The Principle of Res Judicata: Res Judicata, a Latin term meaning 'a matter judged,' is a fundamental principle of law that prevents the re-litigation of an issue that has already been finally decided by a competent court. In India, this principle is primarily codified in Section 11 of the Code of Civil Procedure, 1908 (CPC). Its core purpose is to bring finality to litigation and prevent endless disputes over the same subject matter.

Objectives of Res Judicata:

1. Finality of Litigation (Interest Reipublicae Ut Sit Finis Litium): It is in the public interest that there should be an end to litigation. Once a matter has been decided, it should not be reopened.
2. Prevention of Harassment (Nemo Debet Bis Vexari Pro Una Et Eadem Causa): No one should be vexed twice for the same cause. It protects parties from being subjected to repeated lawsuits over the same issue.
3. Maintenance of Judicial Dignity: It upholds the authority and dignity of the courts by ensuring that their judgments are respected and not challenged repeatedly.

Conditions for Applicability of Res Judicata (Section 11 CPC): For the principle of Res Judicata to apply, the following conditions must be met:

1. Matter in Issue: The matter directly and substantially in issue in the subsequent suit must be the same as the matter directly and substantially in issue in the former suit.
2. Same Parties: The former suit must have been between the same parties or between parties under whom they or any of them claim, litigating under the same title.
3. Competent Court: The court which decided the former suit must have been competent to try the subsequent suit or the suit in which such issue has been subsequently raised.
4. Heard and Finally Decided: The matter directly and substantially in issue in the former suit must have been heard and finally decided by such court.

Applicability Against Co-Plaintiffs and Co-Defendants: Yes, the principle of Res Judicata can be applicable against co-plaintiffs and co-defendants, a concept often referred to as 'Res Judicata inter partes' or 'Res Judicata between co-defendants/co-plaintiffs.' However, its application in such cases is not automatic and is subject to specific circumstances.

Circumstances for Applicability Against Co-Parties: For Res Judicata to operate between co-plaintiffs or co-defendants, the following conditions, as established by judicial precedents, must generally be satisfied:

1. Conflict of Interest: There must have been a conflict of interest between the co-plaintiffs or co-defendants in the former suit. This means that their respective claims or defenses were adverse to each other.
2. Necessity of Decision: It must have been necessary to decide that conflict of interest between the co-parties to grant the relief claimed by the plaintiff in the former suit. The decision on the inter se rights of the co-parties must have been essential for the final adjudication of the suit.
3. Final Decision: The question between the co-parties must have been actually raised and finally decided by the court in the former suit. A mere finding or observation not essential to the final decree will not operate as Res Judicata.
4. Proper Parties: The co-parties must have been necessary or proper parties to the former suit, and the issue between them must have been within the scope of the suit.

Example: Consider a suit where a plaintiff sues two defendants (D1 and D2) for a property. D1 claims ownership, and D2 claims a right of easement over the same property, which D1 disputes. If the court, in deciding the plaintiff's claim, also finds it necessary to determine the respective rights of D1 and D2 regarding the property (e.g., whether D2 has an easement over D1's claimed portion), and this issue is finally decided, then that decision can operate as Res Judicata between D1 and D2 in a subsequent suit concerning the easement. The key is that the decision on the inter se rights of the co-parties must have been integral to the judgment in the former suit.

In essence, Res Judicata applies to co-parties when their rights or liabilities inter se were directly and substantially in issue, were necessary to be decided for the final decree, and were actually decided by a competent court.