Here’s a Philippine-context explainer on the relationship between international law and Philippine domestic law—what the Constitution says, how treaties and customary norms operate in courts and agencies, what happens when norms conflict, and how lawyers actually use (or defend against) international rules in practice.

One-minute snapshot

The Constitution is supreme domestically. Nothing—treaty, statute, or executive agreement—can override it.
Customary international law (“generally accepted principles of international law”) is automatically part of Philippine law via the Incorporation Clause (Art. II, Sec. 2).
Treaties and international agreements bind the Philippines internationally once validly concluded; domestically, a treaty has the force of statute after the constitutional process (usually: presidential ratification + Senate concurrence by 2/3). Executive agreements can bind internationally but cannot amend statutes or the Constitution.
Courts distinguish self-executing treaties (judicially enforceable without further legislation) from non-self-executing ones (which need an implementing law or regulation).
If a treaty/statute conflict arises, they are of coordinate rank in Philippine law; last-in-time (the later norm) typically governs domestically, but the State may still incur international responsibility if it breaches its treaty by applying a later statute.
Jus cogens (peremptory norms) and certain erga omnes obligations sit at the top of the international plane. Domestically, courts give them serious weight, especially in human-rights and humanitarian-law issues.

The constitutional architecture

1) Supremacy of the Constitution

The Constitution is the highest domestic norm. Any treaty, statute, regulation, ordinance, or executive act void if contrary to it.
A treaty cannot be a backdoor constitutional amendment; if a treaty obligation requires a constitutional change, Congress or the people must amend the Constitution first.

2) Incorporation Clause (Art. II, Sec. 2)

“The Philippines… adopts the generally accepted principles of international law as part of the law of the land…”
Result: customary rules (e.g., pacta sunt servanda, immunity of states and diplomats, non-refoulement as a widely accepted standard, rules of state responsibility, basic IHL customs) are judicially cognizable even without a statute or treaty, provided their customary character and relevance are shown.

3) Treaty-making and consent to be bound

President negotiates/ratifies; Senate gives concurrence by 2/3 of all members for treaties/international agreements contemplated by Art. VII, Sec. 21.
Executive agreements (made by the President without Senate concurrence) are valid international commitments if within executive authority and not contrary to statutes/Constitution. Domestically, agencies and courts will respect them but they cannot repeal/amend a statute.

4) Judicial power and justiciability

Courts can invalidate domestic acts that violate the Constitution or exceed statutory power—even if those acts purport to “implement” a treaty.
Courts may invoke international law to interpret ambiguous statutes/constitutional provisions (the presumption of conformity with international obligations).

Sources of international law in Philippine fora

A. Customary international law (automatic incorporation)

How it reaches court: Plead and prove the two elements—general practice + opinio juris—through scholarly works, state practice, UN materials, and comparative jurisprudence.
Typical use cases:
- Immunities: state immunity from suit (unless consent), diplomatic/consular immunities.
- IHL & core crimes: war crimes, crimes against humanity, genocide (now also implemented by statute).
- Human rights baselines: dignity, freedom from torture, non-discrimination (often used as interpretive guides).

B. Treaties and international agreements

Becoming domestic law: After valid Philippine consent (e.g., Senate-concurred treaty) and publication/circulation, courts treat the treaty like a statute.
Self-executing vs. non-self-executing:
- Self-executing: Text supplies a sufficiently definite rule (e.g., jurisdiction clause, immunities clause, status-of-forces venue rules). Courts can apply it directly.
- Non-self-executing: Framework/aspirational language (e.g., “States shall take measures to…”). Needs implementing legislation (e.g., anti-torture, IHL, child protection, environmental regimes).
Executive agreements: Enforceable on the executive branch and persuasive to courts, but cannot defeat a statute or cure a constitutional defect.

C. Domestic implementing laws (transformation)

Congress often transforms treaty obligations into local statutes (e.g., international humanitarian law, anti-torture, enforced disappearance, trafficking). Courts then apply the statute; the treaty remains background context.

Conflicts and hierarchies: which rule wins?

Constitution vs. Treaty/Custom
- Constitution wins domestically. The State must then seek reservation/derogation internationally or face responsibility.
Treaty vs. Statute (same rank)
- Domestically, apply harmonization first. If irreconcilable, last-in-time controls in Philippine courts.
- Internationally, the Philippines cannot invoke internal law to justify non-performance of a treaty; it may incur international responsibility.
Statute vs. Executive Agreement
- Statute controls domestically. The executive agreement cannot amend or repeal it.
Custom vs. Statute
- Courts try to reconcile; where Congress clearly intended to depart from a customary norm not of jus cogens character, the statute may control domestically (again with possible international responsibility consequences).
Jus cogens (peremptory norms)
- No treaty, statute, or executive act validly derogates at the international level. Domestically, courts give such norms paramount weight when interpreting rights and duties.

