Immediate inference

An immediate inference is an inference which can be made from only one statement or proposition. For instance, from the statement "All toads are green." we can make the immediate inference that "No toads are not green." There are a number of immediate inferences which can validly be made using logical operations, the result of which is a logically equivalent statement form to the given statement. There are also invalid immediate inferences which are syllogistic fallacies.

Valid immediate inferences

See also: Categorical proposition § Operations on categorical statements

Converse

Main article: Converse (logic)

• Given a type E statement, from the traditional square of opposition, "No S are P.", one can make the immediate inference that "No P are S" which is the converse of the given statement.
• Given a type I statement, "Some S are P.", one can make the immediate inference that "Some P are S" which is the converse of the given statement.

Obverse

Main article: Obversion

• Given a type A statement, "All S are P.", one can make the immediate inference that "No S are non-P" which is the obverse of the given statement.
• Given a type E statement, "No S are P.", one can make the immediate inference that "All S are non-P" which is the obverse of the given statement.
• Given a type I statement, "Some S are P.", one can make the immediate inference that "Some S are not non-P" which is the obverse of the given statement.
• Given a type O statement, "Some S are not P.", one can make the immediate inference that "Some S are non-P" which is the obverse of the given statement.

Contrapositive

Main article: Contraposition (traditional logic)

• Given a type A statement, "All S are P.", one can make the immediate inference that "All non-P are non-S" which is the contrapositive of the given statement.
• Given a type O statement, "Some S are not P.", one can make the immediate inference that "Some non-P are not non-S" which is the contrapositive of the given statement.

Invalid immediate inferences

Cases of the incorrect application of the contrary, subcontrary and subalternation relations are syllogistic fallacies called illicit contrary, illicit subcontrary, and illicit subalternation. Cases of incorrect application of the contradictory relation are so infrequent, that an "illicit contradictory" fallacy is usually not recognized.

Illicit contrary

• It is false that all A are B, therefore no A are B.
• It is false that no A are B, therefore all A are B.

Illicit subcontrary

• Some A are B, therefore it is false that some A are not B.
• Some A are not B, therefore some A are B.

Illicit subalternation (Superalternation)

• Some A are not B, therefore no A are B.
• It is false that all A are B, therefore it is false that some A are B.

See also

• Transposition (logic)
• Inverse (logic)