The following traditional summary of their relations in respect of truth and falsity is much more to the purpose: (1) If A.is true, I.is true, E.is false, O.is false.
(2) If A.is false, I.is unknown, E.is unknown, O.is true.
(3) If I.is true, A.is unknown, E.is false, O.is unknown.
(4) If I.is false, A.is false, E.is true, O.is true.
(5) If E.is true, A.is false, I.is false, O.is true.
(6) If E.is false, A.is unknown, I.is true, O.is unknown.
(7) If O.is true, A.is false, I.is unknown, E.is unknown.
(8) If O.is false, A.is true, I.is true, E.is false.
Where, however, as in cases 2, 3, 6, 7, alleging either the falsity of universals or the truth of particulars, it follows that two of the three Opposites are unknown, we may conclude further that one of them must be true and the other false, because the two unknown are always Contradictories.
Sec.10.Secondary modes of Immediate Inference are obtained by applying the process of Conversion or Obversion to the results already obtained by the other process.

The best known secondary form of Immediate Inference is the Contrapositive, and this is the converse of the obverse of a given proposition.

Thus: DATUM.

OBVERSE.

CONTRAPOSITIVE.
A._All S is P_ .'.