But whatever advantage there is in occasionally changing the Quality of
a proposition may be gained by the process of Obversion (chap. vii. §
5); whilst to use only one Quality would impair the elasticity of
logical expression. It is a postulate of Logic that the negative sign
may be transferred from the copula to the predicate, or from the
predicate to the copula, without altering the sense of a proposition;
and this is justified by the experience that not to have an attribute
and to be without it are the same thing.

§ 3. A. I. E. O.--Combining the two kinds of Quantity, Universal and
Particular, with the two kinds of Quality, Affirmative and Negative, we
get four simple types of proposition, which it is usual to symbolise by
the letters A. I. E. O., thus:

A. Universal Affirmative -- All S is P.
I. Particular Affirmative -- Some S is P.
E. Universal Negative -- No S is P.
O. Particular Negative -- Some S is not P.

As an aid to the remembering of these symbols we may observe that A. and
I. are the first two vowels in _affirmo_ and that E. and O. are the
vowels in _nego_.

It must be acknowledged that these four kinds of proposition recognised
by Formal Logic constitute a very meagre selection from the list of
propositions actually used in judgment and reasoning.

Those Logicians who explicitly quantify the predicate obtain, in all,
eight forms of proposition according to Quantity and Quality: