning the equality of some affirmative and negative propositions. According to this article, the negation used in a negative proposition is referred to as ‘negating predication’, the predicate of negative propositions as negative predicate and the negative proposition as containing a negative predicate. It has tried to show that negative proposition, negation of predication of the proposition with negative predicate and ' that which contains a negative predicate, ma' adula al-mahmool (a proposition in which its predicate is negative), du al-mahmool al-oduli (a proposition in which its predicated is diverted) and negation are equivocal rather than univocal. Thus, when logicians have taken some negative propositions to be identical with affirmative propositions, they have confused negative predication with one of the propositions that have negative or diverted predicates- a thing that has caused contradiction in their words. This is because some have taken negative proposition to be identical with affirmative propositions at a time when they have considered negative propositions as devoid of predication and the affirmative propositions as having predication.