Note Footnote 6: — Chapter III
Reading edition
(c) Another mode of Conversion, known by mediæval logicians following Boethius as Conversio per contra positionem terminorum, is useful in some syllogistic manipulations. This Converse is obtained by substituting for the predicate term its Contrapositive or Contradictory, not-P, making the consequent change of Quality, and simply converting. Thus All S is P is converted into the equivalent No not-P is S.6