One of the theoretical and practical difficulties in civil proceedings is determining the instances in which the defendant, in defending themselves, is required to file a counterclaim, as well as the cases in which they can defend themselves without the need to file a lawsuit. Providing a clear criterion for distinguishing these two methods of defense is necessary because a counterclaim is filed by submitting a lawsuit, paying legal fees, etc., while a substantive defense does not require compliance with these requirements. By providing in Article 18 of the Code of Procedure of the General and Revolution Courts in Civil Affairs that any assertion, such as set-off, rescission, settlement, and the like, which constitutes a defense, is not a lawsuit and therefore does not require litigation, the legislature has left the issue shrouded in ambiguity. For this reason, the Doctrine has sought to resolve the ambiguity in the law and provide a clear standard for judicial practice by proposing criteria such as the “possibility of normal adjudication”, the “establishment of a new situation”, the “declaratory or executory nature of the defense”, and the “challenge to an established legal position”. However, it appears that these efforts have failed to achieve this objective and have been unable to provide meaningful guidance to judicial practice. After critically examining the foregoing approaches, this article concludes that the “defendant's direct challenge to the constituent elements of the plaintiff's claim” constitutes an appropriate criterion for distinguishing between the two and resolving the problem.