Note 162 — Chapter IV
Reading edition
Similarly, granting the validity of obversion, the contraposition of O may be established by a syllogism in Datisi as follows:—
Given Some S is not P, then we have
All S is S,by the law of identity,
andSome S is not-P,by obversion of the given proposition,
therefore,Some not-P is S.
It will be found that, adopting the same method, the contraposition of E is yielded by a syllogism in Darapti.