B is a species of A A is the father of B .'.

B is a child of A._ But not every relational expression has only one definite reciprocal.

If we are told that _A is the brother of B_, we can only infer that _B is either the brother or the sister of A_.

A list of all reciprocal relations is a desideratum of Logic.
Sec.5.Obversion (otherwise called Permutation or AEquipollence) is Immediate Inference by changing the quality of the given proposition and substituting for its predicate the contradictory term.

The given proposition is called the 'obvertend,' and the inference from it the 'obverse.' Thus the obvertend being--_Some philosophers are consistent reasoners_, the obverse will be--_Some philosophers are not inconsistent reasoners_.
The legitimacy of this mode of reasoning follows, in the case of affirmative propositions, from the principle of Contradiction, that if any term be affirmed of a subject, the contradictory term may be denied (chap.vi.Sec.