All A is not-B.
.'. All B is not-A.
Lastly,
All not-A is not-B.
.'. All B is A.
§ 520. The following practical rule will be found of use for the right performing of the process—
Transpose the subject and predicate, and substitute for each its
contradictory term.
§ 521. As concrete illustrations of the above forms of inference we may take the following—
All the men on this board that are not white are red.
.'. All the men On this board that are not red are white.
Again,
All compulsory labour is inefficient.
.'. All efficient labour is free (=non-compulsory).
Lastly,