Thus, even though res judicata may be said to be included in the doctrine of estoppel, as understood in the wider sense of the term, it must be distinguished from estoppel as distinctly provided for in the Indian Law of Evidence.
The doctrine of res Judicata can be distinguished from estoppel, as generally understood, on the following grounds:
The rule of res judicata is based on public policy, i.e., it is to the interest of the State that there should be an end to litigation and belongs to the province of procedure.
Estoppel, on the other hand, is part of the law of evidence and proceeds on the equitable principle of altered situation, viz., that he who, by his conduct, has induced another to alter his position to his disadvantage, cannot turn round and take advantage of such alteration of other’s position.
Res judicata precludes a man from avowing the same thing in successive litigations, while estoppel prevents a party from saying two contradictory things at different times.
Res judicata is reciprocal and binds the parties, while estoppel binds the party who made the previous statement or showed the previous conduct.
Res judicata, as observed by Mahmud, J. in Sita Ram v. Amir Begum, (8 Allahabad, 324), prohibits the court from entering into an enquiry as well as to a matter already adjudicated upon; estoppel prohibits a party, after the inquiry has already been entered upon, from proving anything which would contradict his own previous declaration or acts to the prejudice of another party who, relying upon these declarations or acts, has altered his position.
In other words, res judicata prohibits an inquiry in limine, and bars the trial of a suit while estoppel is only a piece of evidence and emphasises that a man should not be allowed to retrace the steps already walked over.
Res judicata ousts the jurisdiction of the court, while estoppel shuts the mouth of a party.
The doctrine of res judicata results from a decision of the court, while estoppel results from the acts of the parties themselves.