is equal to
All cases of A not being B are cases of C being D.
This is not a case of C being D.
.'. This is not a case of A being B.
§ 767. The other moods of the first and second figures can be obtained by varying the quality of the antecedent and consequent in the major premiss and reducing the quantity of the minor.
§ 768. The invalid destructive moods correspond with the two invalid types of the partly conjunctive syllogism, and have the same fallacies of simple syllogism underlying them. Affirmation of the antecedent of a disjunctive is equivalent to the semi-conjunctive fallacy of denying the antecedent, and therefore involves the ordinary syllogistic fallacy of illicit process of the major.
Affirmation of the consequent of a disjunctive is equivalent to the same fallacy in the semi-conjunctive form, and therefore involves the ordinary syllogistic fallacy of undistributed middle.
Affirmation of Antecedent = Illicit Major.
Either A is B or C is D.
A is B.
.'. C is not D.
is equal to
If A is not B, C is D.
A is B.
.'. C is not D.
is equal to