3. The four common forms of immediate inference are (1) opposition, (2) obversion, (3) conversion, (4) contraversion.
(1) The name opposition stands for certain definite relations which exist between the logical propositions when they are given the same subject and predicate. The one principle underlying opposition is: Whatever is said of the entire class may be said of a part of that class. The two statements which sum up opposition are first, an I may be derived from an A; and second, an O may be derived from an E.
The crucial fact made obvious by the square of opposition is that A and O are mutually contradictory; likewise E and I.
(2) Obversion is the process of passing from an affirmative to its equivalent negative or from a negative to its equivalent affirmative. “Two negatives are equivalent to one affirmative,” is the basic principle of obversion.
The proposition A may be obverted by negating the predicate and changing to an E. “E” is obverted by negating the predicate and changing to an A. “I” is obverted by negating the predicate and changing to an O. “O” is obverted by negating the predicate and changing to an I.
(3) Conversion is the process of inferring from a given proposition another which has as its subject the predicate of the given proposition and as its predicate the subject of the given proposition.
Conversion is limited by the two rules, (1) do not distribute an undistributed term; (2) do not change the quality.
To convert an A interchange subject and predicate, limiting the latter by some, or a word of like significance. This is called conversion by limitation.
The co-extensive A may be converted without limiting the predicate. This is called simple conversion.
An E proposition may be converted either simply or by limitation. When converted by limitation the inference is a weakened one.