Inference by Converse Relation

In one line

If one thing stands in a relation to another, the second stands in the converse relation to the first.

In the wording a student can write in an examination: inference by converse relation is the immediate inference from a proposition asserting a relation between two terms to the proposition asserting the converse relation between the same terms in the reverse order. From "A is the father of B" it follows that "B is the child of A".

The form

"Ravi is the creditor of Bharat" gives "Bharat is the debtor of Ravi."

"This deed is a mortgage by Anil to the bank" gives "The bank is the mortgagee of Anil."

"A is greater than B" gives "B is less than A."

The relation in the conclusion is not the same relation; it is its converse, which is a different relation running the other way. Naming that converse correctly is the whole of the skill.

Why it works, and why it is not conversion

Why it works. A relation holds between two things in an order, as sequence 340 said: "Ravi owes Bharat" and "Bharat owes Ravi" are different propositions. Every relation nevertheless has a converse, the relation that holds between the same two things in the other order, and asserting one is asserting the other. Nothing has been added.

Why it is not conversion. Conversion, at sequence 450, swaps a subject and a predicate and keeps the relation, which is the copula. Here the two terms are both arguments of a relation, and what changes is the relation itself. Traditional logic could not perform this operation at all, because it had no way of representing a relation with two ends, which is failure one at sequence 320. Inference by converse relation is the traditional scheme borrowing something it cannot account for, which is a fair observation to make in an answer.

The relations a lawyer uses

Almost the entire vocabulary of private law consists of pairs of converse relations, and knowing that they come in pairs is worth more than learning them one at a time.

Why this matters practically. Proving one half of the pair proves the other. Evidence that a person advanced money as a loan establishes both that he is the creditor and that the borrower is the debtor, and it is a single piece of evidence and not two. A pleading that alleges one and not the other has not left a gap.