LEGAL PRINCIPLE: EVIDENCE LAW – Nature of Plea of Res Judicata – Res Judicata as Defensive Plea Not Offensive Claim

This principle clarifies the procedural nature and proper use of res judicata as a defensive, not offensive, pleading. Res judicata operates to bar re-litigation of matters already decided, effectively depriving the court of jurisdiction to hear the subsequent action. This jurisdictional effect explains why it must be raised as a defense rather than as a basis for affirmative relief. A plaintiff cannot use res judicata in their statement of claim because they are asserting rights, not defending against them—res judicata denies rights already adjudicated, which is defensive in character. The proper venues for pleading res judicata are: (1) the defendant’s statement of defence, where it operates to bar the plaintiff’s claim as already decided; or (2) the plaintiff’s reply to the statement of defence, where the plaintiff may plead that the defendant’s counterclaim or set-off is barred by res judicata. The characterization as a “formidable weapon” reflects its conclusive effect—when successfully pleaded, it entirely bars the claim without consideration of merits. The principle prevents misuse of res judicata as a sword rather than a shield, ensuring it serves its proper function of preventing vexatious re-litigation while not being weaponized to pursue affirmative claims. Courts should strike out or disregard pleas of res judicata that appear in statements of claim as they are procedurally improper and contrary to the doctrine’s defensive nature.