Public international law and private international law have fundamental differences in their focus, subjects, sources, and dispute resolution mechanisms.

Public international law governs the relationships between states and other international actors. Think of treaties between nations regarding trade, human rights, or environmental protection. Thus, the United Nations Framework Convention on Climate Change (UNFCCC) is a prime example of public international law in action, establishing obligations for signatory states to reduce greenhouse gas emissions. Similarly, when a state brings a case against another state before the International Court of Justice (ICJ) concerning a border dispute or the violation of international law, this falls squarely within the realm of public international law.

The subjects in public international law are sovereign states, international organizations like the UN or the World Trade Organization (WTO). Sometimes, even individuals in the context of international criminal law or human rights law are covered by its scope. The sources of this legal framework are diverse and often less concrete than domestic law, encompassing customary international law (general and consistent state practice followed out of a sense of legal obligation), treaties (formal agreements between states), general principles of law recognized by civilized nations, judicial decisions, and the teachings of the most highly qualified publicists. When disputes arise, they are often resolved through diplomatic negotiations, mediation, arbitration, or adjudication before international tribunals like the ICJ. In extreme situations, embargo, trade barriers, and war -- as a last resort -- can be viewed as dispute resolution mechanisms in the field of public international law.

Private international law, in contrast, steps in when a legal issue involves a "foreign element." This "foreign element" essentially means that the factual scenario connects to more than one legal system -- at least one domestic to the forum and another one foreign to it. Consider a contract between a Filipino and a Japanese. If a dispute arises regarding the terms of this contract, the question becomes: which country's law or laws should govern the interpretation and enforcement of this agreement – Philippine law or Japanese law? This is the core concern of private international law. It does not deal with the law of nations; rather, it deals with the laws of a nation vis-à-vis the laws of the forum state.

If the dispute can be resolved using the laws of the forum state, there is no need to resort to principles in private international law.

Another illustration would be a marriage between a Filipino citizen and a French national. If they decide to divorce or own property in both the Philippines and France, private international law principles in both countries will be crucial in determining which jurisdiction's divorce laws apply and how the properties will be divided.

The subjects of private international law are primarily private individuals and legal entities engaging in cross-border transactions or relationships. The sources of private international law in the Philippines are primarily found in statutory provisions, such as the New Civil Code provisions on conflict of laws, and the pronouncements of the Philippine Supreme Court in interpreting and applying these rules. While international conventions and model laws can influence the development of private international law, their direct application often requires domestic legislative adoption, i.e., consent. Dispute resolution in this field typically occurs within national court systems, where the court applies the relevant choice-of-law rules to determine which substantive law (domestic or foreign) will govern the case.

One unique aspect to consider is the increasing interplay between public and private international law. While traditionally distinct, the rise of globalization and transnational issues has blurred some of these lines. For example, international commercial arbitration, while often involving private parties, can be significantly shaped by international conventions like the New York Convention on the Recognition and Enforcement of Foreign Arbitral Awards, which is a matter of public international law. (See Tuna Processing v. Philippine Kingford, G.R. No. 185582, February 29, 2012; 683 Phil. 276 [Second Division, Per J. Perez]) Similarly, the protection of foreign investments, while involving private investors, is often governed by bilateral investment treaties between states, a clear domain of public international law. This interconnectedness highlights that while the fundamental principles and focus areas remain distinct, the practical application of both fields can often intersect and influence each other in our increasingly interconnected world.

Furthermore, the evolution of principles in both fields reflects the changing dynamics of the international community. In public international law, we see the development of norms related to human rights, environmental protection, and international criminal justice, reflecting a growing consensus on shared values and responsibilities. In private international law, the shift towards more flexible and policy-oriented approaches in choice-of-law rules acknowledges the need to achieve just and equitable outcomes in cases with foreign elements, moving away from rigid and territorialist approaches like the lex rules. These evolutions, while stemming from different sources and driven by different considerations, both aim to create a more predictable and just international legal order.

In sum, public international law deals with rights, obligations, and interactions between and among states and international actors in relation to their membership in the community of nations. On the other hand, private international law deals with the study of the application of the proper law – whether domestic or foreign – on a given set of facts with a foreign element, affecting contracts, property, obligations and other private transactions between private individuals.

From this, it becomes clear that public international law is a whole other world than private international law. The principles behind each subject, the subjects or parties involved, the sources of law, and the remedy or manner of resolution of disputes are totally different. For example, the sources of public international law include customary international law and those ex aequo et bono. Whereas, the sources of private international law are usually provisions of law or pronouncements by the Supreme Court, which are more or less easily determinable in print than international customs or international equity which usually evolve from the views and values of the international community. Although, in private international law, there also occurs an evolution of applicable principles such as the adoption of modern approaches, these rules are more or less anchored on existing statutory provisions, especially insofar as Philippine law is concerned.