All cases of A not being B are cases of C being D.
This is not a case of A not being B.
.'. This is not a case of C not being D.
Affirmation of Consequent = Undistributed Middle.
Either A is B or C is D.
C is D.
is equal to
If A is not B, C is D.
C is D.
is equal to
All cases of A not being B are cases of C being D.
This is a case of C being D.
§ 769. So far as regards the consequent, the two species of complex reasoning hitherto discussed are identical both in appearance and reality. The apparent difference of procedure in the case of the antecedent, namely, that it is affirmed in the partly conjunctive, but denied in the disjunctive syllogism, is due merely to the fact that in the disjunctive proposition the truth of the consequent is involved in the falsity of the antecedent, so that the antecedent being necessarily negative, to deny it in appearance is in reality to assert it.
CHAPTER XXV.
The Disjunctive Syllogism regarded as an Immediate Inference.