_Some not-P is S_ O._Some S is not P_ .'.

_Some S is not-P_ .'.

_Some not-P is S_ There is no contrapositive of I., because the obverse of I.is in the form of O., and we have seen that O.cannot be converted.

O., however, has a contrapositive (_Some not-P is S_); and this is sometimes given instead of the converse, and called the 'converse by negation.' Contraposition needs no justification by the Laws of Thought, as it is nothing but a compounding of conversion with obversion, both of which processes have already been justified.

I give a table opposite of the other ways of compounding these primary modes of Immediate Inference.
A I E O -- ------------------------------------------------------------------------------ 1 All A is B Some A is B No A is B Some A is not B -- ------------------------------------------------------------------------------ Obverse 2 No A is b Some A is not b All A is b Some A is b -- ------------------------------------------------------------------------------ Converse 3 Some B is A Some B is A No B is A -- ------------------------------------------------------------------------------ Obverse of 4 Some B is not a Some B is not a All B is a Converse -- ------------------------------------------------------------------------------ Contra- positive 5 No b is A Some b is A Some b is A -- ------------------------------------------------------------------------------ Obverse of 6 All b is a Some b is not a Some b is not a Contrapos -- ------------------------------------------------------------------------------ Converse of Obverse 7 Some a is B of Converse -- ------------------------------------------------------------------------------ Obverse of Converse of 8 Some a is not b Obverse of Converse -- ------------------------------------------------------------------------------ Converse of Obverse 9 Some a is b of Contrapos -- ------------------------------------------------------------------------------ Obverse of Converse of 10 Some a is not B Obverse of Contrapos -- ------------------------------------------------------------------------------ In this table _a_ and _b_ stand for _not-A_ and _not-B_ and had better be read thus: for _No A is b, No A is not-B_; for _All b is a_ (col.