re eligible,
The virtuous alone are happy. The introduction of a negative particle
into these already negative forms makes a very trying problem in
interpretation. The aequipollence of the Exponibiles was dropped from
text-books long before Aldrich, and it is the custom to laugh at them
as extreme examples of frivolous scholastic subtlety: but most modern
text-books deal with part of the doctrine of the _Exponibiles_ in
casual exercises.

Curiously enough, a form left unnamed by the scholastic logicians
because too simple and useless, has the name AEquipollent appropriated
to it, and to it alone, by Ueberweg, and has been adopted under
various names into all recent treatises.

Bain calls it the FORMAL OBVERSE,[4] and the title of OBVERSION (which
has the advantage of rhyming with CONVERSION) has been adopted by
Keynes, Miss Johnson, and others.

Fowler (following Karslake) calls it PERMUTATION. The title is not a
happy one, having neither rhyme nor reason in its favour, but it is
also extensively used.

This immediate inference is a very simple affair to have been honoured
with such a choice of terminology. "This road is long: therefore, it
is not short," is an easy inference: the second proposition is the
Obverse, or Permutation, or AEquipollent, or (in Jevons's title) the
Immediate Inference by Privative Conception, of the first.

The inference, such as it is, depends on the Law of Excluded Middle.
Either a term P, or its contradictory, not-P, must be true of any
given subject, S: hence to affirm P of all or some S, is equivalent
to denying not-P of the same: and, similarly, to deny P, is to affirm
not-P. Hence the rule of Obversion;--Substitute for the predicate term
its Contrapositive,[5] and change the Quality of the proposition.

All S is P = No S is not-P.
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_.

Thr