How Philippine courts actually use international law

As direct rules (self-executing treaty provisions; incorporated custom).
As interpretive guides to resolve ambiguity in statutes/constitutional text in favor of compliance with international obligations.
To assess validity of executive acts (e.g., status-of-forces, bases/defense arrangements, extradition/MLAT procedures) against due process and equal protection.
To fill gaps in areas where Philippine legislation is silent but a generally accepted international rule exists (subject to constitutional bounds).

Illustrative domains

Defense & Visiting Forces arrangements: Status, custody, jurisdiction, and criminal process often turn on treaty text (sometimes self-executing) read alongside due process guarantees.
Extradition & MLA: Treaties are honored, but the rights of the requested person (notice, counsel, basic due process) are enforced under the Constitution.
Maritime & baselines: Congress implements sea-boundary treaties/UNCLOS through statutes; courts respect the statute while reading it in light of international maritime law.
Human rights & humanitarian law: Many obligations are now doubly anchored—by treaty/custom and by implementing statutes—so courts enforce them as domestic criminal/civil rules.
Refugees/Stateless persons: Even where statutes are skeletal, agencies and courts use treaty/customary standards (e.g., non-refoulement, due process in status determination) to guide decisions.

Practice toolkit: using (or resisting) international norms

If you want to invoke international law:

Identify the source: Treaty (cite text, status, Senate concurrence); custom (show state practice + opinio juris); or implementing statute.
Argue self-execution (text is specific enough), or point to implementing law.
Harmonize with Constitution/statutes; invoke the presumption of conformity (courts avoid readings that place the Philippines in breach).
Show domestic justiciability: Standing, ripeness, concrete controversy; avoid pure policy asks to the judiciary.

If you want to resist it:

Challenge self-execution (treaty is programmatic; needs legislation).
Show statutory displacement (a later, clear statute governs domestically).
Raise constitutional limits (e.g., due process, equal protection, separation of powers).
Contest custom (no sufficiently general and consistent practice; absence of opinio juris; contrary persistent objection—rare but arguable).

Compliance cycle for the Executive and Congress

Join (sign/ratify) → 2) Concur (Senate, if a treaty) → 3) Transform (pass implementing law if non-self-executing) → 4) Implement (issue rules, train agencies, budget) → 5) Report (treaty bodies, UN mechanisms) → 6) Adjust (amend laws/policies when monitoring flags gaps).

Frequently asked questions

Is international law automatically superior to Philippine statutes? No. Domestically, a treaty and a statute are co-equal; the Constitution is supreme. Internationally, the Philippines remains bound to its treaties and may incur responsibility if it defeats a treaty by later statute.

Do courts apply customary international law without a statute? Yes—if it qualifies as a generally accepted principle and does not violate the Constitution or a clear domestic rule. Courts often use it as an interpretive aid or gap-filler.

Are executive agreements “less binding” than treaties? Internationally, both can bind the State. Domestically, an executive agreement cannot override a statute or the Constitution and must stay within executive power.

What makes a treaty self-executing? Language that is present, definite, and judicially manageable (e.g., “shall have X right / Y immunity”), not merely programmatic (“shall endeavor,” “shall adopt measures”).

What if the Philippines withdraws from a treaty? Domestic implementing statutes remain in force until repealed, and past obligations during membership may persist (e.g., cooperation on acts committed while bound).

Bottom line

The Philippines runs a hybrid system: monist for custom (via the Incorporation Clause), largely dualist for treaties (they need constitutional consent and, if non-self-executing, an implementing law).
Constitutional supremacy governs at home; pacta sunt servanda governs abroad. Effective advocacy maps both planes: what courts can enforce now and what the State must honor internationally.
In hard cases, start with hierarchy and harmony: read domestic rules to conform to international obligations where possible—without sacrificing constitutional guarantees.

If you share a specific context (e.g., defense arrangement, extradition, refugee claim, maritime entitlement, or a human-rights petition), I can outline the precise sources, forum, and arguments to deploy—plus whether you’ll need implementing legislation or can proceed on a self-executing/customary footing.