But what is this but an immediate inference by contraposition, coming under the formula
All A is B,
.'. All not-B is not-A,
and followed by Subalternation?
§ 756. The fallacy of affirming the consequent becomes by this mode of treatment an instance of the vice of immediate inference known as the simple conversion of an A proposition. 'If A is B, C is D' is not convertible with 'If C is D, A is B' any more than 'All A is B' is convertible with 'All B is A.'
§ 757. We may however argue in this way
If A is B, C is D,
C is D,
.'. A may be B,
which is equivalent to saying,
When A is B, C is always D,
.'. When C is D, A is sometimes B,
and falls under the legitimate form of conversion of A per accidens—
All cases of A being B are cases of C being D.
.'. Some cases of C being D are cases of A being B.