Explanation:

Section 21 - 21. Proof of admissions against persons making them, and by or on their behalf. –– Admissions are relevant and may be proved as against the person who makes them or his representative in interest; but they cannot be proved by or on behalf of the person who makes them or by his representative in interest, except in the following cases: ––

(1) An admission may be proved by or on behalf of the person making it, when it is of such a nature that, if the person making it were dead, it would be relevant as between third persons under section 32. (2) An admission may be proved by or on behalf of the person making it, when it consists of a statement of the existence of any state of mind or body, relevant or in issue, made at or about the time when such state of mind or body existed, and is accompanied by conduct rendering its falsehood improbable. (3) An admission may be proved by or on behalf of the person making it, if it is relevant otherwise than as an admission.

Illustration (a) - The question between A and B is, whether a certain deed is or is not forged. A affirms that it is genuine, B that it is forged. A may  prove a statement by B that the deed is genuine, and B may prove a  statement by  A that  deed is  forged; but  A cannot  prove a statement by  himself that  the deed  is genuine,  nor can  B prove  a statement by himself that the deed is forged.