وفي مانعة الجمع ينتج القسم الأول دون الثاني
In a disjunction excluding conjunction, the first kind of inference works, but the second does not.
Exclusion of conjunction means the sides cannot both be true. For example: “This shape is a triangle or a square.” It cannot be both in the same respect, but a circle is neither. Here “or” expresses incompatibility; it does not assert that every shape belongs to one of the two classes.
Thus affirming one alternative rules out the other; denying one does not establish the other because a third possibility remains.