Note 291 — Chapter IX
Reading edition
On the assertoric interpretation If A then C merely negatives ACʹ, while If C then A merely negatives AʹC, and hence it is clear that neither of these propositions involves the other; on the modal interpretation the result is the same, for the truth of C may be a necessary consequence of the truth of A, while the converse does not hold good.
A consideration of immediate inferences enables us to shew from another point of view that If A then C and If A then Cʹ are not true contradictories. For the contrapositives If A then Cʹ, If C then Aʹ, are equivalent to one another; and whenever two propositions are equivalent, their contradictories must also be equivalent. But If A then C is not equivalent to If C then A.