Sources of International Law under Article 38(1) of the Statute of the International Court of Justice: A Philippine-Focused Survey

I. Introduction

Article 38(1) of the Statute of the International Court of Justice (ICJ) is universally regarded as the most authoritative catalogue of the formal sources of public international law. It governs the law-applying function of the ICJ, but—by force of custom and consistent State practice—it has also come to express the hierarchy and modes of proof of international norms in general.

For States like the Philippines, a mixed monist–dualist system whose Constitution both requires Senate participation for treaties (Art. VII, § 21) and “adopts the generally accepted principles of international law as part of the law of the land” (Art. II, § 2, the Incorporation Clause), understanding Article 38(1) is indispensable to domestic courts, the political branches, and scholars alike.

II. Primary Sources under Article 38(1)(a)–(c)

Art. 38(1) Provision	Essence	Philippine Reception
(a) International conventions (treaties and equivalent agreements)	Express, written consent of States; binding inter partes	Requires presidential signature and concurrence of “at least two-thirds of all the Members of the Senate” (Const., Art. VII, § 21). Tañada v. Angara (G.R. No. 118295, 1997) confirmed that once these steps are met, the treaty enjoys constitutional stature.
(b) International custom, as evidence of a general practice accepted as law	Objective element: widespread, consistent State practice; Subjective element (opinio juris)	Directly incorporated without need of legislation by virtue of Art. II, § 2. Invoked in Kuroda v. Jalandoni (G.R. L-2662, 1949) and Yamashita v. Styer (G.R. L-129, 1945) to justify war-crimes jurisdiction, and in Magallona v. Ermita (G.R. No. 187167, 2011) to sustain UNCLOS-consistent baselines.
(c) General principles of law recognized by “civilized nations”	Trans-systemic doctrines common to major municipal legal orders (e.g., estoppel, unjust enrichment, good faith)	Rarely applied nomen proprio but implicitly relied on in equity-infused decisions such as Reyes v. Republic (escheat; G.R. No. 135191, 2009) and in many human-rights rulings where “basic notions of justice” are invoked.

III. Subsidiary Means under Article 38(1)(d)

Judicial decisions – The ICJ’s own jurisprudence (e.g., North Sea Continental Shelf, Barcelona Traction, Nicaragua) and that of other tribunals are subsidiary means for determining the existence or content of a rule. Philippine use: The Supreme Court frequently quotes ICJ dicta for persuasive authority—e.g., Magallona relied on Qatar/Bahrain for baselines, while People v. Gozo (war crimes, 2022) cited the Nicaragua criteria for custom.
Teachings of the most highly qualified publicists – Works of authors like Oppenheim, Crawford, and Brownlie. Philippine courts refer to them sparingly to elucidate custom or treaty interpretation (Bayan v. Zamora, 2000).

Because these are only “subsidiary means,” they do not create law strictly speaking, but they fill evidentiary gaps regarding primary rules.

IV. Equity and the ex aequo et bono Power

Article 38(2) allows the ICJ to decide a dispute ex aequo et bono if the parties consent—effectively elevating equity over positive rules. Philippine tribunals cannot decide ex aequo et bono without legislative warrant, but they may apply equitable considerations within the confines of the Civil Code and custom (see Art. 10, Civil Code).

V. Modern Additions and Debates

Concept	Status at ICJ Level	Philippine Engagement
Jus cogens (peremptory norms)	Recognized by Barcelona Traction and codified in VCLT Arts. 53 & 64	Frequently invoked in rights-based litigation (e.g., Vinuya v. Executive Secretary, 2010, comfort-women claims), but still doctrinally unsettled domestically.
Obligations erga omnes	Barcelona Traction, Bosnia Genocide	Echoed in Pimentel v. Office of the Executive Secretary (2005) to explain Article 19 UDHR duties.
UN General Assembly resolutions, soft law, SDGs	Not binding per se; may crystallize custom	Cited as interpretive aid (e.g., environmental cases invoking UNDRIP and SDGs).
Unilateral acts (e.g., declarations, protests)	Binding if intent is clear (Nuclear Tests Case)	The Philippines’ 2011 Notification of Submission to UNCLOS Annex VII Tribunal (South China Sea Arbitration) is treated as binding vis-à-vis dispute settlement.

VI. Doctrinal Interaction with the Philippine Constitution

Treaty clause (Art. VII, § 21) → Executive negotiates and signs; Senate concurs.
Incorporation clause (Art. II, § 2) → Customary international law and general principles are self-executing.
Supremacy interplay → Once in force, both treaties and customary norms are at par with statutes but inferior to the Constitution (Tañada; Bayot v. Sandiganbayan, 1991). A later-in-time statute may override an earlier treaty domestically (the Mortal-enemy Rule), but would put the State in international breach (People v. Gozo acknowledged but declined to apply this conflict).

VII. Philippine Case Law by Source Category

Source Type	Representative Decisions	Key Take-Aways
Treaties	Tañada v. Angara (WTO Agreement), Bayan v. Zamora (VFA), Saguisag v. Exec. Sec. (EDCA)	Senate concurrence indispensable; executive agreements valid for “implementing” or “ancillary” matters.
Custom	Kuroda, Yamashita, People v. Gozo, Magallona	Incorporation clause gives CIL domestic effect even absent statute.
General Principles	Secretary of Justice v. Lantion (due process in extradition), Government v. Sandiganbayan (equitable tolling)	Employed largely to fill statutory lacunae.
Judicial decisions & Teachings	Magallona (ICJ’s Qatar/Bahrain), Vinuya (Nuremberg precedent)	Persuasive, not binding; weight depends on relevance and prestige.

VIII. Interaction with the South China Sea Arbitration

While the 2016 Award was rendered by a UNCLOS Annex VII tribunal, it heavily referenced ICJ rules on evidence of custom and treaty interpretation—rules ultimately derived from Article 38(1). Philippine foreign-policy organs have invoked the Award as “integrated into the corpus of international law”, demonstrating how Article 38(1) logic migrates into specialized fora.

IX. Practical Guidance for Philippine Lawyers and Courts

Prove the source first. Identify whether a rule is treaty, custom, or general principle; attach official texts, State practice, or comparative doctrinal proof.
Observe constitutional channels. Treaties need Senate concurrence; customs do not.
Cite ICJ and arbitral decisions carefully. They persuade but do not bind; explain factual parity.
Mind hierarchy conflicts. A later statute conflicting with an earlier treaty remains valid domestically but may expose the State internationally—advise legislative amendments instead of silent repeal.
Use “soft-law” instruments as evidence of opinio juris, not as binding in themselves.

X. Conclusion

Article 38(1) of the ICJ Statute remains the master key to identifying what counts as international law. The Philippines—through constitutional text, long-standing jurisprudence, and modern foreign-policy practice—has woven those sources into the national legal fabric:

Treaties via the Senate’s political filter;
Custom via the Incorporation Clause;
General principles via the Civil Code’s open texture; and
Subsidiary teachings via persuasive citation.

Because international controversies—from the South China Sea to cyber-governance—turn on the pedigree of rules, Filipino practitioners must stay fluent in the Article 38(1) grammar. Mastery of that grammar ensures both constitutional fidelity at home and credibility abroad.