are interchanged. Note that this is not same to the implicational converse in the modern logic where a material implication statement is converted (conversion) to another material implication statement . The both conversions are equivalent only for A type categorical statements. From a statement in E or I form, it is valid to conclude its converse (as they are equivalent). This is not the case for the A and O forms. Obversion Obversion changes the quality (that is the affirmativity or negativity) of the statement and the predicate term. For example, by obversion, a universal affirmative statement become a universal negative statement with the predicate term that is the class complement of the predicate term of the original universal affirmative statement. In the modern forms of the four categorical statements, the negation of the statement corresponding to a predicate term P, , is interpreted as a predicate term 'non-P' in each categorical statement in obversion. The equality of can be used to obvert affirmative categorical statements. Categorical statements are logically equivalent to their obverse. As such, a Venn diagram illustrating any one of the forms would be identical to the Venn diagram illustrating its obverse. Contraposition Contraposition is the process of simultaneous interchange and negation of the subject and predicate of a categorical statement. It is also equivalent to converting (applying conversion) the obvert (the outcome of obversion) of a categorical statement. Note that this contraposition in the traditional logic is not same to contraposition (also called transposition) in the modern logic stating that material implication statements and are logically equivalent. The both contrapositions are equivalent only for A type categorical statements. See also Square of opposition Term logic Notes References External links ChangingMinds.org: Categorical propositions Catlogic: An open source computer script written in Ruby to construct, investigate, and compute categorical propositions and syllogisms Term logic Propositions