If an attribute belongs to the whole of a subject, whatever fails to exhibit that attribute does not come under the subject. § 524. This statement contemplates conversion by contraposition only in reference to the A proposition, to which the process has hitherto been confined. Logicians seem to have overlooked the fact that conversion by contraposition is as applicable to the O as to the A proposition, though, when expressed in symbols, it presents a more clumsy appearance. Some A is not B. .'. Some not-B is not not-A. Some wholesome things are not pleasant. .'. Some unpleasant things are not unwholesome. § 525. The above admits of analysis in exactly the same way as the same process when applied to the A proposition. Some A is not B. .'. Some A is not-B (by permutation). .'. Some not-B is A (by simple conversion). .'. Some not-B is not not-A (by permutation). The result, as in the case of the A proposition, is the converse by negation of the original proposition permuted. § 526. Contraposition may also be applied to the E proposition by the use of conversion per accidens in the place of simple conversion. But, owing to the limitation of quantity thus effected, the result arrived at is the same as in the case of the O proposition. Thus from 'No wholesome things are pleasant' we could draw the same inference as before. Here is the process in symbols, when expanded. No A is B. .'. All A is not-B (by permutation). .'. Some not-B is A (by conversion per accidens). .'. Some not-B is not not-A (by permutation). § 527. In its unanalysed form conversion by contraposition may be defined generally as--A form of conversion in which both subject and predicate are replaced by their contradictories. § 528. Conversion by contraposition differs in several respects from conversion by negation. (1) In conversion by negation the converse differs in quality from the convertend: whereas in conversion by contraposition the quality of the two is the same. (2) In conversion by negation we employ the contradictory either of the subject or predicate, but in conversion by contraposition we employ the contradictory of both. (3) Conversion by negation involves only two steps of immediate inference: conversion by contraposition three. § 529. Conversion by contraposition cannot be applied to the ordinary E proposition except by limitation (§ 526).