No S is P = All S is not-P.
Some S is P = Some S is not not-P.
Some S is not P = Some S is not-P.
Conversion.
The process takes its name from the interchange of the terms. The Predicate-term becomes the Subject-term, and the Subject-term the Predicate-term.
When propositions are analysed into relations of inclusion or exclusion between terms, the assertion of any such relation between one term and another, implies a Converse relation between the second term and the first. The statement of this implied assertion is technically known as the Converse of the original proposition, which may be called the Convertend.
Three modes of Conversion are commonly recognised:—(a) Simple Conversion; (b) Conversion per accidens or by limitation; (c) Conversion by Contraposition.
(a) E and I can be simply converted, only the terms being interchanged, and Quantity and Quality remaining the same.
If S is wholly excluded from P, P must be wholly excluded from S. If Some S is contained in P, then Some P must be contained in S.
(b) A cannot be simply converted. To know that All S is contained in P, gives you no information about that portion of P which is outside S. It only enables you to assert that Some P is S; that portion of P, namely, which coincides with S.