§ 494. Some propositions occur in ordinary language in which the quantity of the predicate is determined. This is especially the case when the subject is a singular term. Such propositions admit of conversion by a mere transposition of their subject and predicate, even though they fall under the form of the A proposition, e.g.
Virtue is the condition of happiness.
.'. The condition of happiness is virtue.
And again,
Virtue is a condition of happiness.
.'. A condition of happiness is virtue.
In the one case the quantity of the predicate is determined by the form of the expression as distributed, in the other as undistributed.
§ 495. Conversion offers a good illustration of the principle on which we have before insisted, namely, that in the ordinary form of proposition the subject is used in extension and the predicate in intension. For when by conversion we change the predicate into the subject, we are often obliged to attach a noun substantive to the predicate, in order that it may be taken in extension, instead of, as before, in intension, e.g.
Some mothers are unkind.
.'. Some unkind persons are mothers.
Again,
Virtue is conducive to happiness.
.'. One of the things which are conducive to happiness is virtue.