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In traditional logic, obversion is a form of immediate inference in which from a given categorical proposition another proposition is inferred which has as its subject the original subject, and has as its predicate the complement of the original predicate. The quality of the inferred categorical proposition is changed but the truth value is equivalent to the original proposition. The immediately inferred proposition is termed the "obverse" of the original proposition, and is a valid form of inference for all types (A, E, I, O) of categorical propositions.
In a universal affirmative and a universal negative proposition the subject term and the predicate term are both replaced by their complements:
The universal affirmative ("A" proposition) is obverted to a universal negative ("E" proposition).
"All S is P" and "No S is non-P"
"All cats are animals" and "No cats are non-animals"
The universal negative ("E" proposition) is obverted to a universal affirmative ("A" proposition).
"No S is P" and "All S is non-P"
"No cats are friendly" and "All cats are unfriendly"
In the particular affirmative the quantity of the subject term remains unchanged, but the predicate term of the inferred proposition negates the complement of the predicate term of the original proposition. The particular affirmative ("I" proposition) is obverted to a particular negative ("O" proposition).
"Some S are P" and "Some S are not non-P"
"Some animals are friendly creatures" and "Some animals are not unfriendly creatures."
In the obversion of a particular negative to a particular affirmative the quantity of the subject also remains unchanged, and the predicate term is changed from simple negation to a term of the complementary class. The particular negative ("O") proposition is obverted to a particular affirmative ("I" proposition).
"Some S is not P" and "Some S is non-P"
"Some animals are not friendly creatures" and "Some animals are unfriendly creatures."
Note that the truth-value of an original statement is preserved in its resulting obverse form. Because of this, obversion can be used to determine the immediate inferences of all categorical propositions, regardless of quality or quantity.
In addition, obversion allows us to navigate through the traditional square of logical opposition by providing a means for us to proceed from "A" Propositions to "E" Propositions, as well as from "I" Propositions to "O" Propositions, and vice versa. However, it must be noted that although the resulting propositions from obversion are logically equivalent to the original statements in terms of truth-value, they are not semantically equivalent to their original statements in their standard form.
See also
"Obversion"