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: U. Toto-total Affirmative -- All X is all Y. A. Toto-partial Affirmative -- All X is some Y. Y. Parti-total Affirmative -- Some X is all Y. I. Parti-partial Affirmative -- Some X is some Y. E. Toto-total Negative -- No X is any Y. η. Toto-partial Negative -- No X is some Y. O. Parti-total Negative -- Some X is not any Y. ω. Parti-partial Negative -- Some X is not some Y. Here A. I. E. O. correspond with those similarly symbolised in the usual list, merely designating in the predicates the quantity which was formerly treated as implicit. § 4. As to Relation, propositions are either Categorical or Conditional. A Categorical Proposition is one in which the predicate is directly affirmed or denied of the subject without any limitation of time, place, or circumstance, extraneous to the subject, as _All men in England are secure of justice_; in which proposition, though there is a limitation of place ('in England'), it is included in the subject. Of this kind are nearly all the examples that have yet been given, according to the form _S is P_. A Conditional Proposition is so called because the predication is made under some limitation or condition not included in the subject, as _If a man live in England, he is secure of justice_. Here the limitation 'living in England' is put into a conditional sentence extraneous to the subject, 'he,' representing any man.