_B < A_--a diminishing turns into an increasing ratio, whilst the fact predicated remains the same.

Given, then, a relation between two terms as viewed from one to the other, the same relation viewed from the other to the one may be called the Reciprocal.

In the cases of Equality, Co-existence and Simultaneity, the given relation and its reciprocal are not only the same fact, but they also have the same character: in the cases of Greater and Less and Sequence, the character alters.
We may, then, state the following rule for the conversion of propositions in which the whole relation explicitly stated is taken as the copula: Transpose the terms, and for the given relation substitute its reciprocal.

Thus-- _A is the cause of B .'.

B is the effect of A._ The rule assumes that the reciprocal of a given relation is definitely known; and so far as this is true it may be extended to more concrete relations-- _A is a genus of B .'.