When one person has, by his declaration, act or omission, intentionally caused or permitted another person to believe a thing to be true and to act upon such belief, neither he nor his representative shall be allowed, in any suit or proceeding between himself and such person or his representative, to deny the truth of that thing.
Illustrations
A intentionally and falsely leads B to believe that certain land belongs to A, and thereby induces B to buy and pay for it. The land afterwards becomes the property of A, and A seeks to set aside the sale on the ground that, at the time of the sale, he had no title. He must not be allowed to prove his want of title.
Legal Commentary & Landmark Judgments: Section 121
Section 121 codifies the equitable doctrine of Estoppel, preventing a party who has made a representation from subsequently resiling from it to the detriment of another.
Section 121 protects commercial and legal reliance by precluding a party from retracting unequivocal representations that have induced another to alter their position.
Estoppel is founded on good conscience and equity: a party cannot blow hot and cold (approbate and reprobate). Once an individual induces another to alter their position based on a representation, the law estops them from asserting the contrary.
Section 121 is a rule of civil evidence with immense constitutional and commercial ramifications. In administrative law, it prevents governments from arbitrarily withdrawing tax exemptions or tender commitments after commercial parties have invested capital in reliance on state promises.