Obversion

Obversion is a logical operation in traditional logic that transforms a categorical proposition into an equivalent one. It involves two steps. First, the quality of the proposition is changed. This means that an affirmative proposition (A or I) is converted to a negative one (E or O), and a negative proposition is converted to an affirmative one. Second, the predicate term of the proposition is negated. For example, the A proposition "All S are P" becomes "No S are not P." The E proposition "No S are P" becomes "All S are not P." The I proposition "Some S are P" becomes "Some S are not not P." The O proposition "Some S are not P" becomes "Some S are not not P."

Obversion is a valid inference because the obverted proposition is always logically equivalent to the original proposition. This means that if the original proposition is true, the obverted proposition is also true, and vice versa. The operation is useful for clarifying the meaning of propositions and for simplifying logical arguments. It is a fundamental tool in understanding the relationships between different types of categorical propositions. The process of obversion can be applied to all four standard forms of categorical propositions: A, E, I, and O. Each form undergoes the same two-step transformation, resulting in a logically equivalent statement with a negated predicate.