The opening chapter explores the fundamental question of where the stabilising force of res judicata should begin and end, framing it as both a legal and philosophical paradox: while finality in adjudication fosters social peace, excessive rigidity can hinder necessary adaptation, restrict procedural freedom, and ultimately undermine the very stability it aims to protect. Through vivid hypotheticals—from disputes over a single wall to broader community impacts—the chapter illustrates the practical and theoretical stakes of defining res judicata’s scope. The central controversy is whether its binding effect should apply solely to the main issue or also to preliminary determinations. By challenging the traditional divide between the operative part and the grounds of a judgement, the chapter argues that certain preliminary issues—if fully litigated and expressly decided— deserve the same stabilising force. This proposition finds support in Anglo-American collateral estoppel and recent Brazilian procedural reforms. For procedural theorists, this is not a mere doctrinal nuance but an invitation to rethink the architecture of civil adjudication. If res judicata is indeed a constitutional guarantee, the question shifts from whether it may extend to preliminary issues to under what conditions it must—balancing the enduring need for legal certainty with the equally vital imperative of procedural freedom.