All A is B.
.'. Some B is not not-A.
All ingenuous persons are agreeable.
.'. Some agreeable persons are not disingenuous.
§ 513. The intermediate link between the above two propositions is the converse per accidens of the first—'Some B is A.' This inference, however, coincides with that from 1 (§ 508), as the similar inference from E (§ 510) coincides with that from 0 (§ 506).
§ 514. All these inferences agree in the essential feature of combining permutation with conversion, and should therefore be classed under a common name.
§ 515. Adopting then this slight extension of the term, we define conversion by negation as—A form of conversion in which the converse differs in quality from the convertend, and has the contradictory of one of the original terms.
§ 516. A still more complex form of immediate inference is known as
Conversion by Contraposition.
This mode of inference assumes the following form—
All A is B.
.'. All not-B is not-A.
All human beings are fallible.
.'. All infallible beings are not-human.