42. Convertibility of Propositions. -- Conversion consists in transposing the two terms of a proposition so that the new propositions so obtained shall also be true if the original is true.

(1) The universal negative is convertible, for both the terms are universal. E. g.: No mineral is capable of vital functions; no being capable of vital functions is a mineral.

(2) The particular affirmative proposition is convertible, for here, also, the two terms are of the same extension. E. g.: Some sentient beings are endowed with reason; some beings endowed with reason are sentient.

In these two cases the convertibility is evident: the two terms are purely and simply interchangeable.

These are in fact the only cases of interchangeability.

(3) It must be noted, first of all, that singular propositions are never susceptible of any but an apparent conversion, since a determinate individual term, representing in the last analysis a first substance, cannot serve to express a formal predicable idea. E. g., whether I say, "Peter is a learned man," or, "A learned man is Peter," the same Peter, in spite of the inversion, will still be the subject.

(4) The universal affirmative is susceptible of conversion, in the sense that the predicate can take the place of the subject and vice versa, but on condition that the subject turned into a predicate is modified by some mark of particularity with a restrictive sense. The conversion effected on these conditions is said to be imperfect. E. g.: All men are sentient; certain beings endowed with feeling are men.

There must still be an exception in the essential definition, where the idea defined is equal to the definition. This "imperfect" conversion is no true conversion, for this consists in the simple mutual substitution of the two terms. The addition of a sign of particularity which renders the conversion imperfect alters its nature.