The enduring debate surrounding the classification of private international law rules as either substantive or procedural underscores the inherent complexity of navigating legal issues that transcend national borders. In other words, definitively categorizing these rules proves elusive, often falling prey to the broad definitions typically ascribed to substantive and procedural law within a purely domestic context. Substantive law, in its essence, establishes rights and obligations, while procedural law dictates the manner by which these rights are asserted and enforced. However, private international law operates at the intersection of different legal systems, adding layers of complexity to this traditional dichotomy.

The observation that private international law rules merely point towards the application of a foreign legal system highlights their seemingly procedural function. For instance, Philippine choice-of-law rules on succession may direct a local court to apply the national law of a deceased foreigner. This directive, in itself, does not create the right of the heirs to inherit; that right is established by the relevant substantive law, whether it be Philippine law or the identified foreign law. The choice-of-law rule acts as a signpost, guiding the forum towards the applicable substantive legal framework. As further illustration, the concept of renvoi, where the foreign law may refer the matter back to the law of the forum, further illustrates this point. The Philippine court, adhering to its own private international law rules, may be compelled to apply Philippine substantive law, not because its own choice-of-law rule created the right, but because the designated foreign law so directs. In this sense, the choice-of-law rule acts as a procedural mechanism for identifying the governing substantive law.

However, to categorize private international law rules as purely procedural overlooks their significant impact on the substantive outcome of a case. While the choice-of-law rule itself may not create a right, its application directly determines which body of substantive law will govern the dispute and, consequently, what rights and obligations the parties will have. Consider a contract dispute involving parties from different countries. The Philippine choice-of-law rules for contracts might point to the law of the place of performance. This "procedural" step of identifying the applicable law directly dictates whether the contract is deemed valid, what constitutes a breach, and what remedies are available. The substantive outcome hinges entirely on the application of this choice-of-law rule. Without it, the Philippine court would be left to apply its own substantive contract law, potentially leading to a different resolution. Thus, while the "pointing function" of private international law appears procedural, its effect on the substantive rights of the parties is undeniable.

Moreover, the constitutional argument regarding the Supreme Court's power to promulgate rules of pleading, practice, and procedure adds another dimension to the debate. (See Estipona . Lobrigo, G.R. No. 226679, August 15, 2017; 816 Phil. 789 [En Banc, Per J. Peralta].) If private international law rules were strictly procedural, their inclusion in statutes enacted by Congress, such as the New Civil Code and the Revised Penal Code, would arguably encroach upon the Supreme Court's constitutional prerogative. The fact that core choice-of-law principles are embedded in legislative enactments suggests that they are viewed as more than mere procedural directives. They are integral parts of the legal framework that defines the scope and application of substantive rights in cross-border situations.

The notion that private international law rules are sui generis, occupying a middle ground between substance and procedure, offers a compelling resolution to this dichotomy. Their application involves a unique blend of both aspects. The reference to foreign law can be seen as a procedural step, a mechanism for identifying the applicable rules. However, the subsequent application of that foreign law directly engages with substantive matters, determining the rights and obligations of the parties. Without the initial "procedural" step of choosing the applicable law, the relevant substantive provisions would remain immaterial to the case at hand. Therefore, private international law rules act as the crucial bridge, connecting the procedural act of identifying the governing law with the substantive application of that law to resolve the dispute. They are neither purely substantive nor purely procedural but possess a distinct character that enables the effective resolution of legal issues infected by a foreign element.